Soil Nitrate Accumulation and Corn Response to Preceding Small‐Grain Fertilization and Cover Crops

Abstract Potential benefits associated with establishing cover crops, such as reduced NO 3 leaching risk and lower fertilizer N requirements for succeeding crops, will be fully realized only when the cover crop N contribution is accurately accounted for and availability is synchronous with succeeding crop N demands. The objectives of this study were to evaluate spring soil NO 3 −N accumulation patterns and N availability to corn ( Zea mays L.) following annual ryegrass ( Lolium multiflorum L.), oilseed radish [ Raphanus sativus (L.) var. oleiferus (Stokes) Metzg.], red clover ( Trifolium pratense L.), and no cover crop established after either winter wheat ( Triticum aestivum L.) or barley ( Hordeum vulgare L.). The wheat and barley were produced with 0.5, 1.0, and 2.0 times the amount of recommended N fertilizer. Six field trials were conducted on well‐drained Typic Hapludalf soils in southwestern Ontario intermittently from 1989 to 1995. Corn was produced using a spring mulch‐till system with only 10 kg ha −1 of fertilizer N, which was applied as part of the P starter fertilizer. Applying more fertilizer N to the previous year's small‐grain crop rarely increased spring soil NO 3 −N concentrations or corn yields. Soil NO 3 −N concentration increases between the May and June sample dates following annual ryegrass and oilseed radish did not differ substantially from where a cover crop had not been established; following red clover, however, NO 3 −N increases were always at least 2.8 times greater than after no cover crop. Average aboveground corn biomass N at anthesis following annual ryegrass was 25.6 kg ha −1 less than when no cover crop was grown, whereas following red clover it was 40.4 kg ha −1 greater than with no cover crop. Corn yields were consistently the highest following red clover and often the lowest following annual ryegrass; yield response was positively correlated with June soil NO 3 −N concentrations ( r = 0.61‐0.93). These results suggest that N availability to succeeding corn differs among the cover crop treatments evaluated in the order red clover > oilseed radish ≥ no cover crop > annual ryegrass.