Nitrous oxide emissions from yak excreta patches on an alpine steppe and a summary of available data on nitrogen losses from excreta patches in grassland ecosystems

Abstract:

The excreta (urine and dung) deposition is a key way to supply nitrogen (N) to alpine grasslands in the Qinghai-Tibetan Plateau, and the excreta patches are the highly active hot spots of nitrous oxide (N₂O) emissions, but less information is available on the contributions of excreta patches to N₂O emissions from alpine grasslands. A series of experiments on yak excreta N₂O emissions from an alpine steppe were conducted. The results showed that yak dung application resulted in significantly higher cumulative N₂O emission in a short-term field study, but high soil moisture could lead to a reduction of N₂O due to the complete denitrification. Fresh yak urine caused more N₂O loss than urea under equivalent N application rates. In addition, we also found yak dung fragmentation increase the leaching of dissolved organic C and N, and the heavily fragmenting could decrease the leaching of both ammonium-N and nitrate-N after short-term dung decomposition. Full-size dung pats enhanced nirK & nirS genes that contribute to N₂O emissions, but the yield-scaled N₂O emissions decreased with increasing fragmentation of yak dung, the heavily fragmenting treatment was effective to mitigate N₂O emissions. These studies can provide the data on N₂O release from grassland soils at high-altitude and promote sustainable grazing practices on natural grassland ecosystems on the Qinghai-Tibetan Plateau. Moreover, more livestock is being raised globally due to the increasing demand for livestock products, which inevitably results in increased excreta deposition onto grasslands, to better understand the N dynamics excreta patches, available measurements from previous studies were summarized and a comprehensive analysis was performed. We found 0.28%, 0.76%, 0.08%,and 0.35% of N could be lost via N₂O for cattle dung, cattle urine, sheep dung, and sheep urine, respectively. The increase in N₂O emissions after excreta application was mainly related to the enhanced abundance of ammonia-oxidizing bacteria and denitrifying bacteria. Applying additives (nitrification and urease inhibitors, biochar and liming materials), manipulating diet, as well as managing grazed grassland can have great potential to mitigate N₂O emissions from excreta patches and the grazed grasslands.