NITRATE CONCENTRATIONS IN STREAMS AS A FUNCTION OF CROP COVER IN MIDWESTERN AGRICULTURAL WATERSHEDS: ASSESSING THE ROLE OF CORN AND SOYBEANS

Increased availability and reduced cost of synthetic-nitrogen fertilizers have led to excess nitrogen being deposited in reservoirs. The accumulation of nitrogen (N) in reservoirs has negative effects, generating algal blooms, hypoxic zones, and poor drinking water quality. Corn and soybean utilize nitrogen at different rates, resulting in higher nitrogen fertilizer application to fields for corn than for soybean. This work examines whether the nitrate concentration in a stream may be correlated to the percentage of land devoted to growing corn or soybeans in the watershed. To investigate potential relationships, discharge (Q) and nitrate concentration data from nine USGS gauging stations across Indiana, Illinois, Iowa, Kansas, and South Dakota and agricultural land-use data from USDA were analyzed. Watershed areas ranged from 106 km² (Spoon River) to 154,767 km² (Kansas River). Corn was grown on between 14.31% (Kansas River) to 56.07% (Indian Creek) of the land, while soybeans accounted for 7.19% (Kansas River) to 45.39% (Spoon River). Crop percentages were compared to both weighted flow concentrations and nitrate loads per area from 2008 to 2017. For each system, weighted flow concentration equated to the total annual N-load (kg) divided by the total annual Q. Nitrate load per area represented the quotient of annual N-load (kg) to the watershed area (km²). The analyses indicated that as the percentage of corn cultivated in the watershed increased, both the weighted flow concentration and nitrate load per area decreased for all watersheds, except for the Kansas River, which is the largest watershed with the least amount of corn. Collectively, analysis of the data indicated weighted concentrations increase as the percentage of land with corn increases. Opposite trends were observed when the percentage of soybean cultivated in the watershed increased; weighted flow concentration and nitrate load per area all increased with respect to the percentage of soybean cultivated both for individual watersheds and collectively. The one exception being the North Raccoon River. The results imply soybean production has a more direct impact on nitrate concentrations, although corn fertilizer application and total cultivation rates are higher in each watershed.