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

Potential benefits associated with establishing cover crops, such as reduced NO₃ 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 NO₃−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 NO₃−N concentrations or corn yields. Soil NO₃−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, NO₃−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 NO₃−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. Research supported by Agriculture and Agri-Food Canada and Ontario Ministry of Agriculture, Food and Rural Affairs.