Winter Cover Crops and Landscape Position Impact on Northern Great Plains Corn (Zea mays) Yield, Water and Nitrogen Stress, and Soil Biology.

In semi-arid Great Plains soils, fallow was used to save water required for the cash crop.  The need for fallow was decreased by the adoption of no-tillage.  The objective of this paper was to determine the impacts of winter cover crop cocktails that contain plants from the mustard family (Brassicaceae) on corn yields, corn yield losses due to water and N stress, soil bacterial to fungi ratio, and corn gene regulation in responses in 8 selected genes.  The research was conducted at two landscape positions (summit and footslope) at three northern Great Plains sites in 2011 and 2012.  These sites were characterized as low, moderate, and high water stress environments.  No-tillage was used at all sites and the fall cover crops plot were split by 4 N rates (0, 34, 67, and 134 kg ammonium nitrate-N ha-1).  Soil was analyzed for bacteria/fungi ratios, soil moisture, and inorganic N.  Leaf samples, collected at V12 were analyzed for gene regulation and grain samples were analyzed for yield loss due to water (YLWS) and N stress (YLNS) using the 13C stable isotopic technique. In the moderate water stress environment, the cover crops reduced (p=0.09) the corn yield.  These results are attributed to the cover crop using water that could have been used by the crop.   Findings also showed that winter cover crops contributed to a higher bacterium to fungi ratio, reduced nitrate-N in the spring soil samples, increased or did not impact water infiltration, and had a mixed impact on gene regulation.  The cover crop impact on gene regulation was attributed to the cover crop impact on water stress.  In the moderate yield environment, cover crops increased YLWS.  Associated with increased water stress was down regulation in 2 of the 3 genes associated with mineral nutrition and one of the two genes associated with photosynthesis.  Findings from this study suggest that cover crops may reduce yields through increased water stress.  This risk is increased by planting the cover crops early.