Benefits of Grass-Legume Mixture Extend to Nitrogen Gains and Alleviation of Greenhouse Gas Emissions in Wyoming.

Nitrogen (N) fixation by legumes and carbon sequestration by grass are pertinent to forage productivity, soil health, and reduced greenhouse gas emissions. These roles may be subverted by less optimal species abundance in grass-legume mixtures and variations in climate. An experiment was conducted in 2016 and 2017 at the Sheridan Research and Extension Center in Wyoming to determine N fixation and emission of nitrous oxide (N₂O) and carbon dioxide (CO₂) from alfalfa (Medicago sativa L.) monocrop, meadow bromegrass (Bromus riparius Rehm.)-alfalfa mixtures, and meadow bromegrass supplied with 0, 56, and 112 kg N ha^-1. The mixtures were 50-50% and 70-30% grass-alfalfa seeding ratios. Nitrogen fixation, soil mineral-N, and greenhouse gas fluxes were measured in May, June, August, and October each year. Alfalfa fixed more N (p < 0.05) in mixtures (74 to 89% of total plant N) compared to monocrop (55 to 66%) but transferred similar amount of N to grass (116 to 144 kg N ha^-1year^-1 ) regardless of treatment. Alfalfa maintained high N₂O emission (150 and 139 μg N m^-2 hr^-1) similar to 50-50% mixture of meadow bromegrass with alfalfa (125 and 162 μg N m^-2 hr^-1) in 2016 and 2017, respectively. During similar period, 70-30% mixture of meadow bromegrass with alfalfa had lower (p < 0.05) N₂O fluxes of 109 and 93 μg N m^-2 hr^-1 compared to alfalfa monocrop. There were poor relationships between mineral-N and N₂O flux in 2016 (R² = 0.3, p = 0.04) and 2017 (R² = 0.3, p = 0.006). There were no significant treatment effects on CO₂ fluxes (15 to 67 mg C m^-2 hr^-1), however these values were lower than those for similar crops and management in other areas. Overall, 70-30% mixture of meadow bromegrass and alfalfa increases N fixation while reduces trace gas emissions compared to alfalfa monocrop.