Cover Crops Have the Greatest Influence on Belowground Ecosystem Processes Along a Gradient of Cropping System Diversity

Row crop agriculture dominates most of the landscape of the U.S. upper mid-west, and along with constant pressure to increase production are calls to reduce inputs harmful to air, soil and water. A proposed method to achieve these goals is to stimulate soil ecosystem processes by increasing plant diversity within cropping systems. We examined a gradient of rotational and cropping system complexity, consisting of single crop monocultures of wheat, corn or soy, and rotations of up to six plant species, including cover crops.  No external inputs for fertilizer or pest control were added during ten years of management.  Over two growing seasons, we observed clear trends in microbially mediated carbon and nitrogen cycles, particularly in rotations with cover crops.   Soil enzymes assays showed a greater potential to mineralize labile carbon and nitrogen pools, and shifts in 16S rRNA across the gradient suggested the structure of the microbial community shifted with rotation.  Short term incubations for carbon and nitrogen mineralization confirmed the greater capacity of cover cropped systems to turnover nutrients, even when mineral nitrogen and total carbon did not greatly differ along the gradient.  Increases in mineralizable nitrogen were directly correlated with increase emissions of nitrous oxide in cover cropped treatments.  Cover crops enhanced microbial activity associated with carbon and nitrogen cycling, resulting in higher crop yields, more than increased diversity of crop rotation per se.