The Minnesota Pollution Control Agency is developing a new set of standards for nitrate nitrogen in surface waters. The EPA’s Science Advisory Board has also called for a 45% reduction in both N and P in the Mississippi River to reduce the Gulf of Mexico hypoxic zone. A watershed-level nitrogen reduction planning tool (Excel spreadsheet) will be discussed that compares the effectiveness and cost of combinations of nine different “best management practices” (BMPs) for reducing N loads leaving a Minnesota watershed.
The BMPs are: reducing corn N fertilizer rates to extension recommended rates, changing fertilizer application timing from fall to spring preplant or sidedressing, seeding cover crops, installing tile line bioreactors or controlled drainage, planting riparian buffers, restoring wetlands, or converting some corn and soybean acres to a perennial crop. The N loads are calculated in three categories: drainage tile discharges, leaching from cropland, and runoff.
This tool suggests that it will be difficult to achieve a 45% reduction with the BMPs considered, even at high adoption rates. Reducing N fertilizer rates on corn down to extension-recommended levels and shifting from fall to spring or sidedressed applications tend to be among the cheaper BMPs to adopt, but the results vary across watersheds and weather scenarios. Various other factors such as crop and fertilizer prices also affect the results, hence the need for a computer tool.