Nitrous oxide emission factors from cattle urine and dung, and dicyandiamide (DCD) as a mitigation strategy in subtropical pastures

The Brazilian cattle herd (>200 million) contributes, via excreta, 41% of the national emission of nitrous oxide (N₂O), but little country-specific information exists on related emission factors and mitigation options. The aims of this study were: (i) to determine the N₂O emission factors (EF) for dairy cattle urine and dung in a subtropical pasture and compare them with the 2% EF adopted by the IPCC; (ii) to evaluate the efficacy of the nitrification inhibitor dicyandiamide (DCD) to reduce those N₂O emissions; and (iii) to determine if the best mode of DCD application is dissolved into or sprayed on urine and dung. An experiment was conducted in a kikuyu grass (Pennisetum clandestinum) pasture over a haplic Cambisol, in southern Brazil. Fresh urine and dung from Friesian dairy cows were applied with or without application of DCD; DCD was dissolved into the excreta before its application onto the soil, or was sprayed on the excreta patch. Treatments were applied four times, once per each season of 2014 (in different spots, same area). After excreta application, N₂O fluxes were monitored in 10 air sampling sessions over 63–68 days, by using closed static chambers. Urine and dung were important sources of N₂O, with an average EF of 0.34% for urine and 0.11% for dung; this urges the investigation of related mitigation strategies for Brazilian cattle systems. Yet, those emission factors were not as high as the IPCC default value of 2%, which suggests that this default value may need to be revised for the subtropical region. Additionally, this revision should consider the disaggregation of the EF for urine and dung. As a mitigation strategy, the use of DCD had a potential to curb N₂O emission from urine patches, particularly in the cooler seasons of autumn and winter, when it reduced emissions by 60–82% after being dissolved in urine. DCD sprayed on excreta patches significantly reduced emissions in autumn only (by 47%) and thus was less efficient than when dissolved in urine. With respect to dung, there was no clear evidence that N₂O emission from that excreta is mitigated by DCD, either dissolved into the dung mass or sprayed over the dung pat.––