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Title: Winter wheat yield and nitrous oxide emissions in response to cowpea-based green manure and nitrogen fertilization

Author
item KANDEL, TANKA - Oklahoma State University
item Gowda, Prasanna
item Northup, Brian
item ROCATELI, ALEXANDRE - Oklahoma State University

Submitted to: Experimental Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2019
Publication Date: 10/11/2019
Citation: Kandel, T.P., Gowda, P.H., Northup, B.K., Rocateli, A.C. 2019. Winter wheat yield and nitrous oxide emissions in response to cowpea-based green manure and nitrogen fertilization. Experimental Agriculture. 56(2):239-254. https://doi.org/10.1017/S0014479719000334.
DOI: https://doi.org/10.1017/S0014479719000334

Interpretive Summary: Winter wheat is the major grain crop in the U.S. Southern Great Plains, and is mostly cultivated as a monoculture. Although inorganic nitrogen (N) fertilizers are widely applied to winter wheat in the region, interest for cultivation of summer legumes as green N manure is increasing. A well-established cowpea crop can provide enough N to meet the requirements for winter wheat in the SGP. However, to be an effective green manure, the N in cowpea biomass should be transferred effectively to winter wheat which largely depends of synchronization of N mineralization from soil incorporated cowpea biomass with N-demand of winter wheat. The synchronization is also important to mitigate emissions of N2O, a highly potent greenhouse gas, from N-rich legume biomass. In this study, we tested cowpea terminated at maturity as a green N source for a continuous rotation of winter wheat in central Oklahoma. The comparisons included summer fallow treatments with (90 kg N ha-1 as urea) or without (control) fertilization at planting of winter wheat. Growth, yield and grain N concentrations of winter wheat was significantly lower in cowpea treated plots compared to control or fertilized plots. Emissions of N2O was low from all treatments at planting of winter wheat but high from cowpea treatment after harvest of winter wheat. In conclusion, the results suggested that late terminated cowpea may not be an effective source of N for winter wheat due to lack of synchronization of N mineralization from soil incorporated cowpea biomass with N-demand of recipient winter wheat.

Technical Abstract: The aim of this study was to investigate the effects of cowpea green-manure and inorganic nitrogen (N) fertilizers on yields of winter wheat and emissions of N2O. The comparisons included cowpea terminated at maturity, summer fallow treatments with 90 kg N ha-1 as urea (90-N), and no fertilization (control) at planting of winter wheat. Fluxes of N2O were measured with closed chamber methods after soil incorporation of cowpea in autumn and harvesting of winter wheat in summer. Growth and yields of winter wheat and N concentrations in grain and straw were also measured. Cowpea produced 9.5 Mg ha-1 shoot biomass with 253 kg N ha-1 at termination. Although soil moisture was favourable for denitrification after soil incorporation, low concentrations of soil mineral N restricted emissions of N2O from cowpea treatment. Emissions from other treatments were also low at planting of winter wheat. However, increased concentrations of soil mineral N and large rainfall-induced emissions were recorded from cowpea treatment during summer. Winter wheat growth, yield and grain N concentration were lowest in cowpea treatment and highest in 90-N treatment. In conclusion, late terminated cowpea may reduce yield of winter wheat and increase emissions of N2O outside growing seasons due to poor synchronization of N mineralization from cowpea biomass with N-demand of winter wheat.