Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 1, 2000
Publication Date: December 1, 2000
Interpretive Summary: The use of winter annual cover crops has increased in recent years due to their ability to significantly reduce NO3- leaching during winter/spring recharge and/or provide for the nitrogen (N) demands for subsequent crop growth. Unfortunately, no single cover crop has shown to consistently achieve both objectives. Grasses, for example, can significantly reduce N leaching, but generally provide little N for crop growth. Legumes, such as hairy vetch, can supply substantial amounts of N, but their ability to reduce N leaching during winter/spring recharge is minimal. The use of grass/legume mixtures has the potential to fulfill both objectives. We found that denitrification was observed in vetch cores five days after kill as a consequence of available nitrate and high respiration rates. Substantially lower denitrification was observed on subsequent dates or with other treatments due to either limited nitrate availability or lower respiration rates. Results suggest greater potential N losses from vetch than rye or rye-vetch cover crops due to rapid N-mineralization in conjunction with denitrification and leaching, prior to significant crop N- assimilation. This study supports the conclusion that a rye-vetch cover crop mixture may be superior to monoculture vetch or rye cover crops because of intermediate net N-mineralization rates resulting in decreased N leaching and denitrification losses, and better correspondence between N availability and crop N demands.
Technical Abstract: N mineralization, N immobilization and denitrification were deter- mined for vetch, rye, and rye-vetch cover crops using large packed soil cores. Plants were grown to maturity from seed in cores. Cores were periodically leached, allowing for quantification of nitrate and ammonium production, and denitrification incubations conducted before and after cover crop kill. Gas permeable tubing was buried at two depths in cores allowing for quantification of nitrous oxide in the soil profile. Cover crops assimilated most soil N prior to kill. After kill, relative rates of N mineralization were vetch > vetch-rye mixture > fallow > rye. After correcting for N mineralization from fallow cores, net N mineralization was observed in vetch and rye- vetch cores while net N immobilization was observed in rye cores. Denitrification incubations were conducted 5, 15 and 55 days after kill and with adjustment of cores to 75% water filled pore space (WFPS). The highest denitrification was observed in vetch cores five days after kill as a consequence of available nitrate and high respiration rates. Substantially lower denitrification was observed on subsequent dates. Results suggest greater potential N losses from vetch than rye or rye-vetch cover crops due to rapid N-mineralization in conjunction with denitrification and leaching, prior to signif- icant crop N-assimilation. Results were compared with denitrification incubations conducted with intact soil cores obtained from vetch, rye, and fallow field plots 7, 14, and 49 days after cover crop kill. Results were generally similar, although not directly comparable because of lower WFPS (62-66%) and fertilization.