TILLAGE EFFECTS ON LEGUME DECOMPOSITION AND TRANSFORMATION OF LEGUME AND FERTILIZER N-15

The inclusion of legumes in cropping systems raises questions about their effectiveness as a N source. The objective of this study was to determine the effects of tillage on legume cover crop decomposition and transformation dynamics of legume and fertilizer N-15. Nitrogen-15-labeled hairy vetch (Vicia villosa Roth.) residue or fertilizer was added to soil cores contained in open-ended Plexiglas tubes placed in the field. Vetch residues were either placed on the soil surface for no-tillage (NT), or mixed with the soil for conventional tillage (CT). In 1984, (NH4NO3)-N-15-N-15 was applied at rates of 0 and 100 kg N ha - 1 and, in 1985, ((NH4)2SO4)-N-15 was applied at rates of 0 and 150 kg N ha - 1 to both NT and CT cores with a history of winter fallow. Fertilizer N-15 was applied in solution to the soil surface. Within 30 d, 77% of the original vetch residue weight was lost with CT, compared with 45% with NT. Nitrogen lost from the residue by 30 d averaged 89% with CT and 60% with NT. At 15 d in 1985, soil inorganic N-15 recovery for fertilizer was 78% with CT and 57% with NT, and for vetch it was 47% with CT and 12% with NT. Vetch N-15 immobilization averaged 2.3 times greater than fertilizer N-15 with CT and 1.7 times greater with NT. The results indicate that N availability as measured by soil inorganic N is less from vetch residue than fertilizer due to both greater vetch N immobilization and the dependence of vetch N mineralization on decomposition rate.