ABSTRACTASSESSING THE IMPACT OF COMPOST AMENDMENT FOR MANAGING NEMATODES AND THE HEALTH OF MINERAL SOIL UNDER CARROT PRODUCTION By Alemayehu Workalemahu HabteweldIn the absence of resistant cultivars and sustainable alternatives to nematicides, soil nutrient amendment is one of the potential alternatives for managing herbivore nematodes and improving soil health in carrot (Daucus carota L.) production. Using nematodes as indicators of changes in soil ecosystem, the main goal was to develop integrated compost amendment strategies for managing soil nutrient, nematodes, soil health, and carrot yield and quality. Processing carrot ‘Cupar’ and a fresh market cultivar ‘Sugarsnax 54’ were grown with amendments of plant compost (PC) and animal waste-based compost (AC) use under field and/or glasshouse conditions during 2012-2014 to test a series of hypotheses. Nematode community, five times during the growing season at approximately four-week intervals, soil physicochemical properties and respiration at planting and at harvest, and carrot yield were standard parameters. The first series of experiments compared AC and PC applied at 135, 203 and 270, and 112, 168 and 224 kg nitrogen (N) per ha for Cupar and Sugarsnax 54, respectively, in sandy clay loam soil. Urea and non-amended check were included as controls. The hypothesis was that compost amendment would enhance soil food web structure index (SI), improve soil biological and physicochemical properties, and increase carrot yield and quality, but these effects would differ by compost type and rate of application. Compost amendments increased SI from 50 % in Cupar. Higher rates of AC treatments increased soil respiration. Although compost amendments increased yield in 2013 in Sugarsnax 54, most compost amendments decreased yield in 2012. The variable responses relative to compost rate, source and time suggest that the effect of compost based amendments on the soil food web structure and the ecosystem services it provides point away from a one-size-fits-all approach. In the second experiment, the effect of mixtures of urea and PC at 3:1, 1:1 and 1:3 ratios on soil food web enrichment index (EI), SI, biological activity level, and carrot quality and yield relative to single applications of either product was evaluated. Amendments were applied to supply 135 kg N per ha. Urea alone and non-amended check were included as controls. The hypothesis was that mixtures with higher rates of PC would enhance enrichment index (EI) and SI, soil biological and physicochemical properties, and increase carrot yield and quality. Lower rates of urea mixed with higher rate of PC increased soil biological activity. However, there was no difference among the treatments in carrot yield. In addition, no changes in EI due to treatments were detected.In a third experiment, how compost works in sterilized soil was used to simulate biologically degraded soil under glasshouse conditions over three months; however, it appeared that a longer time is needed for compost to be effective. As part of understanding variable responses and integrated amendment use efficiency, a weighted nematode guild abundance concept was introduce to integrate the fertilizer use efficiency (FUE) and soil food web models for better resolution of agroecological analysis relative to soil health. Integrating weighted nematode guild abundance and agronomic data into the FUE model showed that most of the compost treatments were significantly greater than the control. Collectively, the findings help build a foundation up on which soil health-related industry priorities can be addressed through understanding the biological and physicochemical basis of the changes and designing potential solutions that fit variable soil conditions.
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