Cover Crops Have the Greatest Impact On Soil Ecosystem Function Along a Gradient of Cropping System Diversity.

Modern row crop agriculture is under pressure to increase food, fiber and fuel production, while reducing environmental impacts. A proposed method to achieve these goals is to offset external agronomic inputs by stimulating ecosystem processes via increasing plant diversity in cropping systems. We examined a gradient of rotational and cropping system complexity, established ten years prior, consisting of single crop monocultures of wheat, corn or soy, and rotations of up to six plant species, including cover crops.  No external inputs of fertilizer or pest management were used during this time.   During two growing seasons, we observed clear trends along the gradient in nutrient stocks and in patterns of microbially mediated nutrient cycling.    Carbon acquiring enzymes and short term incubations indicated labile carbon as a greater source of soil respiration in rotations with cover crops, while non-cover-cropped treatments more actively mineralized recalcitrant soil carbon.  Cover cropped treatments showed clear differences in nitrogen cycling, with increases in potentially mineralizable nitrogen (PMN), even when mineral nitrogen did not greatly differ along the gradient.  Increases in PMN were directly correlated with increase emissions of nitrous oxide in cover cropped treatments.  The use of cover crops enhanced microbial activity associated with carbon and nitrogen cycling, resulting in higher crop yields.