Nitrogen release from grass and legume cover crop monocultures and bicultures

The use of grass-legume bicultures grown as winter annual cover crops may provide farmers with additional cover crop management options regarding the availability of cover crop residue N. A 2-yr field experiment was conducted to determine dry matter (DM) accumulation, chemical composition, and N release from grass and legume cover crops grown in monoculture (rye, crimson clover, and hairy vetch) and biculture (rye-crimson clover and rye-hairy vetch). Air-dried plant material was placed on the soil surface in 1-mm mesh nylon bags for 1, 2, 3, 4, 6, 8, and 16 wk. Following retrieval, mesh bag contents were analyzed for total N, C, cellulose, hemicellulose, and lignin concentrations. The 2-yr mean cover crop DM production was in the order of rye-hairy vetch > hairy vetch > rye-crimson clover > rye > crimson clover. The greatest cover crop N content (2-yr mean) occurred with hairy vetch monoculture (154 kg N ha(-1)), compared with a low of 41 kg N ha(-1) for the rye monoculture. When grown in biculture with rye, hairy vetch accumulated more DM and biomass N compared with crimson clover, both as a proportion of the biculture and as DM yield. In general, the order of N release rates (rapid to slow) was hairy vetch > crimson clover = rye-hairy vetch > rye-crimson clover = rye. Estimates of N (kg ha(-1)) released from cover crop residue after 8 wk of field decomposition, averaged over 2 yr, were 24 for rye, 60 for crimson clover, 132 for hairy vetch, 48 for rye-crimson clover, and 108 for rye-hairy vetch. Results of this study demonstrate only slight reductions in N release from grass-legume bicultures compared with legume monocultures.