Nitrogen Utilization from Fertilizer and Legume Residues in Legume‐Corn Rotations¹

Abstract Legume‐corn ( Zea mays L.) rotations are common throughout the U.S. Corn Belt, but little information is available about the efficiency of legume residue N utilization, or about fertilizer‐N use efficiency in different cropping sequences. Our objective was to determine the contribution of N from soil, fertilizer, and legume residues for six legume‐corn sequences. The effect of applied fertilizer N rate was also examined. Field studies were conducted on a Udorthentic Haploboroll and a Typic Hapludoll. First‐phase (Year 1) rotation components included alfalfa ( Medicago sativa L.), corn, soybean [ Glycine max L. (Merr.)], or fallow. Nitrogen–15‐depleted (NH 4 ) 2 SO 4 was applied to second‐phase (Year 2) corn. Fertilizer‐N use efficiency for second‐phase corn was 51% for the whole plant and 36% for grain, with no effect of first‐phase treatment. Average use efficiency of legume‐N (uptake of legume‐N/legume‐N incorporated) was 43% for the whole corn plant and 30% for grain. Second‐phase corn derived a greater proportion of its N from residue of alfalfa (36%) than soybean (12%). When the rate of N applied to corn was increased from 56 to 168 kg N ha −1 , the proportion of N derived from legume residue decreased from 44 to 19% in whole corn plants and from 57 to 23% in grain. Results suggest that the N credit commonly attributed to legumes in crop rotation may be inflated by as much as 132%.