A 3-year assessment of nitrous oxide emission factors for urine and dung of grazing sheep in a subtropical ecosystem

Abstract

PURPOSE: Grazing livestock has strong impact on global nitrous oxide (N₂O) emissions by providing N sources through excreta. The scarcity of information on factors influencing N₂O emissions from sheep excreta in subtropical ecosystems such as those of Southern Brazil led us to conduct field trials in three different winter pasture seasons on an integrated crop–livestock system (ICL) in order to assess N₂O emission factors (EF-N₂O) in response to variable rates of urine and dung. MATERIALS AND METHODS: The equivalent urine-N loading rates for the three winter seasons (2009, 2010, and 2013) ranged from 96 to 478 kg ha⁻¹, and the dung-N rates applied in 2009 and 2010 were 81 and 76 kg ha⁻¹, respectively. Air was sampled from closed static chambers (0.20 m in diameter) for approximately 40 days after excreta application and analyzed for N₂O by gas chromatography. RESULTS AND DISCUSSION: Soil N₂O-N fluxes spanned the ranges 4 to 353 μg m⁻² h⁻¹ in 2009, − 47 to 976 μg m⁻² h⁻¹ in 2010, and 46 to 339 μg m⁻² h⁻¹ in 2013. Urine addition resulted in N₂O-N peaks within for up to 20–30 days after application in the 3 years, and the strength of the peaks was linearly related to the N rate used. Emission factors of N₂O (EF-N₂O, % of N applied that is emitted as N₂O) of urine ranged from 0.06 to 0.34% and were essentially independent of N rate applied. By considering a ratio of N excreted by urine and dung of 60:40, a single combined excretal EF-N₂O of 0.14% was estimated. CONCLUSIONS: Our findings showed higher mean EF-N₂O for sheep urine than that for dung (0.21% vs 0.03%), irrespective of the occurrence or not of urine patches overlap. This value is much lower than default value of 1% of IPCC’s Tier 1 and reinforces the needs of its revision.