FATE OF SYMBIOTICALLY-FIXED N-15(2) AS INFLUENCED BY METHOD OF ALFALFA TERMINATION

Alfalfa (Medicago sativa) may contribute appreciable fixed N to subseq uent crops, but the amount may depend on the method of stand terminati on. In a controlled-environment study, alfalfa was labelled by continu ous, prolonged exposure to a N-15(2)-containing soil atmosphere to all ow direct measurement of the fate of fixed N-2 from growing and termin ated alfalfa. The distribution of N-15 in plant and soil components wa s measured 74 d after alfalfa establishment at the time of alfalfa ter mination, and again 33 d later following the growth of barley (Hordeum vulgare) for 4 wk. At time of termination, 88% of the N-15 present wa s in alfalfa and 12% in soil (5% in root fragments, similar to 5% in m icrobial biomass and 2% in root and microbial products). The distribut ion of N-15 after alfalfa termination was primarily a function of resi due placement rather than termination method. In herbicide treatments in which alfalfa topgrowth was retained on the soil surface, 1% of the N-15 present was recovered in barley topgrowth, 8% in soil and 91% in residues; in tillage treatments in which alfalfa topgrowth was incorp orated, 10% of the N-15 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, 3 0% in soil and 60% in residues. Although tillage and herbicide applica tion 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-ter m N release from alfalfa, thereby improving synchrony between N releas e and N uptake by a subsequent crop. Crown Copyright (C) 1998 Publishe d by Elsevier Science Ltd. All rights reserved.