Effect of Legume-Rich and N-Fertilized Grass Diets on Greenhouse Gas Emissions from Animal Excreta.

Livestock production in the southeastern USA is grass-based and nearly always N limited. Nitrogen fertilizer can supply this need, but significant greenhouse gas (GHG) emissions are associated with fertilizer production and use. Legumes are an alternative N source that can decrease enteric methane (CH₄) emissions. However, little is known about their effect on nitrous oxide (N₂O) and CH₄ emissions from animal excreta. Because N₂O is 10 times more potent as a GHG than CH₄, the observed decrease in enteric CH₄ with inclusion of legumes in animal diets may be irrelevant if N₂O emissions from animal excreta increase. The objective of this study was to quantify GHG emissions from dung and urine of animals grazing legume-rich or N-fertilized grass diets. Pastures were ‘Pensacola’ bahiagrass (BG; Paspalum notatum Flügge) fertilized with 50 kg N ha^-1 (BG + N) and a 50-50 percent-by-weight mixture of ‘Florigraze’ rhizoma peanut (RP; Arachis glabrata Benth.) and BG (50RP). Source pastures (diets) were replicated three times and grazed for 1 wk prior to dung and urine collection. Dung and urine samples were composited across replicates within an excreta type and source treatment, resulting in four source pasture by excreta type combinations. Dung and urine were applied to the base of previously installed static chambers, with four replicates per treatment. Four gas samples were collected at 12-min intervals per deployment event. Deployment events occurred daily in the first week and three times a week thereafter. There was an excreta x diet interaction effect on N₂O emissions, with lower emissions from urine of animals grazing 50RP. Variation in emissions was closely related to soil water content. Preliminary results indicate that inclusion of legumes in animal diets may reduce total GHG emissions compared with N-fertilized grass systems.