The aim of this study was to compare the effects of cowpea green manure and inorganic nitrogen (N) fertilizers on yields of winter wheat and soil emissions of nitrous oxide (N2O). The comparisons included cowpea grown solely as green manure where all biomass was terminated at maturity by tillage, summer fallow treatments with 90 kg N ha−1 as urea (90-N), and no fertilization (control) at planting of winter wheat. Fluxes of N2O were measured by closed chamber methods after soil incorporation of cowpea in autumn (October–November) and harvesting of winter wheat in summer (June–August). Growth and yields of winter wheat and N concentrations in grain and straw were also measured. Cowpea produced 9.5 Mg ha−1 shoot biomass with 253 kg N ha−1 at termination. Although soil moisture was favorable for denitrification after soil incorporation of cowpea biomass, low concentrations of soil mineral N restricted emissions of N2O from cowpea treatment. However, increased concentrations of soil mineral N and large rainfall-induced emissions were recorded from the cowpea treatment during summer. Growth of winter wheat, yield, and grain N concentrations were lowest in response to cowpea treatment and highest in 90-N treatment. In conclusion, late terminated cowpea may reduce yield of winter wheat and increase emissions of N2O outside of wheat growing seasons due to poor synchronization of N mineralization from cowpea biomass with N-demand of winter wheat.

The combination of several measures (organic matter accumulated, soil porosity, field soil respiration, CO2 evolved during incubation, and mineral N released) shows clearly that soil biological activity was enhanced in the systems that minimized tillage. Surprisingly, the abundance of earthworms, which are considered very sensitive to damage by tillage, did not always follow the same pattern. We do not know the reason for the response of earthworms, but in part it may have been an artifact of the interaction of the collection technique with soil properties such as infiltration rate and temperature. The most dramatic improvement in porosity, organic matter accumulation and N mineralization ability came from five years' continuous growth of grass sod, underscoring the potential role of grass in sustainable cropping systems and the value of grass for soil conservation and improvement.