Corn Acreage, Fertilizer Use, and Spring Nutrient Discharge in the Mississippi River Basin: Relationships and Impact On Hypoxia.

Hypoxia in the Gulf of Mexico has been monitored annually since 1985. There have been a number of scientific reports on the causes and effects of hypoxia. Discharge of nitrogen and phosphorus via the Mississippi River has been identified as significant associated factors. Spring discharge of nitrogen and phosphorus has been more strongly related to the annual size of the hypoxic zone than the annual flux of these nutrients. Public and scientific concerns about the impacts of expanded corn acreage, nutrient applications, and the demands for biofuels have increased. Will an expanded corn acreage associated with an increased biofuel demand contribute to increased nutrient flux to the Gulf, and a larger hypoxic zone?  The relationships among spring nutrient flux, corn and soybean acreage, fertilizer nitrogen and phosphorus use, and hypoxia were examined for 1985 to 2008. Since the mid-1980s, spring discharge of nitrate-nitrogen, Kjeldahl nitrogen, total nitrogen, and total and orthophosphate phosphorus declined.  The net inputs of nitrogen and phosphorus declined most notably in the Upper Mississippi and the Ohio Sub-Basins, and modestly in the entire Mississippi River Basin. These changes occurred while corn yields increased and fertilizer consumption remained relatively static.  Trends in spring discharge of nitrate-nitrogen, total phosphorus and orthophosphate phosphorus against harvested corn acreage were negative from 1990 to 2008. The vigilance of farmers, the skills of their crop advisers, and the research and transfer of new management technologies are likely to determine the future of crop nutrient use efficiency. Improvements in crop nutrient use efficiency through a 4R stewardship (right source, right rate, right time, and right place) are considered essential to improve  farm profitability and sustainability, to provide global demands for food, fiber, and biofuels, and to protect water and air  resources.