Nitrous oxide and ammonia emissions from beef cattle excreta in palisadegrass pastures with and without fertilizer-N or mixed with forage peanut

Abstract

Grazing pasture is a major livestock production system in Brazil, and the nitrogen excretion by cattle onto pastures has been identified as an important source of nitrous oxide (N2O). In this study we assessed for long term N2O and ammonia (NH3) emissions from cattle urine and dung deposited in the dry and rainy season on three pastures systems: i) palisadegrass {Brachiaria brizantha (Hochst. ex A. Rich.) R.D. Webster [syn. Urochloa brizantha Stapf cv. Marandu]}in monoculture without fertilizer-N application (Grass); ii) palisadegrass in monoculture with 150 kg N ha-1 (Grass+N); and iii) palisadegrass mixed with forage peanut (Arachis pintoi Krapov. & W.C. Greg cv. BRS Mandobi) without fertilizer-N application (Grass+Legume). Two trials were carried out in a tropical region of Brazil, beginning in the dry and rainy season and extending for one year. The experimental design was in randomized blocks and the treatments were arranged in a 3×3 factorial scheme: three excreta types (urine, dung and control without excreta) and three pasture systems, with three replications. In both trials the N2O peaks were associated with rainfall events and the background levels were achieved after 22-28 days after a rainfall event >20 mm day−1. The N2O emission factors (EFN2O) were greater in areas treated with urine compared than dung in all pasture systems. The urine EFN2O was lowest in the Grass system, and there was no difference between Grass+Legume and Grass+N system. Urine EFN2O were 0.51 %, 0.61 %, 0.83 % in dry season and 0.30 %, 0.50 %, 0.40 % in the rainy season, for Grass, Grass+N and Grass+Legume, respectively. The dung EFN2O did not vary between Grass and Grass+N systems, and there was a tendency of lowest dung EFN2O in Grass+Legume system (P = 0.065). Dung EFN2O were 0.22 %, 0.21 %, 0.12 % in dry season and 0.19 %, 0.20 %, 0.09 % in the rainy season, for Grass, Grass+N and Grass+Legume, respectively. The greatest percentage of excreta-N lost by NH3 volatilization (EFNH3) was observed for urine under Grass+N system in dry season. EFNH3 from urine-treated soil during the dry season was 7.9 %, 21.0 % and 11.2 % of the N in the excreta, and in the rainy season was 1.1 %, 4.2 % and 0 %, respectively, for Grass, Grass+N and Grass+Legume. There was no difference between pasture systems and seasons for dung EFNH3 and the mean of dung EFNH3 was 0.6 %. In all pasture systems, urine EFNH3 were greater than dung EFNH3. These results suggest that pasture system, season and excreta type affect differently the EFN2O and EFNH3. EFN2O and EFNH3 from urine were greater in dry season. The lowest urine EFN2O from Grass system indicates that intensifying the system by fertilizer-N or biological nitrogen fixation favored N losses by N2O in urine patches. However, the Grass+Legume system decreased the dung EFN2O. The lower urine EFNH3 from Grass+Legume and Grass systems, compared with Grass+N suggests that mixed pasture may be a strategy to mitigate NH3 volatilization from urine deposited in pasture. The emission factors found in this study are in agreement with those proposed in 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories.