Fate of symbiotically-fixed 15N2 as influenced by method of alfalfa termination

Alfalfa (Medicago sativa) may contribute appreciable fixed N to subsequent crops, but the amount may depend on the method of stand termination. In a controlled-environment study, alfalfa was labelled by continuous, prolonged exposure to a 15N2-containing soil atmosphere to allow direct measurement of the fate of fixed N2 from growing and terminated alfalfa. The distribution of 15N in plant and soil components was measured 74 d after alfalfa establishment at the time of alfalfa termination, and again 33 d later following the growth of barley (Hordeum vulgare) for 4 wk. At time of termination, 88% of the 15N present was in alfalfa and 12% in soil (5% in root fragments, ∼5% in microbial biomass and 2% in root and microbial products). The distribution of 15N after alfalfa termination was primarily a function of residue placement rather than termination method. In herbicide treatments in which alfalfa topgrowth was retained on the soil surface, 1% of the 15N present was recovered in barley topgrowth, 8% in soil and 91% in residues; in tillage treatments in which alfalfa topgrowth was incorporated, 10% of the 15N present was recovered in barley topgrowth, 52% in soil and 38% in residues. Regardless of termination method, 10% of the 15N present in alfalfa roots was recovered in barley topgrowth, 30% in soil and 60% in residues. Although tillage and herbicide application result in a similar degree of N release from alfalfa root tissue, tillage accelerates mineralization of alfalfa topgrowth resulting in a larger short-term supply of plant-available N. These results suggest that termination management may provide a means of altering short-term N release from alfalfa, thereby improving synchrony between N release and N uptake by a subsequent crop.