Nitrogen Recovery from 15 N-Labeled Green Manures: I. Recovery by Forage Sorghum and Soil One Season After Green Manure Incorporation

This investigation was part of a project to evaluate green manure forage legumes as relay intercrops for sweet corn (Zea mays L.) and chile (Capsicum annuum L.) in north-central New Mexico. The study was carried out under irrigated conditions in 1994 on a Fruitland sandy loam (coarse-loamy, mixed, calcareous, mesic Typic Torriorthent) to evaluate the 15N recovery by forage sorghum (Sorghum bicolor [L.] Moench) and soil after the incorporation of 15N-labeled alfalfa (Medicago sativa L. ‘Nitro’) or hairy vetch (Vicia villosa Roth. ‘Madison’). Two application rates of alfalfa (2.7 or 4.0 Mg dry matter ha□1) or hairy vetch herbage (3.2 or 6.4 Mg dry matter ha□1), were incorporated into soil microplots. Additional treatments consisting of 56, 112, and 168 kg N ha□1 (as urea) were used to compare soil inorganic N concentrations with those released from the legumes. Nitrogen-15 recovery by sorghum tops was 16% of the 15N applied, in two harvests. Of this, 80% was recovered at the first harvest. In soil, 15N recoveries were 50 and 3% of the input at the 0–0.3 and 0.3–0.6 m depths, respectively. Maximum inorganic N concentrations were found 14 d after amendment application. At the end of the growing season, inorganic N concentrations in all treatments were essentially the same as the background (8 mg N kg□1). The low 15N recoveries by crop plus soil and inorganic N concentrations in soil at the end of the season suggest that about 12% and 35–58% of the N applied as green manure was lost from the system for alfalfa and hairy vetch, respectively.