|lllllllllll|llllll|lll|ll|lll||l|lllllllll 3 1293 00895 5878 This is to certify that the dissertation entitled SPATIAL DISTRIBUTION AND WATER UPTAKE OF ROOTS IN STRUCTURED GROWTH MEDIA presented by Mariana Amato has been accepted towards fulfillment of the requirements for DOCTOR degree in PHILOSOPHY Gad/57M; Major professor Date November 14, 1991 MS U is an A fflrman've Action/Equal Opportunity Institution O—12771 r “-— WV. LKBRARY Michigan State University K. —_. PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. ‘ E DATE DUE DATE DUE DATE DUE MSU Is An Affirmative Action/Equal Opportunity Institution ' c:\cIrc\datedm.prn3-p.1 SPATIAL DISTRIBUTION AND WATER UPTAKE OF ROOTS IN STRUCTURED GROWTH MEDIA BY Mariana Amato A DISSERTATION Submitted to Michigan State university in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Crop and Soil Sciences 1991 g7?-qs77 ABSTRACT SPATIAL DISTRIBUTION AND WATER UPTAKE OF ROOTS IN STRUCTURED GROWTH MEDIA BY Mariana Amato Root water uptake has classically been modeled based on the assumption that roots are distributed evenly within soil layers. In many instances, though, root distribution is more likely to be clustered than regular or random, and the distance water has to travel from bulk soil to root is larger than average distance between roots. This can imply limitations to water uptake in soil regions far from the root cluster. A study is presented that characterizes root clustering and water uptake in relation to soil structural status. To measure small scale variability in volumetric water content, 21 mm long probes were designed for a time-domain reflectometer. Water content values higher than 0.07 cmP cm'3 were reliably measured in a sandy-clay and in a clay-loam soil. In the clay-loam, at water content higher than 0.29 cm3 cm'3, a few excessively high values of dielectric constant were measured, yielding excessively high values of 6v. In a greenhouse experiment, maize (Zea mays L.) was planted in soils of different structural status and grown on stored water. Growth and water uptake were affected by soil structure; roots.grew'into sieved sandy-clay soil or shrinkage cracks but did not penetrate clay-loam peds beyond 2 cm from the surface, unless biopores were present. Unextracted water was left in peds, even after plants had lost practically all green leaf area, and large gradients in.6v‘were measured as a consequence of root clustering; In a treatment with.uniformly compacted clay-loam soil, very little root and plant growth was measured, and no wilting occurred although water extraction was small. Water outflow from peds was modeled assuming that a ped could be simplified by a cylinder, and that flow was radially symmetrical. Experimentally measured water gradients in peds could be reproduced by assuming that soil water diffusivity in peds ranged from 4.29%10'2 to 10 cm2 day". In a field experiment maize was grown in a swelling soil with three structural treatments corresponding to minimum tillage, tillage to 50 cm, and loosening of the profile up to 100 cm. Plant growth and yield, as well as root and water uptake patterns were related to soil structure. ACKNOWLEDGEMENTS Many are the people who have directly or indirectly contributed to the development and.completion of this project. I thank ‘them. all, and among' them. I ‘wish, to especially acknowledge Dr. J. T. Ritchie, an outstanding advisor, for the enthusiasm and the encouragement in independent thinking; Dr. J. Crum, for generous help and suggestions in more than one project at M.S.U.; Dr. A. Smucker for orientation in the world of root biophysics; Dr. J. Flore for insight in above-ground plant physiology. Special thanks to Brian Baer for always finding a solution 'to computer-related. and other’ problems, and ‘to Sharlene Rhines for helping with all the arrangements necessary for my frequent coming and going between M.S.U. and Italy. Many thanks to my colleagues in.the‘Nowlin Chair group, and to my friends in the department and outside for help at one point or another, and for making my stay at M.S.U. an enriching experience. I wish to acknowledge Prof. F. Basso, and Prof. E. Tarantino for availability of the facilities at Universita' della Basilicata-Italy, and for working around my traveling schedules, and Prof. L. Postiglione for effective iv encouragement from the early phases of the Ph.D. project. Thanks to Prof. C. Ruggiero for useful suggestions about root measurement, and to Dr. E. De Falco for filling in for me on many occasions when I was away from UB. I wish to thank E. Parente and S. Mazzoleni for precious discussion and suggestions on root and water spatial variability, and F. Zucconi, for continuous inspiration and encouragement in this project, and for precious teachings and discussion on plant physiology. Thanks to S. Pagano for help in developing the water flow program and for support in the final phases of my work. TABLE OF CONTENTS List of Tables . . . . . . . . . . . . . . . . . . . . viii List of Figures . . . . . . . . . . . . . . . . . . . . . x Introduction . . . . . . . . . . . . . . . . . . . . . . 1 Water uptake from root systems . . . . . . . . 1 Spatial distribution of absorbing roots . . . . 8 Consequences for water uptake . . . . . . . . . 10 The root as a spatially variable water sink . . 13 Causes of xy (areal) spatial variation . . . . 18 A- plant . . . . . . . . . . . . . . . . . 19 I- root geometry and branching . . . 19 i- geometrical relations between root parts . . . . . . . . . . . . . 19 ii- morphogenesis . . . . . . . 21 II- competition and chemical interactions between roots . . . . . . . . . . . . 22 III- growth relations and partitioning within the plant . . . . . . . . . 23 3- soil . . . . . . . . . . . . . . . . . 23 C- interactions . . . . . . . . . . . . . 25 Statistical methods and indexes . . . . . . . . 25 Techniques for measuring on a relevant scale . 27 A- Root measurements . . . . . . . . . . . 28 8- Soil water content measurements . . . . 29 C- Measurement of soil properties . . . . 30 Structured growth media . . . . . . . . . . . . 30 References . . . . . . . . . . . . . . . . . . 33 Chapter 1” Small scale soil water content with Time Domain Reflectometry (TDR) . . . . . . . . . . 41 Abstract . . . . . . . . . . . . . . . . . 41 Introduction . . . . . . . . . . . . . . . 42 Materials and methods . . . . . . . . . . 44 Results and discussion . . . . . . . . . . 50 Conclusions . . . . . . . . . . . . . . . 58 Acknowledgments . . . . . . . . . . . . . 59 References . . . . . . . . . . . . . . . . 59 ChaPter 2. Plant growth and water uptake in structured growth.media. I: Maize (Zea mays L.) top growth and water use . . . . . . . . . . . . . . . . . . . 61 Abstract . . . . . . . . . . . . . . . . . 61 vi Introduction . . . . . . . . . . . . . . 62 Materials and methods . . . . . . . . . . 63 Results and discussion . . . . . . . . . . 66 References . . . . . . . . . . . . . . . . 82 Chapter 3. Plant growth and water uptake in structured growth media. II. Clustering of maize (Zea mays L.) roots and spatial distribution of water . . . 84 Abstract . . . . . . . . . . . . . . . . . 84 Introduction . . . . . . . . . . . . . . . 85 Materials and methods . . . . . . . . . . 86 Results and discussion . . . . . . . . . . 91 Acknowledgements . . . . . . . . . . . . 129 References . . . . . . . . . . . . . . . 129 Chapter 4. Plant growth and water uptake in structured growth media. III: simulation of water outflow from peds . . . . . . . . . . . . . . . . . . . . 132 Abstract . . . . . . . . . . . . . . . 132 Introduction . . . . . . . . . . . . . . 133 Materials and methods . . . . . . . . . 135 Results and discussion . . . . . . . . . 142 Acknowledgments . . . . . . . . . . . 157 References . . . . . . . . . . . . . . . 158 Chapter 5. Maize (Zea mays L.) growth and.water uptake in a vertic-ustorthents soil. Root spatial variability and water uptake . . . . . . . . . . . . . . 160 Abstract . . . . . . . . . . . . . . . . 160 Introduction . . . . . . . . . . . . . . 161 Materials and methods . . . . . . . . . 163 Results and discussion . . . . . . . . . 168 References . . . . . . . . . . . . . . 198 Summary and Conclusions . . . . . . . . . . vii O O O O 200 LIST OF TABLES Table 1.1. Pairwise t-test.for’the comparison.between.6wm,and 6V about .07 cm3 cm-3. INTRODUCTION The characterization of spatial distribution of roots and. water’ movement can jprovide important information. to quantify plant water uptake and to explain discrepancies between plant-soil-water model predictions and field-scale water measurements (Tardieu and Manichon, 1986; Passioura, 1988). However, the description and prediction of soil water movement in a way relevant to field conditions requires a transition from relatively uniform and well-defined systems to the heterogeneity in space and time that occurs in the field (Hamblin, 1985). The study of small scale soil water distribution has been limited by lack of suitable techniques operational at the required resolution. Dunham and Nye (1973), using a thin section (2 mm) technique, determined the gravimetric water content of soil layers as a function of distance from a plane of onion roots. Hsieh et al. (1972) studied the bidimensional water distribution around root hairs using a non-destructive gamma-ray technique. Hainsworth and Aylmore (1983) quantified small-scale soil water content using computer-assisted 43 tomography with x-ray, with 2 by 2 mm pixels on 5 mm thick planes. Each of these techniques was developed for laboratory measurements in containers. Small scale techniques have not been. developed for field. characterization of soil water content. Time-domain reflectometry is a recent technique for the measurement of the volumetric soil water content that can be used in container studies as well as in the field. It is based on the determination of the soil relative dielectric constant e, from the velocity of propagation of an electromagnetic signal (IMHz - lGHz) in the soil. Since the relative dielectric constant of water (81.5 at.20 'C) is about 20 to 40 times larger than that of the dry soil (2-4), the measured values of e are strongly related to the volumetric water content in soil. The EM signal travels in the soil along a transmission line, whose propagation characteristics (variations of impedance as a function of distance) are displayed on an oscilloscope screen as a trace with length units on the x-axis. The length of the wavetrace relative to 'the transmission line in the sample, is the basis for dielectric constant determination. Topp et al (1980) suggested an empirical equation to calculate the volumetric vwater content from c in mineral soils. A number of works have contributed to define limitations, advantages and possible future developments of the technique. Among the limitations, it has been pointed out that the maximum length of the TDR “J.,—— 44 transmission line is dependent on the soil type, and that in soils with high clay content signal attenuation may limit the maximum length to less than 1 m, while lines up to 20 m long have been used in sandy soils (Topp and Davis, 1985). The use of short transmission lines for small scale measurements is limited by the instrument accuracy, and depends on the line geometry. Topp et al. (1984) reported consistently low values of TDR-determined versus gravimetrically-determined volumetric water content when measuring water content in the 0-5 cm soil layer, using 150 mm parallel balanced metal rods partly inserted in the soil. No other data have been reported on transmission lines shorter than 100 mm. The aim of the present study is to test the performance of short transmission lines for TDR.measurement of soil water content. Since TDR determinations are based on the determination of the wavetrace length, the error in tracelength measurement was determined for traces ranging from 10 to 150 mm. Then soil water content was measured in two soils with 21 mm long transmission lines. MATERIALS AND METHODS A Tektronics 1502 B cable-tester was used. The ‘transmission lines were parallel balanced. A.balun (impedance transformer Anzac TP 103) was used in order to minimize the impedance mismatch at the transition from coaxial to parallel 45 balanced geometry. The transformer was mounted on a fiberglass board, and it was connected to the transmission line through banana plugs (Figure 1.1). In order to evaluate the error in length measurement using short waveguides, the following procedure was used: the velocity of propagation of the signal was set at 99% of the propagation velocity in air, so that tracelengths would be at the same scale as the lengths measured on a transmission line in air and could be compared directly. A 150 mm long parallel balanced transmission line was set in air, by inserting a couple of 200 mm stainless steel waveguides (diameter 5 mm; distance between rods: 50 mm) on a 50 mm styrofoam support so that 150 mm would be left in air. The Time-domain reflectometer was connected to the guides by inserting the balun banana plugs in the styrofoam until contact with the probes was reached. A zero spatial reference was set.by short- circuiting the waveguides at one point with a thin blade, and recording its position on the wavetrace. The rods were marked ‘with a razor blade in 10 mm increments up to 150 mm (measured with a caliper). Lengths were measured with TDR by shortcircuiting the waveguides at the marked points and determining on the oscilloscope the length of the trace :telative to the chosen distance. Measurements were replicated five times. Two soils were used for water content determination: a clay loam (sand 36.5%, silt 24.1%, and clay 39.4%.), and a 46 REFLECTDHETER PULSE GENERATOR SAMPLING RECEIVER UMXMLCMlE BALUN TIMER I PARALLEL LINE DISPLAY (BANANA PLUGS) §|-—Let ——|§ Figure 1.1. Time-domain reflectometer and connections for a parallel-balanced transmission line. 47 sandy clay (sand 52.3%, silt 10.6%, clay 37.1%)(ISSS classification: sand 2 - 0.02 mm, silt .02-.002 mm, clay <.002 mm.). For each soil 101 measurements were made, comparing TDR- determined and gravimetrically determined volumetric soil water content on samples prepared as follows: a 100 by 100 mm wooden frame, 21 mm high, was attached to a plastified cardboard bottom. The box was filled with sieved soil and carefully leveled to 21 mm, and covered with plastic wrap to prevent evaporation during the measurements. The transmission line for soil water content determinations consisted of a couple of stainless steel syringe needles truncated at.21 mm, inserted on.a rubber support at.a.distance of 14 mm (Figure 1.2). The needle plastic sockets were used for connection with the balun-board banana plugs. Two TDR measurements of soil water were made in each box, with the described transmission lines. The relative dielectric constant e of the samples was calculated from wavetrace length according to Topp et al. (1980): ( c . Let T e = ----------- vp . Lr twhere: c = propagation velocity of an electromagnetic signal in void = 3*1081n sec'1 v5 = propagation velocity of the electromagnetic signal in the transmission line (m sec”). vp can be set on the instrument by the user. 1+ = transmission line length (m) Let length of the wavetrace (m) and the volumetric water fraction of the soil was calculated 48 L__ .r_3 PISTON l l PLASTIC SOCKETS [::] [P [:3 L_i 4‘1 RUBBER SUPPORT CYLINDER 21 mm [Lrl l l l F——l4 mm-fi f———— 20 mm-————{ Figure 1.2. a) Transmission line for small-scale TDR water Content measurements in soil; b) Soil sampler for grav1metr1c determinations. 