Nitrogen Recovery from 15N-Labeled Green Manures: II. Recovery by Oat and Soil Two Seasons After Green Manure Incorporation

Abstract

ABSTRACT Forage legumes in crop rotations are often used to supply N for succeeding crops. However, the residual effect of forage legume green manures for second crops has not been thoroughly investigated in high desert regions of the Southwestern United States. The objective of this study was to evaluate the 15N recovery by a second subsequent crop (oat [Avena sativa L.]) and soil from microplots amended with 15N-labeled alfalfa (Medicago sativa L. ‘Nitro’) and hairy vetch (Vicia villosa Roth ‘Madison’). In 1994, two application rates of each legume were incorporated into 1 m2 microplots and forage sorghum (Sorghum bicolor [L.] Moench) was used to evaluate the 15N recovery. Above-ground residues were removed from microplots immediately after each of the two sorghum harvests. In 1995, the 15N recovery by the oat crop and soil was determined. Nitrogen-15 recovery by oat averaged 3% of the 15N applied as green manure. Of this, 95% was found in tops and 5% in roots. Averaged across application rates, oat recovery of 15N was 63% greater in treatments amended with alfalfa than with hairy vetch. Green manures did not affect the total N concentration of oat parts. Most of the residual 15N was found in soil at the 0–0.3 m depth at the end of the oat season. The percent of 15N recovery by soil in plots amended with alfalfa was 238% higher than with hairy vetch. Green manures applied in 1994 had little residual effect on the N nutrition of oat.