NITRATE LEACHING AS INFLUENCED BY COVER CROPPING AND NITROGEN-SOURCE

To minimize groundwater contamination by NO3- and to maximize efficient crop N use, it is necessary to better characterize NO3- leaching in various cropping systems. Our objectives were to compare the contributions of hairy vetch (Vicia villosa Roth) and NH: N sources to NO3- leaching in corn (Zen mays L.) production, and to compare NO3- leaching losses in a corn production system utilizing hairy vetch as a N source and winter cover crop with leaching losses in one utilizing an NH4+ N source and a rye (Secale cereale L.) cover crop. Nitrogen, sometimes enriched with N-15, was applied to corn grown on a Maury silt loam (fine, mixed, mesic Typic Paleudalf) in lysimeters as vetch-or NH4+-N at rates;between 10.5 and 14.0 g N m(-2) yr(-1). Treatments (N source/cover crop) were: NH4+-N/fallow, vetch-N/fallow, NH4+-N/ rye, and vetch-N/vetch; mean 3-yr cumulative NO3- leaching losses were 6.11, 4.85, 0.35, and 2.51 g N m-l, respectively. Total recovery of added N-15 in leachate was <5% for all treatments, and the percentage of leached NO3- derived from labeled N sources was generally <15%. In the winter-fallow lysimeters, effects of N source on the fraction of water input discharged, concentration of NO3-, and quantity of NO3- leached varied with year and time of year; differences in N-source availability and mulch effects were sometimes indicated. Generally, cover crop effects were larger than N-source effects. The NH4+ N source/rye cover system leached consistently less NO3- than the vetch N source/vetch cover system, even though the fraction of water discharged was not consistently different.