49 with the equation: 6mm = -5.3*10'2+2.92*10'2*e-5.5*10”°*ez+4.3*10'6*e3 (Topp et al., 1980) i where Gym,” = TDR-determined volumetric soil water content c = relative dielectric constant. This equation was found to satisfactorily express the relationship between volumetric water content and relative dielectric constant for a range of mineral soils (Topp et al, 1980), and was tested.by Amato et al. (unpublished, b) for the same clay-loam soil used in this experiment, with 150 mm transmission lines. According to Topp and Davis (1985) the area of soil explored by a transmission line with the geometry described is a cylinder having length.of 21 mm and diameter of about 20 mm. In order to provide a comparison for Ova“), after each measurement the soil around each transmission line‘was sampled with a plastic sampler (cylinder + piston, see Figure 1.2) with inner diameter of 20 mm. After inserting the sampler, the area around it was cleared from the soil and.the cardboard bottom was cut so that the soil cylinder sampled could be ejected by presSing the piston. The water content was then determined by weight difference on the sampled volume. Gravimetrically-determined 'volumetric soil water' will be indicated as Gym in the rest of the chapter. Measurements were made at water content levels ranging from. oven-dry' to saturated soil. Samples for’ which. the 50 gravimetric sampling procedure visibly caused problems in volume sampling (loss of soil, excessive compaction) were discarded. A total of 101 values for each soil was obtained. RESULTS AND DISCUSSION The results for length measurements.in.air are summarized in Figure 1.3. The standard deviation of trace length measurements increased with the mean, ranging from 0.84 mm to 3 mm. The coefficient of variation of the length measurements was quite high for 10 and 20 mm measurements (7.5% and 4.6% respectively), and decreased with length increase. In the range 30 - 90 mm, which corresponds to the range of airdry to saturated soil (see below), the coefficient of variation was <= 2.8% Figure 1.4 reports the calculated error for trace length determination, based on the instrument's horizontal accuracy (redrawn from Amato et al. , unpublished, a). The accuracy reported on the data sheet is+-(0.6mm+2% of the reading), therefore the absolute error increases with 'tracelength.( 0.8 to 3.6 mm for tracelengths of 10 to 150 mm), ‘while the percent error decreases, tending to an asymptotic ‘value of 2% (Amato et al., unpublished a). Comparing Figure 1.3 and 1.4 it can be seen that in a few cases the standard deviation values from actual measurements were slightly lower than the calculated absolute error. The reason for it could be that the theoretical error was based on the instrument's 51 150‘ 1 :1._-. .. y.‘ .. ‘3‘". 120- c A _, E ‘ “.o' .5, . ’ £ 90- 0 on ‘ ‘ s . 5) ~ ‘1 60- \ _ a 8 i \ .“o o a I: ‘ ",0 30— ".k; B~ ’8 r B“ . ..-"‘.-. ".‘ N“ "Hu‘ ‘ ,0" 8' ‘_\G‘ """ Kappa; «8 acres 0 Trace lengths . 112.....-I-,...uu-a """ ‘ en ow 10—2 O t-A sd 4 10 I I l I l I I f 1 I I T I I l 0 3D 60 90 120 150 Rod Length (mm) Figure 1.3. Comparison between length of transmission line in air and TDR trace length.CV = coefficient of variation (%) . sd=standard deviation (mm) . 52 150- 120- L O L It L q) 901 ‘0 . a.) “K 4—1 2 ‘ \ 3 a 2 60 \ c . o . R i \c:\ ___‘ ..... A .. a. _, .....A-- 30 a. 1: :‘ififls‘; -6 ~G—n ‘ ...... A """"" . A m-..” ----- F" s—u Percent Error . 10-2 0 k-A Absolute Error (mm m 10) ' l V F l f I I T—l I f 1 0 30 60 90 120 150 Figure 1.4. determination, calculated from data sheet specifications of the Tektronics 1502 B cable-tester. Trace Length (mm) Absolute and percent error for trace length 53 specifications. The actual performance may be better in case of quite short transmission lines. The compar1son between measured values of Gym” and 6v“) is reported in Figures 1.5 and 1.6 for the clay loam and the sandy clay soil respectively. The differences 8v,g)-6v,TDR) were analyzed with a paired-t test, the results of which are presented in Table 1.1. For the clay loam soil there was a good agreement between the two methods. The overall standard deviation was about 0.023 cm3 cm'3, but in some cases the deviation between methods reached values up to about 0.05 cm3 Cm'3. The differences found were not significant for P = 0.01 and P = 0.05. For samples at 6v(g)< 0.07 cm3 cm’3 the standard deviation was higher than the overall, probably due to the higher percent error in trace-length determination, especially for the oven-dry samples. For these, the tracelength was between 0.020 and 0.025 m, and thus it was subjected to a percent error of 3.0 to 4.6%, as discussed above. For dry Samples the errors in gravimetric determination may have had a higher relative importance than at high water content. Besides, with dry samples being quite loose, small volume Sampling may have implied relatively high soil losses. The latter source of error would explain the underestimation of 6v With the gravimetric method observed for many samples at the dry end of the curve. In moist samples (6W9) > 0.29 cm3 cm'3) , about 30% of the 6 v(TDR) values were excessively high (0.14 to 0.40 cm3 cm'3 higher than va ). Such discrepancies can only 54 0.70-1 0 1:13". -i O 0.60- 00 d O O 1‘; 0.50- o l E - o O S n 0.40- 0 00,63 E - 01-! CD& 0 gs?) O v too 1:? E “0 Cb I l l l l l l I I j 030 0.40 0.50 0.60 0.70 6v(g) (C m5 C m '3) Figure 1.5. Comparison between TDR-determined (6vmm) and gravimetrically-determined (Bi/(9)) volumetric water content in Clay loam soil. 55 0.70- ,. . 1:1.-' 060- ' a? 050- AA 0 o 4o~ #3.“ n . A,v’.' A g l Niki/$46“ v 030— by E - Agar“ Q 020- A928 A tar A -i ‘ A O as .10- IJMASBA « A A it A 0.00 FA" T I I T f I l I I I I l l 00b 0.10 0.20 0.50 0.40 0.50 0.60 0.70 6v(g) (cm3 cm-S) Figure 1.6. Comparison between TDR-determined (evtTDR)) and gravimetrically-determined (emu) volumetric water content in sandy clay soil. 56 Table 1.1. Pairwise t-test for the comparison between 0\r(g) and 0v(TDR). 0v(g)-0v(TDR) clay loam soil # sandy clay soil mean -0.004 0.002 sd 0.024 0.023 sdm 0.00241 0.00241 t 0.973 1.582 d.f. 100 92 n.s. n.s. # for the clay loam soil the 8 points with 0v(g)-0v(TDR) > 0.14 cm3 cm-3 were excluded from analysis. sd = standard deviation of differences sdm = standard error d.f. = degrees of freedom 57 partly be explained by sampling problems that occur in swelling soils at high water content levels. Small volume sampling may cause considerable compaction. For the rest of the samples at 0v,9)> 0.29 cm3 cm'3, the standard deviation of the difference 0v,9)-BV,TDR) was comparable with the overall. No excessively high values were reported by Amato et al. (unpublished, b) for the same soil with 150 mm transmission lines. For the sandy clay soil, the overall standard deviation of the difference Eng-Ova“) was 0.024 cm3 cm3, and it was higher for both the dry and the wet end of the range. The excessively high values of Ova“) recorded for the other soil at high water content were not found. Therefore, it is probably safe to attribute the errors to problems in sampling and length determination. Also, for all samples, volume sampling for the gravimetric determination was made on the assumption that the region explored by the TDR technique was a cylinder with diameter 1.4 times the distance between waveguides, as reported by Topp and Davis (1985) on an empirical basis, but the actual distribution of the EM field around the transmission line would be more complex, and Baker and Lascano (1989) report a region of lower sensitivity larger than that described above. This would be source of discrepancies between 6mm and 0 in case of heterogeneous vtg) water distribution, because the two measures would refer to different soil volumes. Although the boxes for this experiment 58 were prepared so to have uniform water content, some spatial variation may have occurred, especially in the degree of soil compaction, that may have had some influence on the spatial heterogeneity of volumetric water content. There are not many data reported in the literature on centimeter-scale water content determination with TDR. Topp et a1 (1984) , obtained consistently low readings using Time Domain Reflectometry with probes inserted in the soil for 50 nun only. Their error is discussed in relation to travel time accuracy with short tracelength, although the consistent bias would suggest a problem of calibration at small scale, txossibly due to the transmission lines used: the TDR probes Lused for the small scale measurements were long, only partly iJaserted.in soil, and tapered, which can cause a higher error .in.end-of-trace determination. The probes used in the present experiment are parallel, of constant diameter and less spaced, which contributes to a higher resolution in the measurements (Topp and Davis, 1985). CONCLUSIONS Time-domain reflectometry can be used for the determination of soil water content at small scale with the probes described. The measurements in air and in dry soil Proved that in case of tracelength lower than about 0.03 m (ovendry to airdry soil for the transmission lines used) the 59 percent error is quite high (4.7% and up), so the technique is less reliable. For water content higher than 0.07 cm3 cm-3 (tracelength higher than 0.03 m) the percent error in length determination was below 2.8% . In the clay-loam soil, at 0v,9)>0.29 cm3 cm'3 some of the 0v(TDR) values were excessively high, more than expected from errors in length determination or sampling problems, possibly due to problems of signal transmission in conductive soils. This can cause problems for high water content determination with small probes in such soils, although some of the values found were so high that they could be eliminated by a visual screening. ACKNOWLEDGMENTS I wish to thank the Laboratory of Irrigation-C.N.R.-Na- Italy, for the use of the Tektronics cable-tester, Dr. S. Pagano and Dr. B. Olivieri from C.N.R.-Italy, for useful discussion and suggestions on TDR tracelength problems, and Dr. F. Pierce, M.S.U. , for critical reading of the manuscript. REFERENCES Amato, M., De Lorenzi, F., and Olivieri, B. a) Riflettometria nel dominio del tempo per la misura dell'umidita' volumetrica del terreno. I: principi generali ed applicazioni. Amato, M., De Lorenzi, F., and Olivieri, B. b) Riflettometria nel dominio del tempo per la misura dell'umidita' volumetrica del terreno. II: misure in substrati diversi e profili idrici. 60 Baker, J.M., and Lascano, R.J. 1989. The spatial sensitivity of Time-Domain Reflectometry. Soil Sci. 147(5):378-384. Dunham, R.J., and Nye, P.H., 1973. The influence of soil water content on the uptake of ions by roots. I. Soil water content gradients near a plane of onion roots. J. Appl. Ecol. 10: 585-98. Hainsworth, J.M. , and Aylmore, L.A.G. , 1983. The use .of computer-assisted tomography to determine spatial distribut1on of soil water content. Aust. J. Soil Res. 21(4): 435-443. Hamblin, A., 1985. The influence of soil structure on water movement, crop root growth and water uptake. Adv. Agron.: 95- 157. Hsieh, J.J.C., Gardner, ‘W.H., and Campbell, G.S., 1972. Experimental control of soil water content in the vicinity of root hairs. Soil Sci. Soc. Amer. Proc. 36:418-421. Passioura, J.B., 1988. Water transport in and to roots. Ann. Rev. Plant Physiol. Plant Mol. Biol. 39:245-65. Tardieu, F., and Manichon,H., 1986. Caractérisation en tant que capteur d'eau de l'enracinement du mais en parcelle cultivée. I.-Discussion des criteres d'étude. Agronomie 6,4:345-354. Topp, G.C., and Davis, J.L., 1985. Time-domain reflectometry (TDR) and its application ‘to irrigation scheduling. in: Advances in irrigation (D. Hillel,ed.) Academic press, New York 3:107-127. Topp, G.C., Davis, J.L., and Annan, A.P., 1982. Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Resources Res. 16(3):574-582. Topp, G.C., Davis, J.L., Bailey, W.G., and Zebchuck, W.D., 1984. The measurement of soil water content using a portable TDR hand probe. Can J. Soil Sci. 64:313-321. CHAPTER 2 PLANT GROWTH AND WATER.UPTAKE IN STRUCTURED GROWTH MEDIA. I: MAIZE (ZEA.MAYS L.) TOP GROWTH AND WATER.USE. ABSTRACT A study on maize (Zea mays L.) growth and.water uptake in structured soil material was conducted with the objective of quantifying the effect of soil structure on water availability for plants. Five treatments were compared in 100 cm deep containers: C (clay-loam soil), S (sandy-clay soil) S+LA (sandy clay with large clay-loam peds embedded), S+SA (Sandy clay with small clay-loam peds embedded), CC (compacted clay- loam soil). The plants were grown on stored water until near total loss of green leaf area. An irrigated control was established for treatments C, S, and S+LA. Plant height, leaf area, and leaf number'were determined weekly. Plant.dry matter 'was determined at the end of the experiment. The total water “uptake was determined by weight difference of the containers. Plant growth was initially faster in the S treatment, but ended rapidly when the water supply was exhausted. In the CC treatment, plant growth was slow, and water uptake minimal, 61 62 resulting in a quasi-stationary situation with no signs of wilting. In the other treatments, water deficiency affected growth, leaf rolling and the rate of leaf appearance. In the C, CC, and S+LA treatments the total water extracted was relatively small, suggesting that unextracted water was left in the soil. The root/shoot ratio ranged from about 0.16 to about 0.20 in the dry treatments and from 0.12 to 0.14 in the irrigated ones. Irrigation increased the plant top and root size, and decreased the root/shoot ratio in all treatments, but the growth rates for C were smaller than those for S and S+LA even after irrigation, indicating that possibly factors other thanwwater deficiency affected plant growth.in the clay- loam soil. INTRODUCTION The effect of soil structural status on plant water uptake has been studied.mainly in relation to the influence of tillage on plant water relations. Soil compaction is believed to cause a reduction in water uptake due to limited penetration of roots (Hamblin, 1985) . More recently the effect of soil compaction on root clustering has been taken into account to explain limited water extraction within layers colonized by roots (Tardieu, 1977) . Such experimental evidence could be the result of complex soil-plant interactions, including effects of hypoxic soil conditions associated with ' .w—r" 63 compaction (Blackwell et.al., 1985) on root.growth (Schumacher and Smucker, 1984; Vorhees et al., 1975), and root activity (Everard and Drew, 1987), as well as direct effects of soil compaction on plant growth. It has been reported (Masle and Passioura, 1987) that high soil strength has an effect on plant growth that is independent of water and oxygen availability in the soil, and that root hypoxia can reduce shoot growth (Smit et al., 1989). In that case, a reduced water uptake may be partly a consequence and partly a cause of limited plant size in compacted soils. The present work aims to compare plant growth and water use in different types of structured soil materials in order to study the effects of soil structure on.root spatial distribution, water’uptake, and plant behavior in water-limited conditions. The first article