Nitrogen in the Mississippi River Basin: How Do Inputs and Balances Relate to Riverine Export of Nitrate?.

Nitrate export by the Mississippi River is a major cause of hypoxia in the Gulf of Mexico. Much of this nitrate is from tile-drained corn and soybean fields in the upper Midwest, based on sub-basin stream loads. Our objective was to determine the primary factors leading to nitrate export. We examined N inputs (fertilizer, fixation, deposition) and outputs (grain, hay, consumption by humans and animals) for each county in the Mississippi River Basin (MRB) from 1997 to 2006, as well as determined spring nitrate loads for 175 watersheds in the MRB. Past work on larger scales has indicated that net N inputs (NNI) relate well to riverine N export, but we found that NNI was not well related to exports across the basin. Decreasing corn protein concentrations limited the reduction in NNI, even as fertilizer rates remained steady and grain harvests increased, although NNI values declined in the counties with the greatest corn and soybean acreage and yields. Fraction of the county in row crops, fertilizer N per ha, and fraction of crop area tile-drained were all highly correlated to each other as well as to riverine nitrate export. A partial N balance (NNI without soybean and hay fixation along with their harvest) was also strongly related to riverine export. Various non-linear models with watershed runoff, fertilizer N, manure, and deposition were able to predict riverine export. Overall results indicated that dominantly tile-drained counties with corn and soybean production in the upper Midwest were the greatest contributors to riverine nitrate export suggesting that the intensification resulting from engineering these soils is the underlying driver contributing to nitrate export. These results have important agricultural policy implications related to Gulf hypoxia, as our current policies reward high productivity systems despite large nitrate losses.