Nitrous oxide emission factors for urine and dung patches in a subtropical Brazilian pastureland

Abstract

Cattle urine and dung (faeces) patches are nitrous oxide (N2O) sources in pasturelands with impacts in the global N2O budget, but specific information about those emissions are still missing for Brazilian subtropical and tropical regions. We conducted a sequence of 3 field-trials (summer, winter and spring, 90 days each) to evaluate the N2O emission and emission factor (EF) after the deposition of 3 volumes of cattle urine or 3 weights of dung (½, 1 and 1½ time the mean urination volume or defecation weight of Friesian cows) on a free-drained Cambisol of a subtropical pastureland of Brazil. The N2O emission peaks (3198μgN2O-Nm−2h−1 after urine in summer was the highest) occurred on average 17±9 days after application (DAA), both for urine and dung, and dropped to the background levels 41±10 DAA of urine and 49±10 DAA of dung. The highest contents of NH4+-N in soil (200–250mgNkg−1) occurred one day after urine application and 10–14 days later for dung (100–200mgNkg−1). Nitrate peaks occurred from 23 to 26 DAA in urine patches (∼40–50mgNkg−1) and 19–50 DAA in dung patches (∼40–50mgNkg−1). The N2O emission peaks for urine coincided with soil NH4+-N peak in winter but with soil NO3−-N peak in spring. For dung, the emission peak seemed to be more associated with soil NO3−-N than to NH4+-N, either in winter or spring (inorganic-N was not assessed in summer). It was not possible to conclude whether nitrification or denitrification was the dominant process in N2O production, but it seemed that both played relevant roles. The EF for urine, averaged across the seasons, diminished with increments in urine volume, from 0.33% in ½ volume to 0.19% in 1½ volume, possibly because urine percolated deeper into the soil and proportionally less N remained available for N2O production in the top layer. The EF for dung was 0.19%, 0.12% and 0.14% for ½, 1 and 1½ weight, respectively, showing no clear trend with increment in dung weight. The lowest EFs for urine and dung occurred in winter, possibly because of lowest temperatures and soil water-filled pore space. The average EF for dung (0.15%) was lower than that of urine (0.26%), because urea-N of urine is more readily available for the hydrolysis than organic N forms of dung. This result suggests that these two excreta should be addressed separately in national greenhouse gases inventories or communications. Our results suggest that the default 2% EF proposed in IPCC Guidelines for cattle excreta are overestimated for subtropical Brazil.