presents total water uptake and shoot growth. The spatial distribution of roots and water is discussed in the second article (Chapter 3). MATERIALS AND METHODS A greenhouse experiment was conducted at Potenza (Italy) in which maize (Zea mays L.) Dekalb Vitrex 200L was grown in PVC cylindrical tubes. The tubes, 100 Cm high and 25 cm in diameter, were split in half longitudinally and then reassembled with tape so that at the end of the experiment they could be taken apart for soil sampling. Plastic fabric 64 was attached to the bottom and 3 cm of fritted clay were packed on the bottom to allow drainage. Five growth media were tested. They are as follows: 1) C : clay-loam soil (sand 36.5%, silt 24.1%, clay 39.4%)1 taken from a vertisol in four 25 cm layers; the soil was packed in the tube in 25 cm layers, and each layer was allowed to settle by two wetting-drying cycles to an average bulk density of 1.2 g cma; 2) CC : compacted clay-loam soil. The same soil as in C, wet- compacted to a bulk density of 1.5 g cm“; 3) S : sandy-clay soil (sand 52.3%; silt 10.6%; clay 37.1%)1 taken from the field in four 25 cm layers, sieved and packed to a bulk density of 1.0 g cmd; 4) S+LA : sandy clay embedded with large clay-loam peds. Three sub-prismatic peds per tube, of 14-18 cm smaller side, were used. The average bulk density of the final mixture was 1.1 g cm3; 5) S+SA : sandy clay embedded.with small clay loam peds. Eight kg of sub-prismatic peds, of 6-10 cm smaller side were used jper tube. The average bulk density of the final mixture was 1.1 g cmfi. The containers were filled with water, covered with jplastic and allowed to drain for four weeks. One maize plant ‘was transplanted in each container at the 4-leaf stage, and fertilized with 2.3 g NH,NO3. Each soil treatment had an E l Percentages in weight. ISSS classification: sand 0.02-2.00 mm; Sllt 0.002-0.02 mm; clay < 0.002 mm. 65 unirrigated water treatment in which plants were grown on stored water. Treatments C, S, and S+LA had an irrigated control in which plants were irrigated.every other day from 20 days after transplanting with 15 mm of water. Each treatment was replicated six times. On each plant the following measurements were taken weekly starting at 13 days after transplanting: plant height to the top ligule, leaf length from tip to ligule, and the number of fully expanded leaves. For 10 plants grown out of the experiment, the area of each leaf and the length from the leaf tip to the ligule was measured, and.a power regression equation was chosen, based on I? maximization, to express the allometric relation between the two leaf characters: Log10 Area = -1.217 + 2.09 * Log10 Length R2=0.95. Based on the obtained relationship, green leaf area was calculated for each sampling date. The experiment was terminated when the green leaf area of the plants was reduced to less than 20 cm3 per plant for the treatments C, S, S+LA, and S+SA. Treatment CC was ended on day 41 from transplanting, —<’ 0.05‘ / / x x“ LL] A x r z x .2 / m ,/ x / V x w w x w x x A x A .z‘ r 0.00 C CC 3 S+LA S+SA Treatment Figure 2.2.b. Extracted water from planting to termination of the experiment. Vertical bars represent twice the standard deviation. 70 treatments, the contribution of the clay-loam peds was responsible for an initial water content higher than that of the S tubes. The total water extracted, though, was equal or less than what measured in S. For the C treatment the total amount of water extracted was even smaller, and at the end of the experiment the soil had not been dried to the calculated lower limit of plant available water: Comegna et al., 1990, report for the same clay-loam soil, that a volumetric water content of about 0.28 cm3 cm'3, as found in the C treatment at the end of the experiment, corresponds, in laboratory determinations, to a pressure head of about - 0.5 Mpa, that is likely well above the roots ability to lower their potential. Figure 2.3 reports the time—course of plant height for the five treatments under study. Initial plant height was quite uniform, around 6 cm, and it increased rapidly in the S treatment, reaching 15.5 cm at 27 days after transplanting, but it thereafter decreased quite rapidly, while in the S+LA and S+SA treatments it reached a maximum at 34 days. The highest values were found in S+SA. The clay loam soil (C) Showed lower values, and a slower decrease after day 34 of transplanting. Plant height in the CC treatment showed a small increase after transplanting (7.8 cm at 13 days), and remained tEmit—e constant thereafter. A very similar trend was shown in leaf area time-course, as shown in Figure 2.4. Green leaf area fell to less than 20 cm2 after 34 days from transplanting for the S treatment, and after 41 days for S+LA, S+SA and C. Table 71 501) t o-o CC - Ge 0 H s 40-0‘ A—A S+LA E _ A—A S+SA O V 44 30'O_ /I ......... 3 5% - ,./" (D ’1" I dd 20.0— ,,/,./lr-'---§—"-—l C / . -O o . ~z§;——cmr~-——e~#"” Ei gagi//e//// 0,0 l r I I I I l l I l j o 10 2o 30 4O 50 Days after Tansplanting Figure 2.3. Plant height to the uppermost leaf collar in the. unirrigated treatments. Vertical bars represent twice the Standard deviation. Where vertical bars are not present, they are smaller than symbols. 72 800- - o-o cc i 700- 3‘9 C / _ BK—BK S \ A—A S+LA . /% 600‘ t—A S+SA _/ \ \ \ 500— // \ \ 400-1 F \ / \t it 200— \ a“ .. ."u'w §.\,\\§\. ............ ' 100d \. 3; z . M Leaf Area (cmz) 0 1O 20 30 4O 50 Days after Transplanting Figure 2.4. Green leaf area per plant in the unirrigated treatments. Vertical bars represent twice the standard deviation. Where vertical bars are not present, they are Smaller than symbols. 73 2.1 indicates the time at which plants visually started showing signs of wilting (beginning leaf rolling), and the time at which severe leaf rolling was recorded. Table 2.1. Time at which beginning and severe leaf rolling were observed (days from ‘transplanting). Horizontal bars indicate treatments for which the phenomenon was not observed. Treatment Beginning Severe Leaf rolling Leaf rolling s _ 20 S+SA 17 22 S+LA 13 20 C 13 27 CC and S+LA were the first to show some leaf rolling at 13 days after transplanting, and the S+LA showed severe leaf rolling 1 week later. Leaves of the C treatment did not roll until 27 days after transplanting, showing a slower development of water deficit. In the 8 treatment leaf rolling was not perceivable at 13 days, but at 20 days it was already severe. The S+SA treatment started leaf rolling at 17 days. The only treatment in which leaves maintained turgor was the compacted clay loam in which plants kept their green leaf area after terminal stress in the other treatments. Plants in soil 8 lost 74 their leaves earlier than those in other soils. As regards the number of leaves (Figure 2.5), it was not dissimilar in treatments 8, S+LA and S+SA at 13 days of transplanting; after that time, development in the S tubes slowed down until plant death, and it proceeded at the same pace for S+SA and S+LA up to 27 days, after which date the soil with small aggregates (S+SA) produced 0.5 leaves more than the other, on average. This differentiation corresponds with.the reported superiority of the S+SA treatment for height and leaf area, and can be related to the higher amount of water actually extracted by S+SA plants. Differences in development between treatments, though, occurred at a later time than what shown for growth. The C plants had a slower development than those in the above treatments, and ended with only 7 to 8 leaves having appeared at 40 days, about 1.0 and 1.5 less than S+LA and S+SA respectively. Plants in the CC tubes had the lowest number of leaves at each date and apparently growth was reduced such that new leaves did not appear out of the whorl after 27 days from transplanting, about 7 days after height and leaf area had reached a stationary point for this treatment. The final above and below-ground plant dry matter for each treatment is shown in Figure 2.6. It was highest for the S, S+SA, and S+LA treatments, and lowest for the CC, in which treatment roots were not found in the 75-100 cm layer. The root/shoot ratio was highest for the sandy-clay soil (around 75 100-] it??? Leaf Number Days after Transplanting Figure 2.5. Leaf number in the unirrigated treatments. Vertical bars represent twice the standard deviation. Where vertical bars are not present, they are smaller than symbols. 76 90.0— 3 ' A ° <> V " m 60.0— ‘9 m _ A O 0 E - . o 2‘ - o 0 _ >< CC +’ I S+SA O a E 30.0. A S+LA U) _ A irrigated S+LA _ I O S . ‘foo O irrigated S l o O C >l< O irriatedC 0.0 . . I I . I , . I , f3 1 I O 3 6 9 12 Root Dry Mass (g) Figure 2.6. Shoot and root dry mass at experiment termination for irrigated and unirrigated plants. 77 19) and lowest for S+SA (around 0.14). Some developmental ctors confounded this result, since the S plants died and Ire collected 7 days earlier than the others, and this may ave affected the root relative contribution to dry mass in oung plants. The effect of irrigation from day 20 on plant growth and development is shown in Figures 2.7-2.9. The results show that all measured characters were higher with irrigation, and that the S and S+LA treatments had similar trends and final values, while in the clay loam soil both growth (leaf area and height) and development (leaf number) were lower than in the other treatments. This was the result of both the initially lower values of the C treatment at day 20, when irrigation was applied, and of a slower rate of growth and development after the application of irrigation. Plant behavior after irrigation indicated that the unirrigated treatments were water-limited and the loss of green leaf area could be attributed to water deficit, but growth and development limitations found in the C treatment were probably due to non water-related causes as well. The root/shoot ratio of the irrigated treatments was lower than that of the dry ones, and ranged from 0.12 in the S+LA to 0.14 in C, but the absolute values of root dry matter were higher than those of the dry treatments. A larger contribution of the root to the total plant dry matter in water-deficient plants has been reported by many authors, and lately by Mayaki et al. (1976) for soybeans and maize, Blum 78 1000— O'O irrigated C 'CVO C 6-0 irrigated S 80.0— H S _ A—A irrigated S+LA Ari S+LA Plant Height (cm) rl O 10 20 3O 4O 50 Days after Transplanting Figure 2.7. Plant height to the uppermost leaf collar in the irrigated treatments, compared with unirrigated ones. Vertical bars represent twice the standard deviation. Where vertical bars are not present, they are smaller than symbols. 79 2500 3 G—G irrigated C - Om. C - 6*49 inigated S 2000‘q H S . f? . A—A irrigated S+LA r," E H S+LA " .9, 1500— O . (D L i < .I .4_ 1000- ,3 8 : ‘J I A I: O ' I r I . I 'I I I :50 4O 0 1 O 20 50 Days after Transplanting Figure 2.8. Green leaf area per plant in the irrigated treatments, compared with unirrigated ones. Vertical bars represent twice the standard deviation. Where vertical bars are not present, they are smaller than symbols. 80 15.01 O-O irrigated C 14.0— ... C 13.0- <>—<> irrigated S H S 12.0- H irrigated S+LA H S+LA 11.0— 10.0-I Leaf Number Days after Transplanting Figure 2.9. Leaf number in the irrigated treatments, compared with unirrigated ones. Vertical bars represent twice the standard deviation. Where vertical bars are not present, they are smaller than symbols. 81 and Ritchie (1984) for sorghum, Hamblin et al. (1990) for wheat. Differences in root distribution between irrigated and dry treatments will be presented in chapter 3. The overall behavior of plants in treatments C, S, S+LA, and S+SA indicates that water deficit limited plant growth, slowed their development, and finally led to almost total loss of green leaf area. The faster initial growth and more rapid plant water deficit reported for the 8 treatment was presumably due to faster access to the soil water, and consequently more rapid depletion. The clay loam contribution in the different growth media was to increase the amount of water held, and to make it more slowly accessible. For the CC treatment the plants had not died, nor did they show signs of wilting at the end of the experiment, but their size and rate of development were remarkably reduced. There is evidence in the literature. (Masle and Passioura, Smit et al, 1989) suggesting that in compacted soils there is aldirect effect of soil strength on plant growth. Also, the air-filled porosity of the treatment was low, and poor aeration is likely to have contributed to the reduced growth rate. In this treatment little water ‘was extracted. by small but turgid. plants, suggesting that factors other than water availability itself were limiting and, in that case, incomplete water extraction may be more correctly seen as a consequence rather than a cause of limited plant growth. Another possibility is that early-developed severe water deficiency reduced growth and led 82 to sufficient osmotic adjustments to maintain leaf turgor. In the other treatments where water extraction was less than expected (C, S+LA), the plants were apparently near death, and little if any green leaf area was left after severe wilting. This indicates that water availability had been a limiting factor, in spite of the average water content of the tubes being above the limits of root water extraction. In the second article, root length density, root clustering and water content spatial variability within the tube will be examined as possible causes of incomplete extraction in growth media where root penetration in soil peds or compacted soil was limited. REFERENCES Blackwell, P.S., Ward, N.A., Lefevre, R.N., and Cowan, D.J., 1985. Compaction of a swelling clay soil by agricultural traffic; effects upon conditions for growth of winter cereals and evidence for some recovery of structure. J. Soil Sci. 36:633-50. Blum, A., and Ritchie, J.T., 1984. Effect of soil surface water content on sorghum root distribution in the soil. Field Crop Res. 8:169-176. Everard, J. D., and Drew, M.C., 1987. Mechanisms of inhibition of water movement in anaerobically treated roots of Zea mays L. J. Exp. Bot. 38:1154-1165. Hamblin, A.P. , 1985. The influence of soil structure on water ‘movement, crop root growth, and water uptake. Adv. Agron. 95- 157. Hamblin, A.P., Tennant, D., and Perry, M., 1990. The cost of stress: dry matter partitioning changes with seasonal supply of water and nitrogen to dryland wheat. Pl. Soil 122:47-58. Mayaki,W.C., Stone, L.R., and Teare, I.D., 1976. Irrigated 83 and nonirrigated soybean, corn, and grain sorghum root systems. Agron. J. 68:532-34. Masle,J., and Passioura, J.B., 1987. The effect of soil strength on the growth of young wheat plants. Aust. J. Plant Physiol. 