Cereal-legume cover crop mixtures composed of cereal rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth) have the potential to combine the unique strengths of the component species while taking advantage of interspecific synergies. However, management practices and environmental conditions can influence both species interactions and residue decomposition. The overarching objective of this research was to evaluate strategies for optimizing rye-vetch mixture performance within vegetable cropping systems. The main component of this research evaluated 1) how the relative proportions of rye and vetch sown in mixtures influenced cover crop biomass production, winter annual weed suppression, vetch winter survival, and vetch N fixation; and 2) the interactive effects of rye-vetch residue characteristics and the use of black polyethylene mulch (PM) on soil N dynamics, microbial communities, and vegetable yields during production of bell pepper (Capsicum annuum L.) and slicing cucumber (Cucumis sativus L.). Following a replacement series design, main plot treatments consisted of a gradient of seven rye-vetch mixture proportions ranging from 100% vetch to 100% rye, in addition to a no-cover-crop control. Following cover crop termination, subplots consisted of pepper and cucumber grown either with or without PM. Density and biomass composition in the mixtures were highly correlated with rye and vetch seeding rates, with little evidence of substantial interspecific interference. Total shoot biomass in all mixtures was equal to or greater than that of either monoculture, but no differences were detected in vetch winter survival or the efficiency of N fixation. Changing the proportions of rye and vetch in the mixtures resulted in important tradeoffs among some of the services provided by the cover crop. Increasing vetch in mixtures generally led to greater fixed N accumulation, soil inorganic N levels, and vegetable yields, but also led to increased seed costs, pore water nitrate concentrations below the crop root zone, and reduced winter annual weed suppression. Yields and soil N levels were generally higher when PM was used, but the differences between PM and bare ground treatments varied between the two years due in part to differences in the magnitude and timing of precipitation events. Lower microbial biomass was observed under PM compared to bare ground 3 wk after plastic application, but cover crop and mulching treatments did not explain the majority of variation observed in patterns of carbon substrate utilization from Biolog-EcoplateTM data. In a separate field experiment, we also investigated how fall planting dates influenced rye-vetch cover crop biomass quantity and quality in the spring, and evaluated whether staggering (delaying) rye seeding could improve vetch performance in mixtures. Treatments consisted of a two-way factorial of three vetch planting dates (late August, mid September, and late September) and three lengths of rye seeding stagger (co-seeded, short stagger, and long stagger). Later planting of co-seeded mixtures generally led to reduced total shoot biomass and lower proportions of vetch biomass, resulting in cover crop residues with less fixed N and a higher total C:N. For earlier planting dates, delaying rye seeding until vetch emergence (short stagger) increased vetch shoot biomass by 760 - 1,060 kg ha-1 (30-36 kg vetch N ha-1) relative to co-seeding. Staggered seeding provided no benefit to vetch biomass at later planting dates, and delaying rye seeding until the vetch 3-4 leaf stage (long stagger) resulted in significant reductions in vetch winter survival compared with co-seeding.
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