Do rotations with cover crops increase yield and soil organic carbon?—A modeling study in southwest Germany

Conservation agriculture practices of crop rotation with permanent soil cover have been widely promoted for improving long-term agroecosystem resilience in the face of changing climate. However, there has been no comprehensive evaluation of site-specific agroecosystem services of soil health and crop yield in response to improved crop rotations with and without cover crops (CCs) on field and spatial scales. We calibrated and applied a process-based agroecosystems model to determine the effects of improved crop rotation and cover cropping on soil organic N content and mineralization rate, soil organic carbon (SOC) change rate, soil CO2 efflux, and crop yields. A 10-year cropping systems dataset from six sites in southwest Germany was used to calibrate and evaluate the DSSAT model and to provide the typical management practices of the conventional farming system in the region as the business-as-usual (BAU) scenario for model application. A 4-year crop rotation was then designed with and without the inclusion of commonly grown non-legume and legume CCs and applied in three cycles at the research sites and the surrounding region. Crop rotation without CCs treatments provided the no-CC scenario, therefore the effect of CC inclusion could be tested. Relative to BAU and no-CC, the inclusion of CCs in crop rotation on annual rate, resulted in 12% and 3% higher soil organic N and 6% and 8% higher SOC change rate, respectively. Additional advantage of cover cropping on soil organic N and C was more pronounced by legume CCs while non-legume CCs were more efficient in reducing N leaching. Combined positive rotational and cover cropping effects were observed on winter wheat and oilseed rape yields at the research sites. However, we observed spatial variability of these results on regional scale, suggesting management by environment interactions that should be considered for site-specific management recommendations. Crop rotation with CCs significantly increased water productivity of cereal crops, but did not produce higher yield of winter and spring barley or silage maize compared with BAU unless only legume CCs were used in certain areas that are vulnerable to N losses. Our findings highlight the C sequestration potential of improved crop rotations and cover cropping emphasizing the need for site-specific management for agronomically improved and environmentally sound cropping systems.