14:643-56. Schumacher, T.E., and Smucker, A.J.M., 1984. Effect of localized anoxia on Phaseolus vulgaris L. rootlgrowth, J. Exp. Bot. 35:1039-1047. Smit, B.A., Neuman, D.S., and Stachowiack, M.L., 1989. Root hypoxia reduces leaf growth. Role of factors in the transpiration stream. Plant Physiol., 92: 1021-1028. Tardieu, F., 1987. Etat structural, enracinement et alimentation hydrique du mais. III. - Disponibilite des réserves en eau du sol. Agronomie 7(4): 279-288. Vorhees, W.B., Farrell, D.A., and Larson, W.B., 1975. Soil strength and aeration effects on root elongation. Soil Sci. Soc. Amer. Proc. 39:948-953. CHAPTER 3 PLANT GROWTH AND WATER UPTAIG: IN STRUCTURED GROWTH MEDIA. II: CLUSTERING OF MAIZE (ZEA HAYS L.) ROOTS AND SPATIAL DISTRIBUTION OF WATER. ABSTRACT The spatial arrangement of maize (Zea mays L.) roots and of residual water at plant death were studied in structured soil materials in order to quantify the effect of root clustering on water uptake in water-limited plants. Five treatments were compared in 100 cm-high containers: C (clay- loam soil), 8 (sandy-clay soil) S+LA (sandy clay embedded with large clay-loam peds), S+SA (sandy clay embedded with small clay-loam peds), CC (compacted clay-loam soil). The plants~ were grown on stored water until extreme wilting. The structure of the growth media was characterized by structural mapping on five horizontal planes, and by bulk density determinations. Maps of root and water content at the end of the experiment were made on 2 x 2 cm grids, on the same planes used for structural mapping, and across peds. Volumetric water content was measured with time-domain reflectometry. Root length density was also determined at the end of the 84 85 experiment with the technique of Newman (1969) , separately for the peds and the bulk soil. Root growth was highest and quite uniform in the 8 treatment, while in the others root growth was restricted due to little penetration in the peds, particularly beyond the 2 cm outer layer. Water uptake was limited.in.the large peds not penetrated.by roots. This caused pockets of unextracted water in the C and S+LA treatments, and to a lesser extent in S+SA. Limited root growth and water uptake was measured in the CC treatment, and were likely due to direct and indirect effects of compaction. INTRODUCTION Root water uptake has classically been described based on average root length density in a soil layer. Such an approach implies that roots are parallel and distributed randomly or regularly in each layer of the soil, so that each root exclusively draws water from a cylindrical region whose diameter is the mean distance between roots (Gardner, 1960, Newman, 1969). Based on such calculations, the distance water has to travel from the bulk soil to the root is often of the order of millimeters (Tardieu and Manichon, 1986a). In many instances, though, the root horizontal distribution is more likely to be clustered than regular or random. In that case, the distance water has to travel in order to reach the root can be relatively high, and certainly larger than the mean 86 distance between roots. Depending on the soil characteristics, such a path length can be limiting for water movement within a time-frame useful for the plant to overcome water deficiency stress. Although the most common root sampling techniques do not allow for root clumping to be measured, the phenomenon has sometimes been documented by root mapping studies, and recently quantified by a few authors (Tardieu and Manichon ,1987; Tardieu, 1988a; Pettygrove et al., 1989). In those studies where soil water content was measured, a lower uptake was found in case of clustering (Tardieu, 1987; Tardieu, 1988b) but the distribution of water around roots was not reported. One of the reasons is to be found in the lack of adequate experimental techniques. The present work is aimed to quantify root growth and clustering in maize plants grown in structured growth media, and its relation with water uptake patterns under water- limited conditions. MATERIAIS AND METHODS A greenhouse experiment was conducted in which maize (_Z_e§ gmays L.) plants were grown on media with different structural characteristics. The experimental design is described in the first article of this series (Chapter 2) . The treatments were: C (clay-loam soil), S (sandy-clay soil) S+LA (sandy clay 87 embedded with large clay-loam peds) , S+SA (sandy clay embedded with small clay-loam peds), CC (compacted clay-loam soil). Mapping of roots, water content and soil structure were made at the end of 'the experiment, which corresponded to nearly total loss of green leaf area for the treatments C, S, S+LA, S+SA, and to the cessation of plant growth in treatment CC. Access for taking samples in each layer was obtained by separating the two longitudinal halves of each tube. On three replications (tubes) per treatment, two 100 cm3 soil cores were taken in each of the layers: 0-25, 25-50, 50- 75, and 75-100 cm. The soil in the cores was dried at 110 oC and weighed in order to obtain the bulk density, and was then used to determine the root length density (RLD, cm'3) using the technique of Newman (1969). The root dry mass was determined .after oven drying at 60 °C. The bulk density of the peds was also determined for treatment.S+IA.on.3 peds per tube, and for treatment S+SA on 8 peds per tube, by water displacement after coating the aggregates with liquid paraffin. Root length density and root dry mass were determined for the peds as described above. For treatments C and S the number of 100 cm3 cores sampled was four per layer, and bulk density was also determined on two 200 cm? cores per layer, and on four 8 cm3 samples per layer in order to characterize the soil structural status at different scales. Small-scale characterization of root and water spatial 88 variation was made on the remaining three replication tubes per treatment. Measurements of volumetric water content and root length density were made according to the following sampling scheme: - For the S treatment, measurements were made on three 10 cm transects for each of the 0-25, 25-50, 50-75, and 75-100 cm layers. Volumetric water content was measured with time-domain reflectometry (TDR) every 2 cm, using small probes 20 mm long, and with a distance between rods of 14 mm (see chapter 1 for more details). During the measurements, the soil was covered with plastic film to minimize evaporation. After TDR readings, the transect soil was sampled with the sampler pictured in Figure 3.1, designed to collect five contiguous 2 x 2 x 2 cm soil cubes, on which RLD was determined according to Newman (1969). - For the other treatments, the sampling scheme was modified to suit the structural status: in the S+LA treatment water content measured by TDR, and RLD were determined in the bulk soil (sandy-clay) on three replicates for each of the three layers: 0-33, 33-66, 66-100 cm. In each layer a large clay- loam ped was present, and it was extracted from the soil, and sliced along the central horizontal plane. The surface obtained was covered with plastic film and divided into concentric rings by tracing lines parallel to the ped surface every 2 cm on the film. For each ring, three TDR readings were made with the probes described above. The ped was then 89 l— 2 on ..| SAMPLER BLADE / ,// // /7 // / // // // 0) 20:1 / _L // // // // \ \\ We FMNDLE N O 3 CT I- —-I i Ill BLADE :qnzz vam£ T SAMPLER C)20n l I I l t Figure 3.1. Small-scale soil sampler. a) top view; b) side view: c) front view. 90 sliced again 2 cm below the surface used for measurement, and the 2 cm thick slice obtained was cut into concentric rings following the lines previously drawn on the plastic. The soil from each ring was divided into three subsamples on which RLD was determined. This way, volumetric water content and root length density were measured as a function of distance from the surface of the ped. The internal layers of the peds were often too small to allow triplicate measurements. In those cases the number of samples was reduced to two or one. In the S+SA treatment, sampling was as described for S+LA, but the layers in each tube were 0-25, 25-50, 50-75, and 75-100 cm, and for each layer three small peds were sliced and measured as described for S+LA. In the C treatment shrinkage cracks had formed due to soil drying in the 0-25 and 25-50 cm layers. For these layers root length density and TDR water content were measured on three peds per layer as described above. Small scale measurements were not made in the CC treatment, due to problems in soil sampling and TDR probe insertion in the compacted soil. On one tube per treatment mapping of water content and roots was made on five horizontal planes: at 3, 25, 50, 75, and 95 cm, on a 2 x 2 cm grid. Water content was determined by TDR with the probes described above. Determinations were not made for two single sampling grid points, because the dielectric constant values obtained were excessively high (see 91 discussion in chapter 1). Roots were characterized using the notation described by Tardieu and Manichon (1986b). On the same planes mapping of structural status was also made, using three categories: crack, ped, bulk soil. RESULTS AND DISCUSSION Bulk density The bulk density values measured in the 8, 100, and 200 cm9 samples for the C and S treatments are reported in Table 3.1. In both soils, the bulk density at the end of the experiment increased with soil depth. The measured values as well as their variability (coefficient of variation, CV) increased as the sample size decreased, especially in the clay-loam soil. This was partly due to the higher incidence of sampling compaction in the small samples, and partly also to the less likely presence of structural discontinuities (cracks, biopores) in small volumes of soil. Similar results are reported by Fies and Stengel (1981) who argue that bulk density values at small volumes represent the textural density of soil, since they are less likely to include non texture related features like macropores. Results for bulk density measurements in the CC, S+LA, and S+LA treatments are summarized in Table 3.2. Bulk density of the compacted clay-loam treatment was quite high, increasing slightly'with.depth. Its high CV (ranging from 5.8 92 Table 3.1. Bulk density (g cm3) and coefficient of variation (CV, %) as a function of soil depth and sample size in the clay loam and sandy clay treatments. sample size (cm3) 200 100 8 number of samples (2) (4) (4) Clay-loam (C) Layer (cm) 0-25 mean 1.37 1.59 1.63 CV 3.4 7.9 7.4 25-50 mean 1.45 1.63 1.65 CV 4.5 4.6 11.6 50-75 mean 1.55 1.69 1.83 CV 5.2 4.6 7.7 75-100 mean 1.57 1.69 1.84 CV 4.9 3.5 6.6 Sandv-clav (S) Layer (cm) 0-25 mean 1.13 1.19 1.29 CV 7.5 5.8 14.8 25-50 mean 1.14 1.22 1.26 CV 5.2 3.5 6.6 50-75 mean 1.20 1.29 1.31 CV 5.2 7.0 11.1 75-100 mean 1.21 1.28 1.38 CV 4.1 6.3 7.1 Table 3.2. Bulk density (9 cm5) and coefficient of variation (CV, %) from 100 cmP cores for the soil matrix and from soil 93 peds as a function of soil depth. Treatment CC S+LA S+SA Layer (cm) 0-25 mean 1.74 CV 10.4 25-50 mean 1.75 CV 11.2 50-75 mean 1.87 CV 5.8 75-100 mean 1.84 CV 7.8 Peds mean CV (b \l \0 KO mramraerahia 0 l u:w.>hiaro~JH AI- m.> m \l .b I o o N 01 krdbrdulHLJH O qcomcumnowia H #01 U 94 to 11.2 96) were due to problems in core insertion in the compacted soil. Densities for the bulk (sandy-clay) soil were quite similar between treatments 8, S+SA, and S+LA. The bulk density of the peds was higher than that of the bulk soil, and its values were larger in the small peds compared to the large ones. Two main factors are likely to be the cause of the difference between the bulk density of the small and large peds. Firstly, the bulk density of the peds was calculated on the volume as sampled at the end of the experiment. As will be shown in the following paragraphs, the water content of the inner layers of the large peds was at that time higher than that of the outer layers, while the small peds were more uniformly dry. Therefore, while the density of the small peds corresponded to a lower water content, the density of large peds was an average from layers of different water content, and therefore at an intermediate density between wet and dry. Secondly, large peds are more likely to enclose structural cracks or biopores, that would also explain a lower density. The characterization of soil mechanical impedance in a way relevant to root growth has been discussed in relation to bulk density and resistance to penetrometer insertion, and both features present a high variability, depending on the specific methods used. Their relationship with root growth is also variable (Cockroff et al., 1969). The relations between soil bulk density and root growth have recently been discussed by Jones et al. (1991), who used the soil sand percent by 95 weight to predict the moist bulk density at which rooting is severely impaired (BDX) and that at which there is no inhibition of rooting (BBC). The bulk density values reported for the S treatment, and for the sandy clay soil in the other treatments, were in all cases lower or around BDO. In these soils, therefore, no large effect of strength on root growth is predicted. Bulk densities for the peds and the C and CC treatments were higher than BDO. This would indicate inhospitability for root growth in these soils. Values in the CC soil were higher ‘than BDX; this ‘would imply severe impairment of root growth (Jones et al., 1991). Values of BDX, though, were developed for bulk densities at water content near field capacity, while the ones reported in this paper were measured in drier conditions (except for the CC treatment) at the end of the experiment, this indicating that at initial (wetter) conditions the soil density was less limiting for root growth. Characterizing soil strength with a single parameter, does not allow'to account for soil structure or macroporosity that may provide ways for root penetration, even where the bulk soil strength is high. In this study, the reported decrease in density values with increasing soil volume, especially in the clay loam soil, suggests that cracks were present, in which localized root growth could take place. It should also be pointed.out that local soil conditions (like strength in a soil layer) may affect the hospitability of a particular soil region for root colonization, but the actual 96 presence of roots in that area will also depend on whole plant effects, like general water status or nutrition (Tardieu, 1989) and the time-course of stress development. The conditions in adjacent soil regions will also play a role. Tardieu (1988a, 1989) reports that a soil region where soil strength is not limiting per-se, may have low root density because of local compaction directly above ('shadow effect').In the following paragraphs root density will be shown to be higher in the peds of the clay-loam treatment compared to peds in the other treatments. The observation will be discussed in terms of compensatory growth (since in C roots did not have more hospitable soil to grow into), rather than in relation to differences in local peds conditions. Root length density A summary of the results for the root length density measurements on 100 cm3 cores and on peds is shown in Table 3-3. RLD was highest in the sandy clay soil, and decreased relatively little with depth. It was quite similar in the S+LA and S+LA treatments, and lowest in the compacted clay-loam 505-1, in which it declined markedly below 50 cm. No roots were f011nd in the CC samples below 75 cm. Variability was quite high in all treatments, especially so in the C, S+LA, and S+SA times, if all samples were pooled to calculate mean values for each layer. If samples taken from the bulk soil and from the aggregates were considered separately, for the S+LA and S+SA Table 3.3. Root length density (RLD), root weight, and root weight per unit length in unirrigated treatments. sd=standard deviation. Treatment CC C S S+LA S+SA Soil layer (cm) RLD (cm cm3) 0'25 mean 0.56 0.84 1.56 1.20 1.15 Sd 0.21 0.46 0.40 0.96 0.85 25-50 mean 0.40 0.77 1.28 1.02 1.10 Sd 0.14 0.45 0.29 0.87 0.80 50-75 mean 0.02 0.48 1.04 0.75 0.67 Sd 0.02 0.30 0.28 0.56 0.43 5'100 mean 0.00 0.36 0.68 0.39 0.42 sd / 0.25 0.27 0.22 0.29 Root weight (g cm“) 0-25 mean 0.0058 0.0113 0.0173 0.0132 0.0129 Sd 0.0020 0.0075 0.0052 0.0113 0.0106 25-50 mean 0.0019 0.0097 0.0127 0.0113 0.0116 Sd 0.0007 0.0068 0.0029 0.0102 0.0095 50‘75 mean 0.0000 0.0035 0.0077 0.0059 0.0049 Sd 0.0001 0.0026 0.0019 0.0046 0.0037 5'100 mean 0.0000 0.0021 0.0045 0.0028 0.0027 Sd / 0.0015 0.0022 0.0017 0.0023 Root weight per unit length (9 cm”) 0'25 mean 0.000106 0.000127 0.000111 0.000101CL000104 Sd 0.000006 0.000022 0.000016 0.000014 0.000015 25'50 mean 0. 