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Forage legumes in crop rotations are often used to supply N far 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 N-15 recovery by a second subsequent crop (oat [Avena sativa L.]) and soil from microplots amended with N-15-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 m(2) microplots and forage sorghum (Sorghum bicolor [L] Moench) was used to evaluate the N-15 recovery. Above-ground residues were removed from microplots immediately after each of the two sorghum harvests. in 1995, the N-15 recovery by the oat crop and soil was determined. Nitrogen-15 recovery by oat averaged 3% of the N-15 applied as green manure. Of this, 95% was found in tom and 5% in roots. Averaged across application rates, oat recovery of N-15 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 N-15 was found in soil at the 0-0.3 m depth at the end of the oat season. The percent of N-15 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. (C) 2001 by The Haworth Press, Inc. All rights reserved.