Effect of Legume-Rich and N-Fertilized Grass Diets on Nitrous Oxide and Methane Emissions from Bovine Excreta.

Nitrogen is often the most limiting nutrient in grasslands. Fertilizer is a common source of N, but greenhouse gas (GHG) emissions occur due to its production and application. Inclusion of legumes may provide GHG-neutral N to grasslands while reducing enteric fermentation methane (CH₄) emissions and improving forage nutritive value. However, little is known about the effect of legumes on nitrous oxide (N₂O) and CH₄ emissions from animal excreta. This knowledge gap imposes challenges to the use of models for evaluating the effects of management strategies on GHG emissions. The objectives of this study were to quantify the effects of inclusion of legumes in pastures on CH₄ emissions from dung and N₂O emissions from urine and dung and to determine if emissions from dung and urine should be accounted for separately in model estimates. 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 collected separately and composited across replicates within an excreta type and source treatment, resulting in four source pasture by excreta type combinations. GHG emissions were different according to excreta type. Greater CH₄ emissions were found in dung for animals grazing BG + N (140 g CH₄ head^-1 yr^-1) than 50RP (95 g CH₄ head^-1 year^-1). CH₄ emissions reached background levels 8 d after dung deposition. Emission of nitrous oxide was affected by excreta type but not diet, with negative N₂O emissions for urine. These results support disaggregation of emission factors for urine and dung. Results indicate the adoption of forage legumes in warm climates may reduce GHG emissions from livestock excreta.