000048 0. 000117 0. 000100 0.000103 0.000097 Sd 0.000005 0.000024 0.000010 0.000012 0.000017 50-75 mean 0. 000040 0.000070 0.000075 0.000079 0.000071 Sd 0.000009 0.000016 0.000009 0.0000L40.000015 5-100 mean 0.000000 0.000056 0. 000066 0.000068 0.000062 Sd / 0.000012 0.000013 0.000010 0.000008 98 tubes, the distribution became bimodal and the variability decreased (Figure 3.2). In the 0-33 cm layer of the S+LA treatment, for instance, the coefficient of variability was reduced from 79% for the pooled samples, to 40% for roots growing in peds only. This result shows how some of the variation in root sampling can be eliminated if a structured (non random) component is identified, and separated from the total variability. This treatment requires that appropriate sampling schemes are used. Similar treatment was shown by Tardieu (1988 a) to reduce the coefficient of variation for length density of maize roots grown in soil with compacted inter-rows. Root weight is reported in Table 3.3 The separation of samples from bulk soil and aggregates for the S+LA, and S+SA treatments is shown in Figure 3.3. This treatment of data allowed the reduction of sample variability, as reported for the RLD. Table 3.3 reports the calculated root weight per unit length for the same samples. Values were of the order of 10'4 for the 0-25 layer, and decreased with depth, likely the result of a lower incidence of primary and secondary structures. Unlike root length and weight, weight per unit length was similar between peds and bulk soil, although the values measured in peds tended to be a little lower. For this reason, the separation of samples from bulk soil and aggregates had a relatively little effect on reducing 99 2.50- —— 7F __ E “Z- 0 ’ 2.00- E _—__ O V ”-7—”. _— 2; 1.50“ .5 ____. C q) _ -— ‘O : fl“ 5: 1.00- w :H H .- y‘f .0..-— 8‘ . ,1 2 t” ri’t +4 0.50“ V‘ a" 8 : :: . ‘ E peds 0: w 1:5» E L [:1 fine soil / 000 l I I ,, ,, fl El tOtCi 0 25 5O 75 100 Depth (cm) Figure 3.2. Root length density in peds and fine soil. Vertical bars represent the standard deviation. a) treatment S+LA. 100 2.50- -— Q _— 'E ——__ Q 2.00“ _ E 7 -- O ---- __ b 1 5’0- -_L- '5 ____ c .1 . 8 I - M r— 1.00 1% i ‘ :1 ,a Li" “m“ — C" ,r pf 5 r T , — 050— ‘; ‘T j; """ J—L E ped ...J M ' ' .I__ 3 CO) “Lg" E j E— l E E1 LE. :21 fine soil e: 000 [j g 7. E" E: HIE. 2:110th ' a 2‘5 50 75 100 Depth (cm) Figure 3.2. Root length density in peds and fine soil. Vertical bars represent the standard deviation. b) treatment S+SA. 101 0.03- _ A r!) _ E __ U P.— C —F_ 33 0.02— \ —- C1 V 2 R3 a? 0.01— )4 5 — E: +1 a T _, 1": a". __ g “E 33 , — E - '. I —-- H __ —I_-". pecs C: 2. :11:— fl = . é "T; I: fins 331i 5 r“. E" .' -—-..—_- .,;, E- : total 0.00 , V, l . 7 I I o 25 so 75 100 Depfi1(cn0 -gure 3.3. Root weight in peds and fine soil. Vertical bars zpresent twice the standard deviation. a) treatment S+LA 102 0.055" A H? E .— o —_— O —— E 0.02“ " \ :71 V g ' c.- __ ‘6 0.01— ' I E _' 3 I I ”fl -‘- -5 Ni ii .‘ _;T— O r11 -— "T"; .-—— ;'_ | ,— E peds K ‘r'f'? iii 1"! é ' ': Eé !L_ : fine soil 000 fl '5: ? II E 7r “5' 45—57 a:- total ' 6 2'5 5'0 75 100 Depth (cm) Lgure 3.3. Root weight in peds and fine soil. ‘Vertical bars apresent twice the standard deviation. b) treatment S+SA. 103 'ariability (Figure 3.4) . Root weight per unit length was also :imilar between treatments, with the exception of CC, in which ower values were found. Values of root weight per unit engthwere slightly higher in the first 50 cm of the C reatment, probably because of the large structures growing on he face of peds, while the roots growing into peds looked imilar in thickness to those found in peds in the other reatments. The results indicate that roots growing into ompacted soil were thinner than the others. One reason may be he presence of only lower order branches in the peds. It has een reported (Schuurman, 1965; Vorhees et al., 1975), that a ontrasting interface enhances root branching in the soil so aat the ratio of laterals to primary roots increases. Large Dots have a lower probability of penetrating the narrow pores E compacted soil unless they can exert radial pressure to ilarge them (Hamblin, 1985) . There are contrasting reports in 1e literature: roots growing in compacted soils are sometimes >und to be thicker and other times thinner than those growing 1 soils of lower strength. Eavis (1972) tried to separate :chanical strength from aeration effects to explain the .screpancy. From a visual analysis of roots in this study, Lrge root structures were limited to the bulk soil and ped eraces, while only the finer roots penetrated peds. Table 3.4 summarizes root data collected from the frigated tubes. Root length density was higher after 'rigation, and decreased with depth more than in the dry 104 T 0.0002- E u 0'7 V E» c E I ‘s— E 00001 I‘m-y- T ’— - 3 .1 .a. E '7" é I 5 : ‘ g y: T,___' c2. ,4 ./ «pi. ; pm | _. 0 0 :1 E —:. c E c * — g E E E a E Epcds .._. z‘ E / j E :Ifinc soil 0 a 0 0.0000 3 M = ' . a total C O 25 SO 75 Depth (cm) Figure 3.4. Root weight per unit length in peds and fine soil. Vertical bars represent twice the standard deviation. a) treatment S+LA 105 ‘_\ T 0.0002- ,. C U C3 J :6» E ._:.- - I fl' ‘7— 3 0.0001- g; "T‘ Tm. = 'E ' T ——‘-T' E . E “’4? "E“ Fill—=1:— :7— : r‘: E 3 E ....J.—'5_ .._.=____F"j*;_=_'_ Z x: E {1' If; E P"! :E E” .91 E c i— K' '5 H is .2 f" E '4 E C E a/‘i TE Epoch: , ’1 = f": = E r. 15 in “oii .1 I . E A t—-. a H__ C' l__ :f C.) D OOOCC g = "f E , -. ’E I E E total 5 ' 0 2'5 5'0 7'5 100 Death (cm) igure 3.4. Root weight per unit length in peds and fine soil. ertical bars represent twice the standard deviation.b) reatment S+SA. 106 able 3.4. Root length density (RLD), root weight, and root eight per unit length in irrigated treatments. sd= standard aviation. treatment irrigated C irrigated S irrigated t-LA ail layer (cm) RLD (cm cm3) -25 mean 4.47 5.57 5.10 Sd 1.59 1.38 3.85 5-50 mean 2.46 3.68 3.25 sd 1.28 0.82 2.69 0-75 mean 1.47 2.21 1.37 Sd 1.00 0.59 1.07 5-100 mean 0.36 0.54 0.39 sd 0.25 0.26 0.23 Root weight (g cm3) -25 mean 0.0666 0.0796 0.0694 Sd 0.0444 0.0242 0.0597 5-50 mean 0.0290 0.0481 0.0415 Sd 0.0202 0.0109 0.0375 3-75 mean 0.0058 0.0124 0.0074 Sd 0.0043 0.0031 0.0057 5-100 mean 0.0064 0.0058 OJXWZ sd 0.0048 0.0028 OJXM4 Root weight per unit length (g cm“) '25 mean 0.000149 0.000143 0.000136 Sd 0.000026 0.000020 0.000019 5-50 mean 0.000118 0.000131 0.000128 Sd 0.000024 0.000013 0.000015 )-75 mean 0.000039 0.000056 0.000055 Sd 0.000009 0.000007 0.000010 5-100 mean 0.000181 0.000107 0.000186 sd 0.000040 0.000021 0.000028 107 treatments. Values were higher in the S and S+LA treatments, compared to C. Root weight showed a similar pattern, and root weight per unit length was not very different between irrigated and dry tubes In irrigated treatments root weight was greater in the top layers and smaller in the bottom ones, compared to unirrigated treatments. The standard deviation values reported.in the table were higher than those of the dry tubes, but the CV, independent of the treatment mean, were of the same order. The comparison of root data between dry and irrigated treatments confirms reports that the root of droughted plants deviates from the classical exponential distribution found in well-watered plants (Merrill and Rawlins, 1979) because of a lower proportion of roots in the dry superficial soil layers. The higher density of roots in deep layers of drying profiles has been discussed in terms of compensatory growth (Jordan et al., 1979), and Sharp and Davies (1985) speculated about the fine mechanisms of root proliferation in depth, suggesting that aeration effects and abscisic acid may play an important role. Small scale root and water characterization Figure 3.5 reports the values for RLD and TDR-determined volumetric water content in the 25-50 cm layer, across a 10 cm transect in the S treatment. Values of RLD across transects 3 ranged from 0.94 to 1.43 cm', and volumetric water content ranged from 0.126 to 0.133 cmF cm3. No clear trend in space 108 was detected for either RLD or 6w Figure 3.6 summarizes the results for RLD and 6v measured outside and across peds in the S+LA treatment in the 33-66 cm layer. Root length density in the bulk soil averaged 1.58 cm cm’3 . In the ped, roots were found in the 0-2 cm (superficial) layer, where the average RLD was lower than in the bulk soil (0.74 cm cm'3), whereas in the internal layers of the peds roots were found only occasionally, probably growing in biopores. The corresponding 6v values show a water content gradient within peds, with values ranging from 0.246 cm3 cm'3 in the external layer (0-2 cm) to .350 cm3 cm'3 and higher in the layers beyond 4 cm from the ped surface. Values for volumetric water content of the sandy-clay soil outside the peds were lower than those of the ped superficial layer, but in this case the discrepancy is to be attributed to the different texture and density of the two soils. The soil water potential values were likely much closer than the values of 6w The corresponding figure for the 25-50 cm layer of the C treatment (Figure 3.7) shows very similar trends to that in treatment S+LA, although the actual values for RLD at the ped surface are higher, probably as a result of some compensatory growth, since in the C treatment roots could not grow in fine soil. A large proportion of the roots measured in the 0-2 cm were in fact located on the face of the peds. The root spatial distribution across peds was similar for the S+SA treatment 109 (140- ¢?‘ - I E _ 0 C130 g (120* v CD - O O o 9 O (110— A 2.00- T E 1.50— _, O E 1.00— 7 _ 1 O . v Q 0.50— E 0.00 I I T I I I T I l 1 0-2 2—4 4—6 6—8 8—10 Distance ocross Tronsect (cm) Figure 3.5. TDR-determined.volumetric water content, and root length density across a 10-cm soil transects in the 25-50 cm layer of treatment S. 6 (cm3 cm-S) RLD (cm cm-S) (140- (130- (120- (110‘ 2.00? 1.504 1.00— 0.50— C100 110 If; bulk I I . _L _L T 0—2 2—4 4—6 6—8 Distance from Ped Surface (cm) Figure 3.6. TDR-determined volumetric water content, and root length density in the bulk soil and across peds in the S+LA treatment. deviation. Vertical bars represent twice the standard 6 (cm3 cm-s) RLD (cm cm-S) 0.401 (130- (120* (110— ZLOO— 1.50--1 1.00- (150- (100 111 .32. 0—2 2—4 4—6 6-8 Distance from Ped Surface (cm) bulk Figure 3.7. TDR-determined volumetric water content, and root length density in the bulk soil and across peds in the C treatment. deviation. Vertical bars represent twice the standard 6 (cm3 cm-J) RLD (cm cm-J) (140- (130- (120- (110‘ 2100* 1.50— 1.00- (150- (100 112 , I I I J— l' —L 0—2 2—4 4—6 Distance from Ped Surface (cm) T Figure 3.8. TDR-determined volumetric water content, and root length density in the bulk soil and across peds in the S+SA treatment. deviation. Vertical bars represent twice the standard 113 (Figure 3.8), for which little if any roots were found beyond the superficial layer of the aggregates. For both C and S+SA treatments, the measured gradient in By across peds was smaller than that found in S+LA. Values of TDR-determined.6v for the bulk soil and across peds for all layers are reported in Tables 3.5 to 3.8 for each treatment. Bulk.soil volumetric water content.was quite low in some of the samples in the top layer of the tubes due to soil evaporation at the surface. Gradients across aggregates for all soil layers were of the order of magnitude of those reported in Figures 3.5 to 3.8. In the clay-loam soil below 50 cm shrinking cracks were not clearly detected, therefore sampling across structural units was not possible. The values of standard deviation associated with 6v measurements were lower than 0.028 cm? cmq, except for the internal layers in the S+SA treatment, in which the standard deviation was higher. The reason for a higher variability in those layers is that peds in the S+SA treatment ranged from 8 to 10 cm in size, therefore the internal layer was actually at different distances from the ped surface in the different peds. No excessively high.dielectric constant values were found, as the ones reported for the clay-loam soil in 21 mm-high layers in chapter 1, and the consistency of the measurements, even at high 6v, suggests that none of the measured values was an artifact of the TDR technique. Values of RLD for the bulk soil and across peds for all 114 Table 3.5. TDR-determined 6v (cm3 cm'3) across peds and in bulk soil for the S+LA treatment. n= number of samples. sd=standard deviation. CV = coefficient of variation. bulk soil ped layer (cm) Soil layer (cm) 0-2 2-4 4-6 6-8 0-33 n. 24 24 18 8 2 mean 0.088 0.240 0.288 0.342 0.353 sd 0.010 0.019 0.028 0.019 0.010 CV 11.0 8.1 9.7 5.6 2.8 33-66 n. 24 22 14 6 2 mean 0.143 0.246 0.292 0.351 0.363 sd 0.016 0.028 0.016 0.022 0.023 CV 11.2 11.4 5.6 6.3 6.2 66-100 n. 24 21 12 6 1 mean 0.154 0.233 0.276 0.353 0.318 Sd 0.010 0.025 0.016 0.021 / cv 6.7 10.6 5.8 6.1 / 115 Table 3.6. TDR-determined 6v (cm3 cm'3) across peds and in for the C treatment. n= number of samples. sd=standard deviation. CV = coefficient of variation. bulk soil ped layer (cm) Soil layer (cm) 0-2 2-4 4-6 0-25 n. 9 9 9 mean 0.212 0.277 0.316 sd 0.020 0.022 0.007 CV 9.4 7.9 2.3 25-50 n. 9 9 9 mean 0.223 0.285 0.324 sd 0.016 0.014 0.011 CV 7.2 4.8 3.4 50-75 n. 9 mean 0.345 sd 0.030 75-100 n. 9 mean 0.389 sd 0.043 CV 11.1 116 Table 3.7. TDR-determined 9v (cm3 cm'3) across peds and in bulk soil for the S+SA treatment. n= number of samples. sd=standard deviation. CV = coefficient of variation. bulk sail ped layer (cm) Soil layer (cm) 0-2 2-4 4-6 0-25 n. 9 25 12 3 mean 0.085 0.220 0.272 0.283 sd 0.016 0.014 0.014 0.033 CV 18.7 6.4 5.2 11.7 25-50 n. 9 26 15 1 mean 0.142 0.229 0.267 0.306 sd 0.009 0.017 0.012 / cv 6.2 7.4 4.6 / 50-75 n. 9 26 15 2 mean 0.163 0.221 0.279 0.304 sd 0.011 0.015 0.015 0.051 CV 6.9 7.0 5.5 16.8 75-100 n. 9 26 14 3 mean 0.175 0.223 0.280 0.324 Sd 0.016 0.014 0.014 0.022 CV 9.1 6.2 4.9 6.9 117 Table 3.8. for the S ‘treatment. n= number' of samples. deviation. CV = coefficient of variation. TDR-determined 6v (cm3 cm'3) across 10 cm transects sd=standard distance on transect (cm) Soil layer (cm) 0-2 2-4 4-6 6-8 8-10 0-25 n. 9 9 9 9 9 mean 0.084 0.090 0.091 0.088 0.087 Sd 0.012 0.008 0.010 0.010 0.010 CV 14.3 9.3 10.9 11.0 11.2 25-50 n. 9 9 9 9 9 mean 0.132 0.133 0.128 0.133 0.125 Sd 0.024 0.019 0.013 0.013 0.014 CV 18.0 14.6 10.0 9.5 10.9 50-75 n. 9 9 9 9 9 mean 0.156 0.152 0.152 0.154 0.156 sd 0.019 0.018 0.016 0.020 0.020 CV 12.2 11.8 10.3 12.9 12.9 75-100 n. 9 9 9 9 9 mean 0.196 0.194 0.187 0.184 0.191 sd 0.024 0.026 0.021 0.017 0.020 CV 12.3 13.2 11.1 9.3 10.6 Table 3.9. sd=standard deviation. CV 118 coefficient of variation. Root length density ( cm cm3) across peds and in bulk soil for the S+LA treatment. n= number of samples. bulk soil ped layer (cm) Soil layer (cm) 0-2 2-4 4-6 6-8 0-33 n. 9 9 9 8 2 mean 1.93 0.76 0.05 0.00 0.35 sd 0.371 0.123 0.100 0.000 0.501 CV 19.2 16.2 212.1 0.0 141.4 33-66 n. 9 9 9 7 1 mean 1.49 0.74 0.03 0.08 0.00 sd 0.298 0.210 0.094 0.184 0.000 CV 20.1 28.4 300.0 216.0 0.0 66-100 n. 9 9 9 5 1 mean 0.68 0.65 0.11 0.08 0.42 sd 0.210 0.085 0.221 0.190 0.000 CV 31.0 13.0 201.0 223.6 0.0 119 Table 3.10. iRoot length.density ( cm cmfi) across peds for the C treatment. n= number of samples. sd=standard deviation. CV = coefficient of variation. bulk soil ped layer (cm) Soil layer (cm) V 0-2 2-4 4-6 0-25 n. 9 9 9 mean 0.91 0.05 0.04 sd 0.348 0.142 0.118 CV 38.3 300.0 300.0 25-50 n. 9 9 9 mean 0.88 0.06 0.02 sd 0.366 0.165 0.071 CV 41.5 300.0 300.0 50-75 n. 9 mean 0.45 sd 0.184 CV 41.0 75-100 n. 9 mean 0.28 sd 0.265 CV 93.5 Table 3.11. 