Effects of alpine meadow degradation on nitrifying and denitrifying microbial communities, and N2O emissions on the Tibetan Plateau

Meadow degradation is often accompanied by significant changes in nitrogen (N)-cycling and nitrous oxide (N2O) emission potential, and leads to challenges in meadow management. However, the mechanisms of soil N-cycling and N2O emissions remain poorly understood, especially in alpine ecosystems. In this study, we investigated the soil N-cycling process in four alpine meadows on the Tibetan Plateau along a degradation gradient using real-time quantitative polymerase chain reaction and amplicon sequencing to elucidate the mechanisms. Compared to non-degraded meadows, meadow degradation reduced N2O emissions by 38.5-140.2%. Meadow degradation reduced the abundance of amoA of ammonia-oxidising archaea (AOA) and ammonia-oxidising bacteria (AOB), whereas light and moderate degradation increased the abundance of genes nirS, nirK, and nosZ. Lightly degraded meadow exhibited the highest alpha-diversity of both nitrifiers and denitrifiers, but this higher diversity was not accompanied by higher N2O emissions, with only 32.3% of the microbial taxa identified as predictors of N2O emissions, suggesting that functional redundancy exists in the N-cycling process in meadow ecosystems. Nitrosospira and Mesorhizobium from the AOB and nirK communities, respectively, were identified as the key taxa that may contribute to N2O emissions. Soil properties, especially N reaction substrates, including ammonium-N, nitrate-N, dissolved organic N, and total N, were the primary drivers for N2O emissions via mediation of the N-cycling community, especially nitrifiers. Our results emphasised the importance of environmental factors in shaping nitrifying, denitrifying, and N2O emissions, providing insights for the restoration of degraded meadow ecosystems.