Impact of a forage legume or nitrogen fertilizer application on ammonia volatilization and nitrous oxide emissions in Brachiaria pastures

HIGHLIGHTS

NH₃ volatilization losses and N₂O emissions from urine were higher than from dung

NH₃ loss from cattle urine on N-fertilized grass was greater than on mixed pasture

Grass-legume pasture showed total N₂O emissions lower than N-fertilized pasture

NH₃ and N₂O emission from excreta were similar to or below the IPCC default values

Soil N₂O emission factor of N fertilization was under half the IPCC default of 1 %.

ABSTRACT

The largest proportion of greenhouse gas (GHG) emissions in the Agriculture sector of the Brazilian national GHG inventory is derived from the large (>200 million head) herd of cattle. The greatest contribution to these emissions comes from the enteric methane from cattle, but the direct and indirect emissions of nitrous oxide (N₂O) from cattle excreta and N fertilizer are responsible for approximately 9 % of all national anthropogenic GHG emissions. Ammonia (NH₃) can be volatilized from N fertilizer and cattle excreta and deposited in sites remote from the source, constituting an indirect source of N₂O. This study aimed to determine whether direct N₂O emissions and NH₃ volatilization from N-fertilized pastures were greater than those derived from a mixed grass-legume pasture without N fertilizer addition. Emissions of N₂O and NH₃ from excreta and N fertilizer from a Palisade grass (Urochloa brizantha cv. Marandu) monoculture fertilized with 2 × 60 kg N ha^-1 yr^-1 urea were compared to those from a mixed Palisade grass-forage peanut (Arachis pintoi) pasture. Dung and urine were collected from these cattle, and NH₃ losses and N₂O emissions from the excreta and from N fertilizer were monitored using static chamber techniques. Volatilization of NH₃ and N₂O emissions were found to be greater from urine than from dung. Ammonia losses from excreta and urea fertilizer were low, not exceeding 6.8, 1.1, and 4.7 % of the N applied as urine, dung, and fertilizer, respectively. The N₂O emissions showed a tendency to be greater for the urine from the N-fertilized compared to the mixed grass-legume pasture, and the N₂O emissions from the urine of the N-fertilized pasture ranged from 0.08 to 0.94 % of applied urine N. The N₂O emission from the N fertilizer was at maximum 0.46 % of the applied N. The direct N₂O emissions and the loss of NH₃ by volatilization (indirect N₂O emission) from the excreta of cattle grazing the mixed grass-legume pasture were similar to, or lower than, the grazed grass monoculture fertilized with 120 kg N ha^-1 yr^-1. As the mixed pasture received no N fertilizer and hence no GHG emission from its manufacture or application, introducing forage peanut to the Urochloa brizantha pastures shows potential to be responsible for lower GHG emissions than the N fertilized grass pasture.

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