120 n: Root length density ( cm ems) across peds and in bulk soil for the S+SA treatment. sd=standard deviation. number of samples . CV = coefficient of variation. bulk soil ped layer (cm) Soil layer (cm) 0-2 2-4 4-6 0-25 n. 9 25 16 3 mean 1.92 0.68 0.07 0.07 sd 0.444 0.145 0.150 0.123 CV 23.1 21.3 225.3 173.2 25-50 n. 9 26 18 2 mean 1.75 0.73 0.04 0.00 sd 0.434 0.185 0.085 0.000 CV 24.8 25.4 240.1 0.0 50-75 n. 9 26 15 2 mean 1.12 0.57 0.06 0.14 sd 0.229 0.170 0.162 0.200 CV 20.5 29.6 264.8 141.4 75-100 n. 9 26 15 4 mean 0.68 0.49 0.03 0.11 sd 0.237 0.130 0.106 0.196 CV 34.7 26.5 400.9 184.8 121 Table 3.12. Root length density ( cm cm'3) across 10 cm transects for the S treatment. n= number of samples. sd=standard deviation. CV = coefficient of variation. distance on transect (cm) Soil layer (cm) 0—2 2-4 4-6 6-8 8-10 0-25 n. 9 9 9 9 9 mean 1.49 1.30 1.43 1.60 1.89 sd 0.556 0.347 0.443 0.480 0.541 CV 37.4 26.7 31.0 29.9 28.7 25-50 n. 9 9 9 9 9 mean 1.17 1.16 1.05 1.43 0.94 Sd 0.410 0.391 0.273 0.424 0.300 CV’ 35.0 33.6 25.9 29.6 31.8 50-75 n. 9 9 9 9 9 mean. 0.94 0.94 0.92 1.05 0.83 sd 0.274 0.381 0.279 0.262 0.400 CV' 29.2 40.7 30.3 25.0 48.5 75-100 n. 9 9 9 9 9 :mean. 0.53 0.66 0.65 0.61 1.53 sd 0.151 0.253 0.125 0.283 2.807 CV' 28.2 38.3 19.4 46.8 183.1 122 layers is reported in Table 3.9 to 3.12, for each treatment. Measured RLD in the bulk soil of treatments S+LA (Table 3.9) and.S+SA ( Table 3.11) was higher than in 8 (Table 3.11) probably as a result of compensatory growth, since growth in peds was limited in treatments S+LA and S+SA. Roots were found beyond the surface 2 cm in peds only in biopores or secondary cracks. In all clay-loam structural units, the volumetric water content of the external layer ranged from about 0.220 to 0.246 cm3 cm'3. According to Comegna et al. (1990) this amount corresponds to a soil matric potential of about -0.8 to -1.0 MPa in this soil. This value is higher than that at which roots can lower their potential, but since it is an average value for the whole 0-2 cm of the external layer, the actual value around roots may be lower. Dunham and Nye ( 1973) and Hsieh et al. (1972) report large water content gradients across the first 5 mm from a root plane, and if measurements had been made over smaller space increments in this experiment, steeper gradients might have been found. A visual analysis of the samples suggested that roots in the Superficial ped layer were actually not uniformly distributed in the 0-2 cm layer, but density was higher close to the surface of peds. 123 Soil mapping Some of the results for soil mapping on horizontal planes are reported in Figures 3.9 to 3.12 for the C, S, S+LA, and S+SA treatments at various depths. Mapping showed that the distribution in space of roots and volumetric water content was strongly related to the structural status of soil: roots developed quite uniformly at the centimeter scale in the sandy-clay, and could not penetrate peds beyond the 0-2 cm layer, unless biopores or cracks were present. In the clay- loam treatment, up to 50 cm, where structural units were distinguishable, root growth was higher on ped surfaces and in the 0—2 cm superficial layer of peds, but some root growth was found within peds, in secondary cracks. Below 50 cm, root growth was quite variable spatially, but no structural unit was clearly distinguishable due to the higher water content of the layers. Root indexes determined within ped layers were quite uniform. The water content was quite uniform, within .02 cm3 cm'3 in the sandy-clay soil, except for the areas close to the container walls in the 3 and 25 cm layers, where 6v was lower than the layer average, presumably due to evaporation. Across peds, gradients of water were shown. In the sampling planes close to the tube surface (3 cm depth), root density was quite high in the center of the plane, close to the plant crown, due to the higher presence of structures departing from the plant. The water content was higher in that region, presumably because of protection from evaporation due to the 124 o] 25 G [:3 BuU0 where 6: volumetric water content (cm3cnm3) r= radial distance (cm) D= soil diffusivity (cm2 day") t= time (days) R0= maximum cylinder radius The assumption was of initially uniform water content. The initial value was set at 0.40 cm3 cm'3 because it was the volumetric water content of peds at planting, measured on one replicate tube after preparation. Since the time-course of root growth in peds and water extraction was not measured, some assumptions had to be made as to the time at which uptake from peds started. Extraction was hypothesized to begin at the 138 time when plants showed severe leaf rolling, that is day 20 for the S+LA treatment from data in chapter 2. This simplifying assumption was suggested by observations from a field experiment exposed in chapter 5, in which roots were found to proliferate in peds more markedly after colonization and water extraction from less compacted soil regions. Experimental data from chapter 3 were used for comparison with simulations. Results on water gradients in the second soil layer were chosen, to minimize effects of soil evaporation and early root growth that may confound results in the first layer. The boundary condition of water content at 0.19 cm3m_ IS Rodlus (cm) =9 cm, with 1115 tion as a function ia der of rad 1n D=1. Each curve corresponds to one day. i istance from the center of a cyl different values of D(8). Simulations were run for 41 days. Fig 4.2. Time—course of water content var a) case of d 147 (6 ,1!" u.”'.4 d “t: .. Lu J- " A m l E 3 CC " _. ~v (J (’3 p. '_ f.) H A 2‘: .. -‘ ‘d -d | > ~ .. ~::::~ \\ ~ —---~ \‘\ ====:——::--.‘-- =::§ :"=‘-~——==~-—==-»~ ‘tt‘ :=£====::::~-.- -—‘-§b::=.~. r“ so :‘.'.TTT""== $3532" " Cd .::::::5ff§§§:231533;“;:.~:§:~§iiiiéi::i.a;;;-=?3333233 _ F' I:::::::mmmIllumuimllllllllluum lllllllHll) P *- U.LD . [C77] :0 o O- C U) Fig 4.2. Time-course of water content variation as a function of distance from.the center of a cylinder of radius=9 cm, with different values of D(6). Simulations were run for 41 days. b) case ii: 1 G Am- .20 15 18 Radius (cm) =9 cm, with (8). Simulations were run for 41 days. 1us §$§£ WWW... Ezgflammmn a 33.1 ...g mm- Ea...§.§mmm seaslfimm 35.5...." “an...” 1.8.... ..I.II :cuIO ‘00 .9. v0! .\ u. @3th 3‘ ......:...7 fill. may...» w". . . . . \- Ilflv ......r .o I. 'l I I. 1.. ...s... . . fiNnWA/l/thwntc. .. . . 0.30 0.25 an. EUMEUV >m 20 0. 15 18 Radius [cm] funct1on 9 cm, with 1115: iation as a der of rad' 1n 1ons were run for 41 days. rve corresponds to one day. a cyl (6). Simulat Each cu O . as _ Igl‘gig' .. E .fi% '3‘: 0.20 0.15 a 4 6 8 1a Radius (cm) Fig 4.2. Time-course of water content variation as a function of distance from the center of a cylinder of radius=9 cm, with different values of D(6). Simulations were run for 41 days. f)case vi: 4.29*10Q 0.80 - 0.15 r r 10 8 6 4 2 0 Distance From ped surface (cm) Fig 4.3. TDR-determined. water content as a function of distance from peds surface for treatments S+LA.(open circles), S+SA (open rectangles), and C (filled triangles), compared with simulated water content along a cylinder radius after 21 days of extraction for cylinders of radius= 9, 4, and 6 cm respectively. 156 content (0 . 2 0 cm3 cm'3) . Diffusivity was not independently measured, but the values calculated from the experiment were as low as 2*10'1 cm2 day'1 at the dry end. Hsieh et al. (1972) showed that large water gradients can develop between root surface and bulk soil even at quite low bulk soil matric potential (high water content), and over a distance of only 11 mm from the root. Their results can be commented in terms of low diffusivity values, and, although no measurements or calculations were reported, the authors remark that a bed of soil aggregates was used instead of sieved soil, and that would imply a quite low D value. From the gradients measured in this experiment it can be inferred that the actual soildiffusivity was lower than the one suggested by Passioura (1985) for time-constant calculations. Alternatively, the limited water uptake could be ascribed to a high root resistance as a consequence of direct effects of compaction or hypoxic conditions. This latter factor could be important in the case of the CC soil, in which air filled porosity values would be low. A low diffusivity around roots may be due to localized small-scale soil compaction due to root growth pressure or to microscale discontinuities in peds structure. Root shrinkage in drying soil has been reported by Huck et al. (1970) and Faiz and Weatherley (1982), who argue that the gaps created by reduced root-soil contact would decrease dramatically root water extraction. Herkelrath et al. (1977) developed a model that 157 included the case of low root-soil contact to explain discrepancies between calculated and measured water uptake rates. Faiz and Weatherley (1982) found that shrinkage could not account for all of the differences in measured and calculated water uptake in an experiment with sunflower. Passioura (1988) argues that shrinkage has not been shown for small roots, and discounts the importance of this factor for reductions in root water uptake. In summary, for the bottom soil layers of CC a slow water uptake could be predicted from the low RLD, but the unextracted water found in the other cases could not be explained by classical approaches, based on water uptake calculations from half average distance between roots. Results from a clustered root model suggested by Passioura (1985) were closer to what measured, but they could not explain it completely when soil diffusivity was assumed to be equal to 1 cm3 day4. Modeling results suggest that diffusivity of water around roots should be quite low to explain the measured gradients in water content across peds. A low diffusivity may be the result of physical soil features like compaction around roots, or root shrinkage. ACKNOWLEDGEMENTS I wish to acknowledge S. Pagano for help in developing the model, and B. Baer for assistance with computer related 158 problems. REFERENCES Dunham, R.J., and Nye, P.H., 1973. The influence of soil water content on the uptake of ions by roots. I. Soil water content near'a.plane of onion roots. J. Appl. Ecol. 10:585-98. Faiz., S.M.A., and Weatherley, P.E., 1977. The location of the resistance to water movement in the soil supplying the roots of transpiring plants. New Phytol. 78:337-47. Faiz, S.M.A., and Weatherley, P.E., 1988. Further investigations into the location and magnitude of the hydraulic resistances in the soil-plant system. New Phytol. 81:19-28. Faiz, S.M.A., and Weatherley, P.E., 1982. Root contraction in transpiring plants. New Phytol. 92:333-43. Gardner,‘W.R., 1960. Dynamic aspects of water availability to plants. Soil Sci., 89:63-73. Hanks, R.J., and Gardner, H.R., 1965. Influence of different diffusivity-water content relations on evaporation of water from soils. Soil Sci. Soc. Am. Proc. 495-8. Hamblin, A.P., 1985a The influence of soil structure on water movement, crop growth, and water uptake. Adv. Agron. 95-157. Herkelrath, W.N., Miller, E.E., and Gardner, W.R., 1977. Water uptake by plants: II. The root contact model. Soil Sci. Soc. Am. J. 41:1039-43. Hsieh, J.J.C., Gardner; W.H., and. Campbell, G.S., 1972. Experimental. control of soil water content in the vicinity of root hairs. Soil Sci. Soc. Amer. Proc. 36:418-420. Huck, M.G., Klepper, B., Taylor, H.M., 1969. Diurnal variations in root diameter. Plant Physiol. 45:529-30. Jordan, W.R., and.Miller, F.R., 1980. Genetic variability in sorghum root systems: implications for drought tolerance. in: Adaptation of plants to water and high temperature stress. Turner, N.C., and Kramer, P.I. (eds). Wiley, New York:383-399. Passioura, J.B., 1985. Roots and water economy of wheat in 159 Day, W. and Atkin, R.K. (eds) Wheat growth and modelling. Plenum. 185-198. Passioura, J.B., 1988. Water transport in and to roots. Ann. Rew. Pl. Phys. P1. Mol. Biol. 39:245-65. Tardieu, F., 1988. Analysis of the spatial variability in maize root density. II. DistanCes between roots. Plant and Soil 107:267-272. ' Tardieu, F., andeanichon, H. 1986. Characterisation en tant que capteur d'eau de l'enracinement du mais en parcelle cultivée.II.- Discussion des criteres d'etude. Agronomie 6(4):345-54. Zur, B., Jones, J.W., Boote, K.J., and Hammond, L.C., 1982. Total resistance to water flow in field soybeans:II. limiting soil moisture. Agron. J. 74:99-105. CHAPTERS MAIZE (ZEA MAYS L.) GROWTH AND WATER UPTAKE IN A VERTIC- USTORTHENTS SOIL. ROOT SPATIAL VARIABILITY AND WATER UPTAKE. ABSTRACT In a field experiment three structural situations were studied in a vertic-ustorthents soil: MT (corresponding to minimum tillage), T50 (in which undisturbed peds were surrounded by fragmented soil in the first 50 cm of the profile), and T100 (as in T50, but at 100 cm depth). Plant growth and development were measured weekly. Vegetative and reproductive biomass were determined at harvest. Soil bulk density, volumetric water content and root length density were determined on four dates. Leaf extension rate (LER) and roots and water spatial distribution were determined in fragmented soil and across peds on two dates during a period without precipitation. Plant growth and yield varied with soil structure being higher in T100 followed by T50. Leaf appearance rate and tasseling were not.affected. Root length.density was higher in fragmented soil than in peds and root proliferation into peds increased after colonization of the surrounding fine soil. 160 161 Root density within peds was highest in treatment MT, where no fragmented soil was available for root growth. A higher LER in treatment T100 during soil drying was associated with a higher amount of fragmented soil where most of the water uptake occurred. Access to water was lower in peds scarcely penetrated by roots beyond the 2-cm superficial layer. Mapping of roots and water on two dates showed that spatial distribution was related to soil structure. This was especially noted on the late sampling date. These results suggest new possibilities for simplifications in the modeling of water uptake of clustered root systems. INTRODUCTION One of the most studied effects of tillage in fine soils is the improvement of the crop's water relations (Hamblin, 1985). This benefit is often ascribed to the release of mechanical stress that leads to an increase in root density in deep soil layers. Tardieu and Manichon (1987b) argued that low root water uptake in compacted soils can be related to clustering rather than