CONSERVING RESIDUAL CORN FERTILIZER NITROGEN WITH WINTER COVER CROPS

Autumn residual fertilizer nitrogen (FN) can be easily leached into groundwater in humid climates. Winter cover crops were evaluated for their ability to assimilate residual corn FN and thereby reduce N losses. Labelled FN (N-15 depleted) was applied to corn in Maryland in 1986 and 1987 at rates of 0, 168, and 336 kg FN ha-1 on a Mattapex silt loam (fine-loamy, mixed, typic Hapludult). Cover crop treatments following corn harvest were hairy vetch (Vicia villosa Roth), crimson clover (Trifolium incarnatum L.), cereal rye (Secale cereale L.), or annual ryegrass (Lolium multiflorum Lam.), and a weed/fallow control of chickweed (Stellaria media L.). The covers were harvested three times the following spring and dry matter yields (DM), %N, and atom % N-15 were determined to assess FN uptake. Fall labelled N in the soil (to 80 cm) averaged 17 and 114 kg FN ha-1 over both years for the 168 and 336 kg FN ha-1 rates, respectively. However, the quantity of total residual mineral N (soil N plus FN) after the 168 kg ha-1 rate was 87 kg N ha-1, which was comparable to the quantity of labelled N at the high fertilizer rate. The average cover crop FN uptake (kg FN ha-1) in mid-April after the 336 kg N ha-1 treatment was 48 for cereal rye, 29 for annual ryegrass, 9 for hairy vetch, 8 for crimson clover, and 6 kg FN ha-1 for the native weed cover (LSD P = 0.05 of 7 kg FN ha-1). Corresponding percent recoveries of the fall N in the aboveground DM were 45% for cereal rye, 27% for annual ryegrass, 10% for hairy vetch, 8% for crimson clover, and 8% for native weed cover. These results show that grass cover crops conserved the most FN. Cereal rye recovered more FN through mid-April because of its growth in cool weather, although annual ryegrass was equally effective if grown to mid-May. Renewed efforts should be made to utilize grass cover crops to conserve N in humid climates.