Simulated Yak and Tibetan Sheep Urine Deposition Fleetingly Promoted N2O, CO2 and CH4 Emissions in an Alpine Steppe of Northern Xizang Plateau

The urine of yaks (Bos grunniens) and Tibetan sheep (Pantholops hodgsoni) represents a vital source of nutrients in the alpine grasslands on the Qinghai-Xizang Plateau, and its crucial role in affecting soil properties and greenhouse gas (GHG) emissions cannot be underestimated. However, few in-situ experiments examining the impacts of yak and Tibetan sheep urine deposition on the alpine grassland nutrient dynamics and GHG fluxes, and the underlying mechanisms and influential factors associated with GHG emissions in urine deposited grasslands are still not well understood. This study conducted a 33-day simulated urine deposition experiment that include the treatments of yak urine (YU), Tibetan sheep urine (TSU), and a control (CK) without any application in an alpine steppe of northern Xizang Plateau. We collected soil samples at depths of 0-10 cm, 10-20 cm, and 20-30 cm after 11, 21, 33 days of experimental treatments to explore its physicochemical and microbial properties. The GHG samples were collected respectively at 1, 3, 5, 8, 13, 18, 27, and 33 d of the experiment by using the method of artificial static closed chamber and determined the nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) concentration by using a gas chromatograph. The gene abundance of 0-10 cm soil ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), membrane-bound nitrate reductase (NarG), nitrite reductase encoding gene (NirS), nitric oxide reductase gene (NorB), and nitrous oxide reductase encoding gene (NosZ) were determined using real-time fluorescence quantitative PCR. The results showed that cumulative N2O, CO2, and CH4 emissions for YU treatment across the 33-day experimental period amounted to 49.6 g N ha(- 1), 73.8 kg C ha(- 1), and - 176 g C ha(- 1), respectively, with the values significantly (P < 0.05) greater than the emissions for CK treatment (36.4 g N ha(- 1), 23.5 kg C ha(- 1), and - 232 g C ha(- 1)). The deposition of YU and TSU immediately increased soil nutrient availability, anaerobic conditions, and microbial activity, which led to increasing N2O, CO2, and CH4 emissions to vary degrees, and reduced CH4 uptake. Soil CO2 flux for YU treatment exhibited a significant (P < 0.05) negative correlation with NO3--N concentration, while soil CH4 flux exhibited extremely significant (P < 0.01) negative correlation with SOC concentration. Soil N2O flux for TSU treatment was negatively correlated with soil NifH, AOB, AOA, and NirS gene abundances (P < 0.05). In contrast, the cumulative N2O, CO2, and CH4 emissions during the initial 13 days for YU and TSU treatments accounted for a significantly (P < 0.05) higher proportion (49.9% and 46.6%, 66.1% and 64.9%, 40.2% and 49.0%, respectively) of the total emissions compared to the CK treatment (43.8%, 40.7%, and 31.0%). Urine application resulted in 0-10 cm soil AOA, AOB, NirS, NarG, and NosZ gene abundances for YU treatment were significantly (P < 0.05) higher than that for CK treatment. The significant enhancement of 0-10 cm soil functional gene abundance (NirS, NorB, and NosZ), as well as increased levels of NH4+-N and NO3--N concentrations for YU and TSU treatments during the early stage of the experiment (P < 0. 05), proving the strengthen of denitrification activity and soil respiration and therefore resulted in a higher proportion of N2O and CO2 emissions. The conclusions would contribute to a better understanding of the short-term effects and functional mechanisms of urine deposition on alpine steppe GHG emissions, and provide scientific insights for optimising livestock urine management model and facilitating the greenhouse gas emission reduction strategies in pastoral areas.