The impact of fertilization and hydrology on nitrate fluxes from Mississippi watersheds

The watersheds of the Mississippi are some of the most intensively managed agricultural basins in the world. As such, they receive high loadings of nitrogen and export a large amount of nitrate to the drainage networks of the Mississippi River basin and coastal ocean. We find a positive correlation between fertilizer input and stream export of nitrogen. According to the correlation, 34% of applied fertilizer nitrogen is exported to streams and rivers of the Mississippi basin, a fraction that is greater than the global average. The relationship is partly causal, but also reflects indirect effects, as fertilizer application in the Mississippi basin also correlates with agricultural practices, such as row cropping and tile drainage. The overall impact of these agricultural practices is to increase water throughput and decrease water and nitrogen residence time and processing, which, in turn, increase nitrogen export and the percentage of fertilizer that is exported. This response, coupled with a general increase in precipitation in the MidWest, is exacerbating the nitrogen problem and will decrease the efficacy of nitrogen-reduction management.