Contributions of planting legume cover crops after winter wheat to reduction of soil residual mineral N in soil profile and to the yields of consequent crop corn in Southern Ontario, Canada

The fallow period after winter wheat harvest in the humid temperate southern Ontario, Canada, bears a high risk for N leaching out of the crop root zone into the water systems, eventually into the Great Lakes in the region. Cover crops have the potential to reduce N losses and increase cropping system resiliency by providing nutrients to crops and adding organic residues to the soils. This study reports a 3-yr cover crop study including Crimson clover (CC), hairy vetch (HV), red clover (RC), and control. The objectives are to reports: (1) what is the reduction of residual mineral N in soil profile in cover crop fields before winter; (2) how much N can be fixed; (3) what is the contribution of cover crops to the consequent corn yields. The contents of soil mineral N was around 75-80 kg N ha-1 in 0-90 cm after winter wheat harvest for all treatments (including CK). After about 3 month cover crop growth (August – mid-November), the contents of residual soil mineral N diverged (0-90 cm) , with about a 10 kg N ha-1 decrease in the cover crop treatments versus a significant increase (65 kg N ha-1) in no cover crop control treatment. Before freeze-up, the amounts of total N in above ground biomass were 175, 146, and 107 kg ha-1, respectively, in CC, HV and RC treatments and 9.1, 16.9, and 24.8 kg ha-1 in roots for corresponding treatments. The N contents in plant above ground biomass changed to 87, 241 and 207 kg ha-1 in CC, HV, and RC plots before corn planting in following spring. Over winter HV and RC gained substantial amount of N while CC lost N. The corn grain yields were 14363 kg ha-1 for the fertilized conventional control (CK) and only 6566 kg ha-1 for the organic control (CKO). In comparison, the corn grain yields were 10233 kg ha-1 for the CC, 12234 kg ha-1 for the HV and 12315 kg ha-1 for the RC treatments, respectively.