Nitrous oxide emissions as influenced by legume cover crops and nitrogen fertilization
In this study, we measured nitrous oxide (N2O) fluxes from plots of fall-planted hairy vetch (HV, Vicia villosa) and spring-planted broadleaf vetch (BLV, Vicia narbonensis) grown as nitrogen (N) sources for following summer forage crabgrass (Digitaria sanguinalis). Comparisons also included 60 kg ha⁻¹ inorganic N fertilizer for crabgrass at planting (60-N) and a control without N fertilizer. Each treatment had six replicated plots across the slope. Fluxes were measured with closed chamber systems during the period between spring growth of cover crops and first-cut of crabgrass in mid-July. HV had strong stand and aboveground biomass had 185 ± 50 kg N ha⁻¹ (mean ± standard error, n = 6) at termination. However, BLV did not establish well and aboveground biomass had only 35 ± 15 kg N ha⁻¹. Ratio vegetation index of crabgrass measured as proxy of biomass growth was highest in HV treatment. However, total aboveground biomass of crabgrass was statistically similar to 60-N plots. Fluxes of N2O were low prior to termination of cover crops but were as high as 8.2 kg N2O ha⁻¹ day⁻¹ from HV plots after termination. The fluxes were enhanced by large rainfall events recorded after biomass incorporation. Rainfall enhanced N2O fluxes were also observed in other treatments, but their magnitudes were much smaller. The high N2O fluxes from HV plots contributed to emissions of 30.3 ± 12.4 kg N2O ha⁻¹ within 30 days of biomass incorporation. Emissions were only 2.0 ± 0.7, 3.4 ± 1.3 and 1.0 ± 0.4 kg N2O ha⁻¹ from BLV, 60-N and control plots, respectively.
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