CORRELATION OF CROP COVER AND NITRATE IN TWO AGRICULTURAL WATERSHEDS, A PRELIMINARY INVESTIGATION

Increased availability and reduced cost of synthetic-nitrogen fertilizers have resulted in the excess deposition of nitrogen in surface water reservoirs. The accumulation of nitrogen has created deleterious effects, generating algal blooms and hypoxic zones. Differences in the utilization rate of nitrogen and the rate of nitrogen application for corn and soybean suggest that nitrogen concentrations, for this work nitrate, may be correlated to the percentage of land cultivated for corn or soybeans in a watershed. To investigate potential relationships, stream-water nitrate concentrations (NO₃-N) and the percentage of land-use devoted to corn and to soybeans in two agricultural watersheds, a small 6.5 km² (LK1) and a large 35.6 km² (LK7), were analyzed. Five years of data for both nitrate and land-use were available for evaluation: 2006, 2007, 2008, 2015, and 2016. During those years, corn accounted for 23-31% in LK7 and 11-21% of the land-use in LK1. Land-use devoted to soybeans ranged between 19-27% (LK7) and 9-21% (LK1). The ratio of land cultivated for corn as compared to soybeans ranged from 0.87-1.65 and 0.51 to 2.27. For LK1, a positive correlation between stream-water NO₃-N and the land devoted to soybeans, and a negative correlation between NO3-N and land for corn production were observed. For the larger watershed, opposite and weaker correlations were observed. The results indicated there may be a watershed size threshold or alternative factors, i.e. presence of tiles in the watershed, that have a greater influence on nitrate concentrations.