Fate of symbiotically-fixed 15N2 as influenced by method of alfalfa termination
1998
Mohr, R.M. | Janzen, H.H. | Bremer, E. | Entz, M.H.
Alfalfa (Medicago sativa) may contribute appreciable fixed N to subsequent crops, but the amount may depend on the method or stand termination. In a controlled-environment study, alfalfa was labelled by continuous, prolonged exposure to a 15N(2)-containing soil atmosphere to allow direct measurement or the fate of fixed N2 from growing and terminated alfalfa. The distribution or 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 or 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, approximately 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.
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