N2O emissions from residues of oat and grass pea cover crops cultivated in the US Southern Great Plains

Authors: SINGH, HARDEEP - Oklahoma State University; KANDEL, TANKA - Oklahoma State University; Gowda, Prasanna; Northup, Brian; KAKANI, VIJAYA - Oklahoma State University

Submitted to: Frontiers in Sustainable Food Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/31/2020
Publication Date: 2/10/2021
Citation: Singh, H., Kandel, T.P., Gowda, P.H., Northup, B.K., Kakani, V.G. 2021. N2O emissions from residues of oat and grass pea cover crops cultivated in the US Southern Great Plains. Frontiers in Sustainable Food Systems. 4. Artile 604934. https://doi.org/10.3389/fsufs.2020.604934.
DOI: https://doi.org/10.3389/fsufs.2020.604934

Interpretive Summary: Producers in the U.S. Southern Great Plains are showing increasing interest in in recent years on including cover crops in their production systems. Cover crops can provide many benefits to agriculture and the environment, including; reducing soil erosion and improving soil condition, suppressing weeds, and increasing the pools of carbon, nitrogen (N), and organic matter in soils. However, cover crops can also result in emissions of carbon dioxide and nitrous oxide (N2O) as soil microbes decompose their residues. Nitrous oxide is a potent greenhouse gas, and emissions of N2O from decomposing plant residues may offset any potential benefits of cover crops. These emissions largely depend on type of cover used, and the form of management applied to the residues. In this study, we compared the N2O emissions that are produced by the decomposing residues of two spring-grown cover crops; oat (a cereal grass) and grass pea (a legume) that were incorporated into soil. We also compared differences between incorporating biomass of grass pea and removing biomass as a hay crop, and included an untreated control for comparisons. We also measured the amount of N transferred to a following forage crop (crabgrass), to describe fertilizer values of the residues. We found the total amount of N2O emissions produced on the cultivated oat plots was 66% greater than in the cultivated plots of grass pea. Removal of aboveground biomass of grass pea reduced N2O emissions by 63%, compared to the emissions from cultivated plots of grasspea. Biomass yield of following crop (crabgrass) in response to cover crops was reduced by 47 to 56% compared to the 8.2 ton/acre yield generated by the control treatment, though N concentrations were 50 to 75% lower. These results show that higher yield cover crops, including cereal grasses, can lead to greater N2O emissions after soil incorporation, and harvesting aboveground biomass of legume-based cover crops for forage could help limit N2O emissions.

Technical Abstract: Grass pea (Lathyrus sphaericus) and oat (Avena sativa) are potential cover crops in lieu of spring fallow periods within summer crop systems in the US Southern Great Plains (SGP). The main objective of this study was to compare nitrous oxide (N2O) emissions from residues of a legume (grass pea) and a cereal (oat) cover crops. The comparisons included oat and grass pea cultivated solely as cover crops where all biomass was terminated by tillage at flowering (18 May), removal of grass pea biomass for forage use, and a control with plots fallowed during the spring (March to May). Crabgrass (Digitaria sanguinalis) was cultivated as a summer hay crop immediately after termination of the cover crops. Fluxes of N2O were measured with a closed chamber connected to a portable gas analyzer on 23 dates during 3-month growth period of crabgrass. At termination, oat produced more aboveground biomass than grass pea (3.56 vs. 2.17 Mg/ha) but total N in biomass was similar (102-104 kg N/ha) as nitrogen concentrations in grass pea was greater than in oat (4.80 vs. 2.86% of dry mass). Three-month cumulative emissions of N2O from grass pea incorporated plots (0.76 ± 0.11 kg N2O-N/ha; mean ± standard error, n=3) were significantly lower (P < 0.05) than from oat incorporated plots (1.26 ± 0.14 kg N2O-N/ha). Emissions from grass pea harvested plots (0.48 ± 0.04 kg N2O-N/ha) were significantly lower (P < 0.05) than grass pea incorporated plots. Yields produced by crabgrass were similar (P > 0.05) for oat incorporated, grass pea incorporated, and grass pea harvested plots (8.65-10.46 Mg/ha), but yield responses to the control (18.53 Mg/ha) were significantly greater. Nitrogen concentration in crabgrass biomass was greater from oat and grass pea incorporated plots (2.86-2.87 %) than grass pea harvested (1.93 %) and control (1.44 %) plots. In conclusion, the results indicated that greater biomass yields by oat-based cover crops could lead to greater N2O emissions after soil incorporation, and removal of aboveground biomass of grass pea indicated harvesting legume-based cover crops for forage could mitigate N2O emissions.