Correlation of Nitrate and Crop Cover in Row-Crop Agricultural Watersheds

Studying long-term nitrate trends in the Mississippi River Basin (MRB) stems from the development of the hypoxic zone in the Gulf of Mexico to the changes of land use and practices that have taken place in the basin within the last century. Nitrate contamination in the MRB is considered nonpoint source pollution with the most common source coming from a combination of agricultural runoff, and the use of nitrogen fertilizers in areas adjacent to streams. In addition, land use in agricultural watersheds has undergone heavy hydrological modifications. These modifications have been occurring for decades and include channelization of the headwater streams and intensive subsurface tile drainage in fields that efficiently route water to nearby streams. Tile drainage has been in place since the 1860s and continues being replaced and expanded each year, with plastic pipes instead of the original clay pipes being used since the 1950s. Increased use of tile drainage increases the likelihood of downstream surface water contamination from nutrients due to rapid drainage of excess water and nutrients in the soil. Intensive row-crop production of corn-soybean accounts for 90 to 95% of the landscape in the MRB with an average rate of 140lb/acre of N-fertilizers applied to corn in 2010. Less than 50 years ago, corn was grown in rotation with cereal crops and forage legumes such as alfalfa, red clover, and sweet clover with an average rate of 58lb/acre of N-fertilizers applied to corn in 1964. Farming practices began to change with the increased availability of N-fertilizers in the 1960s and 1970s, eliminating the need to incorporate legumes into a crop rotation to restore nutrients into the soil for an optimal yield. Thus, introducing row crop production of corn and soybeans that is practiced today. Using United States Geological Survey water quality data, a statistical analysis will be conducted to determine the effect of land cover changes on nitrate load in agricultural watersheds dominated by a subsurface tile-drained system over the last decade for both seasonal and annual changes. This study will focus on answering the following questions: (1) how does crop type effect nitrate loads in surface water? (2) Do nitrate loads differ between planting season and harvesting season