low density of roots. In this case different soil structural situations related to soil tillage should be characterized in terms of their effects on root spatial distribution. Tardieu and Manichon (1987a) reported that 3 types of structure are most commonly found in the soil tilled layer. The first, identified as '0' , is fragmented; the 162 second, 'B', consists of compacted blocks separated by cavities; the third, 'C' , is continuously compacted. In a field study they measured root spatial variability and water uptake for each of the 3 structural situations in loamy soils (Tardieu and Manichon, 1987b and 1987c). Root.distribution was found to be clustered and. water uptake reduced in the compacted areas for structural types B and C. In a glasshouse study (Chapter 3) root and.water spatial distribution were shown to be closely related to soil structure after roots had withdrawn much of the water from the soil. If this is the case in the field, then it may be possible to predict root clustering and water uptake patterns based on soil structure. In vertisols the soil structural pattern is determined by tillage and by natural shrinkage and swelling. Both factors can provide a degree of regularity in structural patterns. The distribution of roots and water will also be regular to the extent to which it is affected by structure. In case of a spatially regular pattern of clustering it would be possible to introduce further simplifications in the modeling of water uptake in structured soils. Field studies are necessary to characterize2the.relationslbetween soil structure and root and water spatial patterns. A study was conducted on root and water distribution in a swelling soil with three different structural situations. Root growth. and. water ‘uptake ‘were. characterized. at ‘the 163 centimeter scale as a function of soil structure. MATERIALS AND METHODS A field experiment was conducted at Guardia Perticara (PZ-Italy) at an elevation of 700 m a.s.l. on a vertic- ustorthents soil in 1989. In order to minimize variability in soil texture an area of 147 n? was chosen in which particle size distribution was reasonably uniform. Results from 8 sampling points at.4 depths each are reported in.Table 5.1. No spatial trend was apparent from sample results. Three soil structural situations were created on 49 m2 plots: 1) MT: minimum soil disturbance (corresponding to minimum tillage: soil was tilled at 5 cm to prepare a seedbed). In this treatment the soil was expected to develop the structural status corresponding to its own patterns of cracking upon drying. 2) T50: the soil of the whole plot was excavated with a backhoe in two 25-cm layers, and about half of the soil was laid between two plastic sheets and fragmented by pounding. The rest was constituted by peds of average size 24 cm. The soil layers were put.back;in.place so that large peds were surrounded by loose soil as in the B treatment described by Tardieu and Manichon (1987a) obtained by tillage in wet conditions. 164 Table 5.1. Soil particle size distribution as a function of soil depth in the experimental field area. Size class Sand Silt Clay (2-0.02 mm) (0.02-0.002 mm) (< 0.002 mm) % % % Soil depth (cm) 0-25 mean 36.5 23.9 39.6 C.V. 2.7 3.9 3.2 25-50 mean 36.0 24.1 39.9 C.V. 6.2 5.7 2.4 50-75 mean 37.3 23.9 38.8 C.V. 4.0 2.7 1.2 75-100 mean 38.0 24.7 37.3 C.V. 3.2 1.8 2.3 3) T100: the soil was prepared as in 2, but up to a depth of 100 cm, in four 25-cm layers. This reduced soil density below normal tillage depth. The treatments ‘were applied. on .April 18. They are illustrated in Figure 5.1. Maize (Zea mays L.) Dekalb Vitrex 200L was sown on July 1, 1989, and thinned at emergence (July 9) to give a population density of 8 plantS'mQ. Plants were fertilized on July 12 and August 15 with 60 kg N ha_1 each time and harvested on November 2, 1989. The temperature and precipitation for the period of the trial are summarized in Figure 5.2. Non-destructive measurements were made on ten plants per 165 AAA/\AAA. IIII AAA/\AA/ undisturbed 801]. W/ Fragmented 801]. Figure 5.1. Structure of the soil profile for the experimental treatments, up to 100 cm depth. a) MT; b) T50; c) T100. 166 ------- Mofinmni ----- Minimum 35.0 - ---- .‘. 1 ......... u . u o ' o 25.0 - 15.0— Temperature (0c) 5.0 — 80.0—1 60.0 - 40.0 " l l 1 T l I I i 1 11 21 31 41 51 61 71 81 91 101 111 121 131 Days from Sowing Precipitation (mm) Figure 5.2. Weekly rainfall and minimum and maximum temperature for the crop growing season 1989. 167 plot. Weekly measurements were made of height to the top ligule, number of fully expanded leaves, and leaf length from tip to ligule. Leaf area was calculated using a relation between area and length presented in a previous paper (Chapter 2). Thermal time from emergence was calculated using a base temperature of 8 °C, according To Ritchie and NeSmith (1991). After July 28 no precipitation was recorded for 16 days. In order to monitor the effect of water deficit.on.the plants, leaf extension rates (LER) were measured between July 31 and August 8 on the laSt three leaves of ten plants per plot. Ears 'were Iharvested. on. all plants individually and weighed after oven drying at 60 °C. Dry mass of stems and leaves were also determined on all plants at harvest. Soil sampling was done on large and small scale. Large samples consisted of four 100 cm8 cores taken in each of the layers: 0-25, 25-50, 50-75 and 75-100 cm. For treatments T50 and T100 samples were taken separately in peds and fragmented soil (two replicates each). There was no fragmented soil in treatment MT; thus samples were taken in undisturbed soil. Water content and bulk density were calculated on the sampled volume after oven-drying at 110 °C. Root length density (RLD) was determined according to Newman (1969) . Sampling dates were: July 25, July 31, August 8, and September 8. Small scale sampling was made on July 31 and August 8 on four 50 x 50 cm horizontal planes for each treatment (at depth of 25, 50, 75, and 95 cm). Soil structural mapping was made 168 recording the location of cracks, loose soil and peds. 0n the same planes, before structural mapping notations were made, soil volumetric water content was measured on a 2 by 2 cm grid with time-domain reflectometry (TDR) using 20 mm long probes of the type described in Chapter 1. 0n the same grid root mapping was also made using the same notation as in Tardieu and Manichon (1986). Root distribution and water content were also measured on 3 peds per layer in each treatment on the same dates, with the procedure described in Chapter 3 for the greenhouse experiment. The layers were 0-25, 25-50, 50-75, and 75-100 cm from the soil surface. RESULTS AND DISCUSSION Plant above-ground growth and development The time-course of plant height is summarized in Figure 5.3. For all treatments height increased quite slowly in the first 47 days from sowing, and more markedly thereafter. This pattern was probably a result of the low amount of precipitation recorded in the first 5 weeks after emergence (about 29 mm), while in the following 3 week period rainfall was more abundant and regular (60.4 mm). Plants were always highest in treatment T100, followed by T50, and lowest in MT. Final plant height ranged from 133.6 cm in MT to 166 cm in T100. A similar pattern of growth was found in leaf area time- 169 I-I MT 160.0- M T50/1 o—o T100 %/,./ . - / A ' E ./ . 8 120.0_ ‘/ is“... z . 22 /- '5 / /- I 80.0— _. g /. *5 .o//_/,:--‘ E .// 0.0 I I I I r I I I I I 20 30 4o so 60 70 Days from Sowing Figure 5.3. Time-course of plant height to the uppermost collar during vegetative growth. Vertical bars represent twice the standard deviation. 170 course (Figure 5.4) with final values of about 5300 cm2 per plant measured in the T100 treatment, while T50 and MT plants reached areas of about 3700 and 3400 cm? respectively. The number of fully expanded leaves is plotted in Figure 5.5 against thermal time from emergence. The rate of leaf appearance decreased after the second sampling and was quite low until the fourth sampling, at 47 days from sowing, as reported for plant height and leaf area. An increase in leaf appearance rate was measured when the rainfall amount was higher, between 47 and 68 days from sowing. Ritchie and NeSmith (1991) reported that the relation between the number of leaf primordia or leaf tips and thermal time is linear, but the number of fully expanded leaves increases non-linearly with thermal time. This phenomenon is discussed in relation to the more rapid expansion of the last internodes, and to the observation that the final size of the last leaves is smaller than in the middle section. A nonlinear relation between the number of expanded leaves and thermal time was reported also by Muchow and Ca-rberry (1989) for tropical maize sown at different dates for fully irrigated and water stressed conditions. Treatments in which the crop was stressed during early vegetative growth showed a lower appearance rate than fully irrigated maize, and a higher one after rewatering so that he final number of leaves was the same for all treatments. The periods during and after stress were analyzed separately so that a linear fit could be used to 171 60001 [I MT ‘ A—A T50 / Leaf Area (cm2) (N 8 c.D Days from Sawing Figure 5.4. Time-course of leaf area per plant during Vegetative growth. Vertical bars represent twice the standard deviation. 172 satisfactorily describe the relation between thermal time and the number of fully expanded leaves. In the present experiment there was little variability in leaf appearance rate between treatments, and the final number of leaves was the same for all structural situations, unlike the other measured plant characters. A linear fit was found to describe adequately the relation between number of leaves and thermal time after emergence for the two separate periods of lower and higher rainfall (respectively before and after the fourth sampling), similarly to what reported by Muchow and Carberry (1989). The linear regression lines and parameters are reported in Figure 5.5. The slope of the line corresponding to the first period is intermediate between the values reported by Muchow and Carberry (1989) for“water stressed and fully irrigated, and.in the second period it is smaller than both, probably because in our case the second period corresponded to a higher amount of rainfall, but.not to full irrigation- Tasseling for 75% of the plants was recorded on September 7 in all treatments. Based on the presented data, the effect of treatments on development were less important than those on growth. Leaf extension rate (LER) for the period August 1 to August 8 is reported in Figure 5.6 for all treatments. Leaf extension on August 1 was highest in treatment T100 and lowest in MT. Elongation decreased with time, and the differences between treatments became more evident, especially between T50 and T100. Final LER values of MT and T50 were 54 and 72% 173 Number 01 Fully Expanded Leaves 2 i . . . i . 1 300 400 500 600 700 800 900 1000 1100 Cumulated Thermal Time from Emergence I MT + T50 9* T100 Figure 5.5. Relationship between the number of fully expanded leaves and thermal time from.emergence for treatments MT, T50, and T100. Each point is the mean.of observations on leplants. The fitted line for the period of lower water supply (before the fourth sampling) is y=0.433+0.0102x (R2=0.92). For the period of higher water supply (after the fourth sampling) it is y=-10.9452+0.0242x (R2=0.99) . 174 100 LER (cm) 20- 10‘ A—A 150 0.0 i i i i 1 2 3 4 5 6 7 8 Date (August 1989) .1 Figure 5.6. Leaf extension rate between August 1 and August 8 in the three experimental treatments. 175 Table 5.2. Final vegetative and reproductive biomass (g) after oven-drying at 60 °C. Stem + leaves Ears Grain Treatment MT 50.5 68.6 51.5 T50 69.6 93.6 74.1 T100 82.2 94.3 75.9 respectively of T100 LER. Final plant biomass and yield data are summarized in Table 5.2. Average stem and leaf biomass was higher in treatment T100, followed by T50 and MT. Ear weight averaged 68.6 g for MT and between 93.6 and 94.4 g in T50 and T100. Soil and root Bulk density values for all treatments are reported in Table 5.3. In all treatments bulk.density was higher on August 8, at low soil water content, and lower on September 8 especially in the surface layers, following a rainfall. The values found are similar to those reported for the same soil in 1988 (Comegna et al., 1990). The sampling depths were different in Comegna et al. (1990), where measurements were made after a 8-year tillage experiment. A direct comparison with values found in this experiment would therefore be 176 Table 5.3. Bulk density (g cm-3) and coefficient of variation (CV, %) From 100 cm9 cores. a) average per layer. Sampling date 7/25 7/31 8/8 9/8 Soil layer (cm) Treatment MT 5-25 mean 1.48 1.48 1.50 1.40 CV 4.67 2.26 5.34 2.73 25-50 mean 1.55 1.51 1.56 1.42 CV 3.84 4.67 5.16 5.88 50-75 mean 1.58 1.54 1.58 1.56 CV 2.84 4.99 2.84 4.67 75-100 mean 1.56 1.58 1.58 1.56 CV 3.63 5.16 4.26 4.23 Treatment T50 5-25 mean 1.36 1.37 1.41 1.44 CV 13.57 9.82 5.80 5.37 25-50 mean 1.44 1.47 1.52 1.49 CV 11.38 10.15 8.99 8.01 50-75 mean 1.56 1.56 1.64 1.61 CV 4.20 3.55 4.20 4.93 75-100 mean 1.58 1.58 1.58 1.57 CV 2.80 6.93 5.55 3.55 Treatment T100 5-25 mean 1.36 1.38 1.38 1.40 CV 9.95 11.65 9.41 8.22 25-50 mean 1.47 1.48 1.49 1.46 CV 10.65 9.65 8.41 6.22 50-75 mean 1.47 1.47 1.49 1.51 CV 6.02 7.90 7.09 9.65 75-100 mean 1.49 1.49 1.48 1.55 CV 8.17 7.37 5.48 5.09 tion 1a fine soil and. peds for b) 177 (g cm-3) and coefficient of var cores . 1’ Bulk densit from 100 cm mama mcfladsmm mm.m mm.m om.~ mm.m mm.6 ~m.m >o em.H ~4.H mm.a H4.H mm.H H4.H some oodums no.4 mm.v m.4 o~.a mm.m mm.¢ >a sm.a ~4.H sm.a mm.a mm.a H¢.H cams meiom ma.6 mm.~ mm.m mm.6 mm.q mm.¢ >o mm.H oa.H mm.a sm.a o6.H mm.a cams omimm o~.4 mm.m no.4 mm.m mm.~ mm.~ >o m¢.H sm.a Hm.H ¢~.H ma.H m~.H some mmim code acmeummue mm.6 mm.o no.4 mm.m mm.m mm.¢ >a H6.H mv.a mm.a mm.H mm.H Hm.a came omimm mm.m om.v mm.m no.4 ma.a mm.~ >o 84.H mm.a mv.a 6~.H mm.a H~.H cams mmim ome ucmsummue A203 spawn HHom mama HHom mcflu mood HHom was“ mama HHom mafia Hm\s m~\s O %) treatments T50 and T100. Table 5.3. (CV. 178 incorrect, but it can be pointed out that a higher value of BD was measured in this experiment for the MT treatment compared to the disturbed treatments in the surface layer, while Comegna et al. (1990) reported a lower value in the 10-20 cm treatment of a minimum tillage plot compared to tilled treatments. This was attributed to the effects of tillage on biological activity, a long term effect that would not be likely to produce measurable differences in the time-frame of the present experiment. Regarding treatment effect in all soil layers, soil bulk density was lower in the plots where the soil had been disturbed but its distribution was bimodal with higher values corresponding to peds and lower to fragmented soil. In a previous paper (Chapter 3) values of bulk density were compared with root growth limiting values calculated according to Jones et al.