Large‐Scale Assessment of Symbiotic Dinitrogen Fixation by Crops: Soybean and Alfalfa in the Mississippi River Basin

Human activities have increased the amount of earth's reactive N, resulting in significant improvements in crop yield and animal production but also in environmental degradation and ecosystem disruption in some areas. For example, agriculture has been cited as a major source of N that contributes to hypoxia in the Gulf of Mexico. Although other sources of N have been well characterized in large ecosystem studies, the contribution of legume crops to the N cycle has not. Furthermore, the role legumes play in reducing excess N is not widely recognized. Symbiotic N 2 fixation is a facultative process that is reduced by plant N uptake from other sources. Using reported and estimated crop yield and protein concentration with published estimates of soil N mineralization and atmospheric N deposition, we estimated spatial patterns of symbiotic N 2 fixation for soybean [ Glycine max (L.) Merr.] and alfalfa ( Medicago sativa L.) across the Mississippi River Basin, the largest in North America. We estimate that alfalfa haylage adds about 20% to total production of dry alfalfa hay and increases total land area in alfalfa by about 11% over that reported in the Census of Agriculture. Our analysis shows wide ranges in N 2 fixation (0 to 185 kg N ha −1 for soybean and 45 to 470 kg N ha −1 for alfalfa), reasonable mean rates (84 kg N ha −1 for soybean and 152 kg N ha −1 for alfalfa), and suggests that about 2.9 million Mg of fixed N is harvested annually in these two cultivated legumes.