The impact of cropland cover on river nutrient levels in the Mississippi River Basin

Aim This study developed high-resolution datasets of the area of maize, soybean and wheat cultivation in the United States for 1950, 1970 and 1992. The datasets were used to examine the relationship between individual crop cover and nutrient levels in rivers across the Mississippi River Basin. Location This study was conducted in the United States, with focus on the Mississippi River Basin. Methods County and state-level estimates of the planted area of maize, soybean and wheat were integrated with a satellite-derived pattern in total cropland to develop 5' x 5' resolution datasets of the fractional area of maize, soybean and wheat in the United States for 1950, 1970 and 1992. Regression analysis was used to determine the relationships between mean levels of nitrogen, phosphorous and silica in 25 rivers across the Mississippi Basin, and the extent of maize, soybean and wheat cultivation in the watersheds. Results The crop datasets provide a spatially explicit representation of the area of the three primary crops in 1950, 1970 and 1992. The datasets clearly depict the rapid expansion of soybean cultivation, the increased specialization of individual agricultural regions, and the accompanying reduction in agricultural biodiversity since 1950. The statistical analysis identified strong correlations between the extent of maize and soybean cover in a watershed and the level of nitrogen (N) and phosphorous (P) at the watershed outlet. In particular, the area of maize appears to be strongly related to the concentration of total-N and the ratio of dissolved inorganic-N and organic-N at the watershed outlet. Conclusions The intensification of cultivation of the three primary crops - maize, soybean and wheat - has altered land cover and water quality across the U.S. since 1950. The high present-day rate of fertilizer use on maize and the expansion of soybean cultivation could be largely responsible for the change in nutrient ratios in the Mississippi River since 1950.