Managing Over-Winter N Losses From Green Manures by Altering Residue Incorporation Timing.

The loss of N from organic residues can be substantial, yet management alternatives to mineral fertilizers in grain production systems are needed. Synchronizing N supply with plant uptake could be better controlled in organic cropping systems by altering the timing of green manure incorporation. Spring incorporation can slow the soil N supply in areas with short growing seasons, leaving an excess of residual NO₃^- not taken up by the successive crop. Similarly, mineralization and nitrification can be substantial following early fall incorporation and newly nitrified NO₃^- has a similar risk for over-winter N loss. As alternatives, red clover incorporation could be performed in fall instead of spring, or fall incorporation could be delayed as late as possible. The objective of this study is to assess fall NO₃^- from red clover incorporation treatments in response to soil conditions and the implications for environmental N loss as leached NO₃^- and N₂O emissions.

Red clover was grown for two years in an experiment established in Truro, Nova Scotia and then incorporated in September, November, or May (i.e., incorporation treatments). A fourth treatment was also incorporated in November, but above-ground red clover was removed leaving roots and residue to be incorporated. Fall residual mineral N (0-30 cm) was lowest from roots (11.21 kg ha^-1) as compared to other treatments (18.68-21.58 kg ha^-1). Monthly flow-weighted concentrations of NO₃^- in drainage water were highest in October when clover was incorporated in September (12.29 mg L^-1), while the remaining treatments were not different (6.97-9.69 mg L^-1). Cumulative loss of leached NO₃^- from October to December was 2.06 to 7.98 kg NO₃^- ha^-1, yet there was no difference among treatments. Sample collection and analysis is ongoing and further water and GHG data will be presented.