Legume residue and soil water effects on denitrification in soils of different textures

Legume cover crops commonly used to supply additional N and reduce potential for over-winter N leaching losses may also influence denitrification depending upon soil water status and soil type. Interrelationships between incorporated hairy vetch ( Vicia villosa) residue and soil water status on denitrification in coarse, medium and fine textured soils were investigated in the laboratory. Repacked soil cores were incubated, 10, 20 and 30 d with and without acetylene (C 2H 2). Denitrification losses were 20–200 μg N kg −1 from each soil when 60% of the soil pore space was filled with water and increased to from 14.0 to 18.6mg N kg −1 at 90% water-filled-pore space (WFPS). Incorporation of vetch residue (2.5 g kg −1) greatly stimulated denitrification (51.1–99.5 mg N kg −1), probably due to greater availability of organic C as indicated by higher CO 2 emissions. The major denitrification losses occurred during the first 10 days and more so in residue-amended soils. The supply of C from incorporated legume crop residue was a major factor influencing denitritication especially when soil wetness restricted aeration and adequate nitrate was present. At similar water contents, rates of denitrification differed greatly in soils of varying texture, but when varying water holding capacity and bulk density were accounted for using WFPS. all soils behaved very similarly. Use of WFPS as an index of aeration status enabled identification that differences in denitrification losses in vetch-amended soils of varying texture resulted in part from varying capacity to supply NO 3 − and metabolize organic matter. These results illustrate the utility of WFPS, compared with soil water content, and its reliability as an indicator of reduced aeration dependent denitrification for soils of varying texture.