Nitrogen management for organic crops using winter-hardy legumes in southern Ontario, Canada.

Soybean-winter_wheat-corn rotation is common in humid southern Ontario, Canada. There is a fallow period after wheat harvest (late July to November) and before the consequent corn (late March to early May). This bears a high risk for nitrate leaching from the crop root zone into the water systems, eventually into the Great Lakes in the region. Cover crops have the potential to scavenge nutrient losses (N and others) and increase cropping system resiliency by providing nutrients to crops and adding organic residues to the soil. The objectives of this study were to determine: (1) how much N can be scavenged in legume cover crop biomass (shoots and roots); (2) what is the reduction of residual mineral N (nitrate and ammonium) in soil profiles of cover crop plots versus non cover crop control plots in early November (crops stop growing); (3) what is the contribution of cover crops to the yield of consequent crop corn.
Method: The wheat stubble was disc-tilled, and the cover crops were planted into the soil in an organic managed soybean-winter_wheat-corn rotation system. The cover crop treatments included Crimson clover (CC), hairy vetch (HV), red clover (RC), sasbenia (S), a non cover crop conventional control (CK) and a non cover crop organic control (CKO). This 3 year study (2013-2015) was arranged in a randomized complete block design with 4 field replicates on Harrow sandy loam soil. No synthetic fertilizer was added to the cover crop plots in either cover crop phase or soybean, winter wheat and corn phase except for the CK plots which N, P, K were added following conventional management protocol. Soil cores (0-90 cm) were collected before planting cover crops and a second set of soil cores were collected again in early to mid-November (before freeze-up). The soil core samples were separated in 0-5, 5-10, 10-20, 20-30, 30-50, 50-70, and 70-90 cm segments and used to determine the amounts soil mineral N (nitrate and ammonium) remaining in the soil profiles. Cover crop biomass samples were collected in the same time as the second set of soil cores was collected. Cover crop biomass was collected again in following year at early May before plowing cover crop field for corn planting. Grain corn fields were organically managed, weeds were controlled using rotary tiller, and corn grain yields were recorded. Total nitrogen in cover crop plant biomass was determined using the dry combustion method with a LECO CN2000 Analyser (Leco Corp., St Joseph, MI, USA). The concentrations of soil inorganic nitrogen (nitrate and ammonium) were measured using the Berthelot reaction method (NH4+) and the Cd reduction method (NO3-) on a TRAACS 2000 (Bran + Luebbe Analyzing Technologies, Buffalo Grove, IL) auto-analyzer.
Results: Sasbenia growth and stands were considerably poor which stopped growing at mid-October with the lowest total biomass N accumulation (41 kg N ha-1) compared with other cover crop treatments. Before freeze-up, the amounts of total N in above ground biomass were 155, 166, and 105 kg ha-1, respectively, in CC, HV and RC treatments and 8.0, 13.2, and 28.8 kg ha-1 in roots for corresponding treatments. The amounts of N (in weeds) were only 4 and 37 kg ha-1 in the CKC and CKO, respectively. 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 (cover crops in 2014 only, cover crop biomass samples for 2015 were collected in May and are underway for analysis). It indicates that the HV and RC gained more N but CC lost N over winter. Compared with the HV and RC, the CC was less winter hardy.
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 re-growth, the contents of residual soil mineral N diverged, with about a 10 kg N ha-1 decrease in the cover crop treatments versus a significant increase (65 kg N ha-1) in the non cover crop control treatment. Based on our model predictions (the CANB-N model and the IROWC-N model, Drury et al. 2016 a,b) that a third to a half residual soil mineral N could be leaching out of the soil profile into the water system in the non-growing seasons, growing cover crops, particularly the HV and RC, in the fallow period can significantly reduce the risk of nitrogen loss.
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. Without the use of chemical N fertilizer, growing CC in the fallow seasons after winter wheat attained 70% of the corn grain yield of the CK and growing HV and RC achieved 85% of the corn grain yield in the CK which received 150 kg N ha-1.
Conclusion: The use of legume winter cover crop after winter wheat harvest has significant contributions to the agro-ecosystems and to the corn production in the region. The HV and RC performed better than the CC in term of N benefits to corn grain yield. The other significance of planting legume cover crops is to reduce soil nitrogen loss during non-growing seasons.