Nitrous oxide emissions as influenced by legume cover crops and nitrogen fertilization

Authors: KANDEL, TANKA - Oklahoma State University; Gowda, Prasanna; SOMENAHALLY, ANIL - Texas A&M Agrilife; Northup, Brian; Dupont, Jesse; ROCATELI, ALEXANDRE - Oklahoma State University

Submitted to: Nutrient Cycling in Agroecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/4/2018
Publication Date: 7/21/2018
Citation: Kandel, T.P., Gowda, P.H., Somenahally, A., Northup, B.K., Dupont, J.I., Rocateli, A.C. 2018. Nitrous oxide emissions as influenced by legume cover crops and nitrogen fertilization. Nutrient Cycling in Agroecosystems. 112(1): 119-131. https://doi.org/10.1007/s10705-018-9936-4(0123456789().,-volV)(0123456789().,-volV).
DOI: https://doi.org/10.1007/s10705-018-9936-4

Interpretive Summary: Agriculture is a major contributor for emissions of nitrous oxide (N2O) which is 265 times more potent than carbon dioxide (CO2) as a greenhouse gas (GHG). Legumes cultivated as sources of green nitrogen normally have low Carbon:Nitrogen ratios and high mineralization rates which are conducive factors to increase N2O emissions. In this study, we measured N2O and CO2 fluxes from fall planted legume hairy vetch and spring planted legume broadleaf vetch cultivated as N sources for crabgrass which is an annual grass cropped as a summer forage in low-input agronomic systems of the Southern Great Plains. Comparisons also included 60 kg ha-1 inorganic nitrogen fertilizer supplied as dry urea and control treatment without cover crops and nitrogen fertilization. Cumulative N2O fluxes from hairy vetch treatment plots (30.3 ± 12.4 kg N2O ha-1) within 30 days of biomass incorporation were significantly higher than other treatment plots (2.0 ± 0.7, 3.4 ± 1.3 and 1.0 ± 0.4 kg N2O ha-1 from broadleaf vetch, 60 kg N/ha and 0 kg N/ha, respectively). Thus, results indicate that highly productive legumes may fix enough nitrogen to meet the requirements of following recipient crops, but they may also increase the emissions of N2O dramatically. Future studies are required to reduce N2O emissions after soil incorporation of leguminous cover crops. Some of the management options include but not limited to removing aboveground biomass for use as forage or other economic purposes and application of nitrification inhibitors prior to incorporation of biomass into the soil.

Technical Abstract: Emissions of nitrous oxide (N2O) from agricultural soils is mostly contributed by soil nitrogen (N) inputs from application of inorganic fertilizers or legume cover crops cultivated as green N sources. In this study, we measured N2O and carbon dioxide (CO2) fluxes from plots (n = 6 in each treatment) of fall-planted hairy vetch (HV, Vicia villosa) and spring-planted broadleaf vetch (BLV, Vicia narbonensis) grown as N sources for following crabgrass (Digitaria sanguinalis) planted as a summer forage. Comparisons included 60 kg ha-1 inorganic N fertilizer supplied as dry urea for crabgrass at planting and a control plot with no N fertilizer. Measurements of gas fluxes were taken with closed chamber systems at two-week intervals until the incorporation of HV biomass in early-May but more frequently (mostly daily) thereafter until the N2O fluxes approximated zero in mid-June. Auxiliary data such as concentrations of soil mineral N (NH4+ and NO3-), crop growth and biomass yield, soil temperature and moisture, and copy numbers of denitrifier genes were also measured. Concentrations of soil NH4+ were higher on HV plots but soil NO3- concentration were higher on 60-N plots after termination of cover crops and fertilization of 60-N plots. Crabgrass grew better on HV plots as evidenced by higher RVI (ratio vegetation index) measured as canopy crop reflectance, but total aboveground biomass at first cut 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 m-2 day-1 from HV plots after termination. The fluxes were also increased by large rainfall events recorded after biomass incorporation. Rainfall enhanced N2O fluxes were also observed in other N treatments, but their magnitudes were smaller. Fluxes of CO2 were also high from HV plots and followed similar dynamics and spatial variations as N2O fluxes. The high N2O fluxes from HV plots contributed to losses of 30.3 ± 12.4 kg N2O ha-1 within 30 days of biomass incorporation. Losses were only 2.0 ± 0.7, 3.4±1.3 and 1.0 ± 0.4 kg N2O ha-1 from BLV, 60-N and 0-N plots, respectively. In conclusion, although HV fixed large amounts of N for the following recipient crop, it also contributed to high emissions of N2O.