(1991) and the meaning of such values was discussed. Values from the present study are intermediate between non-limiting and totally-inhibiting bulk density according to Jones et al. (1991), based on the soil sand percent by weight. This suggests that some reduction in root growth should be a result of bulk density in this experiment. Values found in the surface layer for T50 and T100 on the first two dates are lower than the non-limiting bulk density, indicating that no effect of soil compaction on root growth was to be expected in these cases. Bulk density values in this experiment, though, correspond to a range of soil water 179 contents, as reported below, while calculations proposed by Jones et al. (1991) were developed for soil at field capacity. other limitations of this approach are discussed in chapter 3. Table 5.4 reports the values of volumetric water content as a function of soil depth in all treatments. On all dates 6v was higher in the undisturbed soil and the variations in water content were lower for this treatment. Water content distribution was bimodal in the other plots, where less extraction was measured from peds compared to fragmented soil. This observation agrees with what reported by Tardieu and Manichon (1987c), who measured lower water uptake from compacted soil regions in loamy soils with a 'B' structural type, which compares to the structure of the T50 and T100 treatments. The RLD (Table 5.5) varied between treatments in amount and distribution along the profile. In treatment MT it ranged from about 0.8 to about 1.4 cm cm“ on different dates in the top 25 cm, and decreased dramatically with depth. In T50 the distribution was similar to that found in MT 25 days after sowing, but subsequently it ‘was Ihigher up to 50 cm, corresponding to the disturbed soil layers. In T100 the decrease of root density with depth was less pronounced and the final total RLD was higher than in the other treatments. As described in chapter 3, if root density values were disaggregated into peds and fragmented soil the distribution appeared.bimodal with higher values in the fine soil and lower 180 Table 5.4. Volumetric water content (cm3 cm’3) on 100 cm3 cores.a) average per layer. sd= standard deviation. Sampling date 7/25 7/31 8/8 9/3 Soil depth (cm) treatment MT 5-25 mean 0.285 0.278 0.259 0.266 sd 0.023 0.015 0.012 0.025 25-50 mean 0.330 0.325 0.320 0.280 sd 0.025 0.026 0.018 0.026 50-75 mean 0.358 0.354 0.352 0.341 sd 0.028 0.027 0.018 0.019 75-100 mean 0.378 0.376 0.367 0.349 sd 0.023 0.019 0.029 0.018 Treatment T50 5-25 mean 0.285 0.263 0.244 0.273 sd 0.079 0.105 0.119 0.077. 25-50 mean 0.295 0.290 0.283 0.260 sd 0.106 0.091 0.088 0.071 50-75 mean 0.341 0.337 0.286 0.290 sd 0.021 0.019 0.015 0.023 75-100 mean 0.365 0.365 0.360 0.350 sd 0.017 0.019 0.029 0.026 Treatment T100 5-25 mean 0.235 0.266 0.247 0.287 sd 0.114 0.093 0.108 0.053 25-50 mean 0.294 0.293 0.287 0.256 sd 0.100 0.099 0.088 0.069 50-75 mean 0.320 0.315 0.256 0.262 sd 0.085 0.088 0.123 0.064 75-100 mean 0.371 0.368 0.304 0.267 sd 0.016 0.043 0.059 0.075 181 wNo.o #No.o 3N0.0 mmo.o wNO.o vmo.o Om wqm.o Nwm.o owm.o wmm.o mam.o owm.o COTE ooalmb mmo.o nmo.o mmo.o mmo.o mmo.o nNo.o Um mgm.o me.o 35m.o mmm.o owm.o owm.o COTE whiom OH0.0 hHo.o mHo.o Odo.o mHo.o bHo.o Um mvm.o mNN.o mom.o MNN.O mom.o nNN.o COTE omimm mHo.o omo.o mHo.o mHo.o mHo.o omo.o Um vmm.o HhH.o mmm.o oom.o hem.o wwa.o COTE mmim OOHB uCTEuOTHB @Ho.o vmo.o ¢H0.0 HNo.o vHo.o HN0.0 Um mwm.o HNN.O ¢mm.o mNN.o Ohm.o omm.o COTE omimm HNo.o NNo.o mac.o omo.o mHo.o ONo.o Om wmm.o ooa.o 3mm.o mmH.o Hem.o mNN.o COTE mmim omB uCTEUOTHE Asoc spawn Edam l and.peds for treatments T50 and T100. sd=standard mama afiom mcflu moan aflom Tsflu mama Hwom Tswu w\w Hm\s mmxs TuOO OCMHQEOm 1ne 801 Table 5.4. Volumetric water content (cm3 cm'3) on 100 cm3 cores deviation. D) f Table 5.5. Root length density (cm cm“) and coefficient of 182 ‘variation (CV, %) on 100 cm? cores.a) average per layer. Sampling date 7/25 7/31 8/8 9/8 Soil depth (cm) treatment MT 5-25 mean 0.68 0.78 0.85 1.37 CV 45.50 43.00 58.05 55.80 25-50 mean 0.20 0.23 0.36 0.68 CV 35.20 37.60 70.20 60.76 50-75 mean 0.08 0.12 0.15 0.20 CV 41.20 43.20 87.75 75.64 75-100 mean 0.00 0.07 0.07 0.08 CV / 68.00 65.40 67.10 Treatment T50 5-25 mean 0.70 0.76 0.81 1.38 CV 55.35 52.89 63.86 59.71 25-50 mean 0.30 0.50 0.71 1.21 CV 60.27 63.96 77.22 65.01 50-75 mean 0.08 0.17 0.22 0.29 CV 57.34 62.40 76.50 73.20 75-100 mean 0.01 0.06 0.06 0.12 CV 65.10 61.88 67.40 73.81 Treatment T100 5-25 mean 0.67 0.87 1.00 1.60 CV 57.56 55.01 62.58 61.50 25-50 mean 0.36 0.50 0.75 0.99 CV 59.06 66.52 79.54 63.71 50-75 mean 0.15 0.20 0.45 0.65 CV 59.06 25.40 73.98 70.78 75-100 mean 0.02 0.08 0.10 0.30 CV 64.97 71.39 65.00 68.00 183 ma.ma Hm.m~ \ sa.~m \ HH.m~ >0 mo.o 8H.o oo.o ma.o oo.o ao.o some OOHims m¢.m~ Hm.mm \ n¢.om vm.qm so.m~ >o so.o mm.o oo.o mm.o mo.o m~.o came meiom m6.- m~.m~ 6m.¢~ mm.mm \ mm.- >0 HH.o m~.H No.6 mm.o oo.o 86.6 came omimm 66.mm mm.om as.ma om.m~ 4m.mm nn.6~ >0 ma.o Hm.a 80.6 om.a oa.o m~.H cows mmim ooae ucmsummue ov.mm oa.m~ oo.vm mm.w~ oo.sa sa.- >o so.o sm.H mo.o mm.o Ho.o vm.o came omimm oo.m~ oo.Hn oo.aa om.a~ oo.m~ sm.6~ >o mH.o sm.a no.6 mv.a ao.o m~.H cams mmim ome usmEuOmuB Isoc zuawa Haom mama aflom mafia mama Hfiom mafia mama Hwom mcflu m\m Hm\s mm\n mama UCHHQEOm Table 5.5. Root length density (cm ch) on 100 cm? cores.b) fine soil and peds for treatments T50 and T100. 184 in the peds. The time-course of root distribution shows that the percentage of roots found in peds increased with time. Thus root proliferation in peds increased after colonization in the less compacted areas. Root distribution in and around peds on July 31 and August 8 is summarized in Tables 5.6 to 5.8 for all treatments. As reported for large scale sampling, RLD was higher outside peds. Values of RLD in peds increased remarkably between the first and second sampling date, especially in deep soil. Data show that root penetration beyond the ped's superficial layer was only occasional, as reported in chapter 3 for a greenhouse study with the same soil. Values of root density in peds were higher in MT (Table 5.6) than in other treatments (Tables 5.7 and 5.8). As for the greenhouse study in chapter 3, this is interpreted in terms of compensatory'growth: sinceino fragmented soil was available in treatment MT for roots to grow, proliferation inside peds was higher. However roots did not generally penetrate peds in MT beyond the superficial 2-cm layer and in fact a large part of the roots were at the ped surface. Water content distribution across peds is summarized in Tables 5.6 to 5.8. On July 31 in all treatments peds in the superficial 50 cm showed a gradient in water content between the surface and the center. This indicates that water was being extracted from.peds“ The peds surface layer had a lower water content in treatment MT, probably in relation to the higher root density around peds, that constituted a stronger 185 Table 5.6. Root length density and volumetric water content across peds deviation. in treatment MT on two dates. sd=standard ped layer (cm) 0-2 2-4 4-6 7/31 RLD (cm cm3) 0-25 mean 1.20 0.00 0.00 Sd 0.22 0.00 0.00 25-50 mean 0.35 0.00 0.00 Sd 0.06 0.00 0.00 8 (cm31 mm Shrlnkoge crock (1mm Figure 5.10. Mapping of structure, roots and water at 25 cm from soil surface for treatment MT on August 8. a) structural mapping; b) root density; c) TDR-determined volumetric water content. 193 0) b) L0) , 2 ® 6\\ [:3 Fragmented SOIL EIIIIDed .Figure 5.11. Mapping of structure, roots and water at 25 cm ‘from.soil surface for treatment T50 on August.8. a) structural 'mapping; b) root density; 0) TDR—determined volumetric water content. 194 a) [:3 Fragmented soll Ped Figure 5.12. Mapping of structure, roots and water at 25 cm from. soil surface for ‘treatment T100. on .August 8. a) structural mapping; b) root density; c) TDR-determined volumetric water content. 195 few cases cavities were observed. Root distribution was related to soil structure as follows. Roots were found in the bulk soil, and in the 0-2 cm layer in peds, but only occasionally inside peds. Root distribution in the bulk soil and at ped surfaces was more regular on the second date of sampling. Water content on the first date was quite uniform in the bulk soil while in peds it increased with distance from the surface. Within the ped's superficial 2-cm layer, water distribution was not uniform, but it was lower where roots were present. On the second date, both root and water distribution were more uniform in peds as a function of distance from surface. In summary, plant growth, yield and leaf elongation varied with soil structure, while leaf appearance rate and tasseling were less affected. Many studies in the literature report that in compacted soils plant growth and yield are reduced (see Hamblin, 1985 for a discussion). Many physical and chemical properties are associated with compaction. In several cases yield reduction in compacted soils is associated with water stress (Hamblin, 1985) . Philips and Kirkham (1965) POinted out that fertilization can alleviate plant growth and Yield reductions associated with compaction. This is Classically interpreted as indicating that compaction reduces the soil volume explored by roots making less resources (Water, nutrients) available. Poor aeration is also commonly found in compacted soils, and its effects both on root growth 196 ( Voorhees et al., 1975, Schumacher and Smucker, 1984) and functionality (Everard and Drew, 1987) , and on top growth (Smit et al., 1989) are documented. Studies in controlled conditions, though, showed that soil strength can play a direct role on reducing plant growth, independent of water and oxygen availability (Masle and Passioura, 1987) . In the field it is not always possible to quantitatively discern the effects of the single factors because soil strength increases as soil drying occurs and both amount and functionality of roots are affected. Data on higher top growth in treatments T50 and T100 can be partly attributed to higher availability of fragmented soil. Water uptake was found to occur mainly in the finer soil, and low water access in peds was responsible for the plants experiencing water deficit even though the average water content of the soil was still high. Values for the average 6" in each soil layer (Table 5.4.a) were for the most part higher than 0.280 cm3 cm‘3. According to Comegna et al. (1990) this corresponds to a soil matric potential of above 0.5 MPa, based on laboratory determinations for the same soil used in this experiment. Water deficiency symptoms such as Severely reduced leaf extension would not be predicted from these values. Only in a few instances 6v values were as low as °°244 to 0.273 cm3 cm'3 (corresponding to about 0.8 to 0.5 MPa a"3"301'ding to Comegna et al. , 1990) . An analysis of data taken separately for fragmented soil and peds (Table 5.4.b) , and of water content gradients across peds (Tables 5.6 to 5.8), 197 though, shows that water content values were much lower ( as low as 0.160 to 0.225 cm3 cm'3) in the areas where root density was higher (fragmented soil and first layer of peds). This would explain why the plants were experiencing water deficiency stress even though regions of higher water content were present in the soil, since those region were inaccessible for roots. A lower leaf elongation rate in a period of no rainfall in treatments MT and T50 was therefore likely caused by'a lower access to water in compacted areas, although.direct effects of increasing soil strength in drying soil around roots may have played a role. Non—water related effects of soil strength on growth reduction were not investigated, but cannot be excluded in this experiment. Root and water spatial distribution.were related to soil structure, and in the August 8 mapping more than on July 31. The relation between structure and root and water spatial distribution needs further investigations, especially regarding its time-course. It has been argued (Passioura, 1985) that water uptake in clustered roots does not depend on RLD but on distance between clusters, provided root density in the cluster is large enough for the roots to constitute a uniform sink (i.e. a surface around peds). In that case water uptake could.be modeled solely based on the clusters geometry, rather than on root density measurements. Where root clusters are related to a fairly regular soil structural pattern, as in the presented cases, this could introduce further 198 simplifications in water uptake modeling. It was shown in this experiment that on the first mapping date root distribution around and inside the surface of peds was not uniform, but it was more so on the second date. Further characterization of such time-course is needed to discern cases in which uptake could indeed be modeled based on structural patterns. REFERENCES Crank, J., 1975. The mathematics of diffusion. Clarendon, Oxford pp 321. Everard, J.D. , and Drew, M.C. , 1987. Mechanisms of inhibition of water movement in anaerobically treated roots of Zea mays L. J. Exp. Bot. 38:1154-65. Jones, A.C., Bland, W.L., Ritchie, J.T., Williams, J.R., 1991. Simulation of root growth. in in Modeling Plant and Soil Systems. Hanks, J., and Ritchie,.J.T. (eds). A.S.A., C.S.S.A., S.S.S.A., Madison: 91-124. Hamblin, A.P., 1985a The influence of soil structure onrwater movement, crop root growth, and water uptake. Adv. Agron. 95- 157. Masle, J., and Passioura, J.B., 1987. The effect of soil strength on the growth of young wheat plants. Aust. J. Plant Physiol. 14:643-56. Muchow, R.C., and Carberry, P.S., 1989. Environmental control of phenology and leaf growth in tropically adapted maize. Field Crops Res. 20:221-36. Passioura, J.B., 1985. Roots and water economy of wheat in Day, W. and Atkin, R.K. (eds) Wheat growth and modelling. Plenum. 185-198. Philips, R.E., and Kirkham, D., 1965. Soil compaction in the field and corn growth. Agron. J. 29-34. Ritchie, J.T., and NeSmith, D.S., 1991. Temperature and crop 199 development. in Modeling Plant and Soil Systems. Hanks, J., and RitChie’ JCT. (edS). A.S.A., C.S.S.A., S.S‘OSOAO' Madison: 5-30. Schumacher, T.E., and Smucker, A.J.M., 1984. Effect of localized anoxia on Phaseolus vulgaris L. root.growth. J. Exp. Bot. 35:1039-1047. Smit, B.A., Neuman, D.S., and Stachhowiack, M.L., 1989. Root hypoxia reduces leaf growth. Role of factors in the transpiration stream. Plant Physiol., 92:1021-28. Tardieu, F., and Manichon, H., 1986. Caractérisation en tant que capteur d'eau de l'enracinement du ma'is en parcelle cultivée. II.- Une méthode d'etude de la repartition verticale et horizontale des racines. Agronomie, 6(5): 415-23. Tardieu, F., and Manichon, H., 1987 a). Etat structural, enracinement et alimentation hydrique du mais. I.-Modélization d'états structuraux types de la couche labourée. Agronomie 7(2):123-31. Tardieu, F. , and Manichon, H. , 1987 b) . Etat structural, enracinement et alimentation hydrique du mais. II.- Croissance et disposition spatiale du systeme racinaire. Agronomie 7(3):201-11. Tardieu, F., and Manichon, H., 1987 c). Etat structural, enracinement et alimentation hydrique du mais. III.- Disponibilite des reserves an eau du sol. Agronomie 7(4) :279- 88. Voorhees, W.B., Farrell, D.A., and Larson, W.B., 1975. Soil strength and aeration effects on root elongation. Soil Sci. Soc. Amer. Proc. 39:984-53. SUMMARY SUMMARY AND CONCLUSIONS The main objective of this research was to study the effect of soil structure on root clustering and its consequences for water uptake in water-limited conditions. A time-domain reflectometry technique was tested for the determination of small scale spatial distribution of water in the 3 were soil.‘Volumetric water content values higher than 0.07 cm3