Modeling Surface Water Quality Impacts of Biofuel Crop Alternatives in South Central Minnesota.

Corn ethanol production in Minnesota has increased to nearly 2.3 billion liters per year, requiring corn grain from roughly 17% of Minnesota’s corn acreage.  Minnesota corn acreage increased  14% between 2006 and 2007.  Most of this increase was due to loss of soybean acreage, and shift from a corn-soybean to a corn-corn-soybean rotation.  As a result of this shift to corn acreage, Minnesota producers applied more nitrogen and phosphorus fertilizers. The objective of our research was to quantify the impacts on water quality of increases in corn acreage and changes in crop residue management. The Soil Water Assessment Tool (SWAT) model was used to evaluate the overall impacts of increased corn acreage on water quality due to sediment, nutrient and pesticide losses.   The study was conducted in the Le Sueur watershed of South Central Minnesota, where a corn-soybean rotation is the dominant cropping system. The SWAT model was calibrated over two years and validated over four years monthly and annual water yield, sediment yield, total nitrogen and the total phosphorus outflows. Water quality impacts were studied with a baseline corn-soybean rotation in comparison with a corn-corn-soybean rotation and a biofuel production alternative involving extensive plantings of switchgrass on environmentally sensitive landscapes. Impacts of various crop residue removal rates were also studied. Simulation results showed that increased corn acreage had no significant impact on the annual water yield but increased sediment yield slightly. NO₃-nitrogen and total N losses increased substantially with a shift to increasing corn acreage.  Losses of total P and orthophosphate increased slightly in the expanded corn acreage scenarios. Significant water quality benefits were observed with the switchgrass scenario.