Including winter cover crops in agricultural rotations has shown a great potential to enhance soil carbon (C) content and to supply nitrogen (N) for subsequent main crops. However, the benefits of cover crops have been mostly examined in small-scale research plots and the actual performance of cover crops in farmer practiced agricultural fields is not well understood. Large spatial variations in performance of cover crops and soil properties in agricultural fields cause uncertainty about the cover crop-induced benefits. My overall goal in this study was to analyze the variations in cover crop performance across topographically diverse agricultural fields and to assess how those variations control cover crop influences on soil organic C and mineral N. In chapter 2 I used power analysis to assess how useful the total organic carbon (TOC), particulate organic carbon (POC) and short-term mineralizable carbon (SMC) can be in predicting changes induced by cover crop management. I collected soil samples from ten fields under management with and without cover crop and analyzed them for TOC and for two active C pool characteristics, such as POC and SMC. The results showed that to be able to detect differences between the management systems with an acceptable type II error of 0.20 an 80% difference in TOC and a 50% difference in SMC were needed. The statistical power for POC was never in an acceptable range. I recommend SMC as the preferred C fraction for detecting differences between treatment induced changes in C processes in agricultural field experiments.In chapter 3, I, first, assessed the spatial distribution of TOC, POC and SMC across 20 fields under conventional, low-input, and organic managements, where the latter two contained cover crops in their rotation. The results showed that POC was significantly lower on topographical slopes of fields under conventional agricultural management compared to managements with cover crops. Then, I monitored the specific contribution of cover crops to POC and SMC across topography during 2011, 2012 and 2013. I established microplots with and without cover cops on three topographical positions of "depression", "slope" and "summit" where I collected soil samples and measured cover crop biomass inputs. Red clover had significant positive effects on SMC in slopes and summits. Rye had significant positive effects on SMC only on summits. The cover cropping practice has a potential to increase soil C, hence soil quality, in depressions and summits and maintain it in slopes. In chapter 4, I monitored the spatial and temporal distributions of soil NO3-N in 20 agricultural fields. The results showed that temporal and spatial distributions of NO3-N were mainly controlled by soil organic C content. Then, I assessed the contribution of cover crops to controlling NO3-N and potential mineralizable nitrogen (PMN) across topography. Red clover had significant positive effects on NO3-N across all topographical positions. Rye had significant negative effects on NO3-N only in depressions. The spatial variations in amounts of residue from cover crops were found to be an important factor controlling the effects of cover crops on soil mineral nitrogen across topographically diverse fields.
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