Vegetation degradation and its progressive impact on soil nitrogen mineralization in the Qinghai-Tibet Plateau's alpine wetlands: Insights from a three-year study.

The soil nitrogen (N) cycle in the alpine wetland of the Qinghai-Tibet Plateau (QTP) has been strongly affected by vegetation degradation caused by climate change and human activities, subsequently impacting ecosystem functions. However, previous studies have rarely addressed how varying degrees of vegetation degradation affect soil net nitrogen mineralization rates and their temporal dynamics in these sensitive ecosystems. Therefore, we conducted a three-year field-based soil core in situ incubation mineralization experiment on the northeastern margin of the Tibetan Plateau from 2019 to 2021 to assess the variations in soil net ammonification, nitrification, and mineralization rates during the growing season (June to October). The main aim was to determine the dynamic effects of different degrees of vegetation degradation (non-degraded, lightly degraded, moderately degraded, and severely degraded) on soil net nitrogen transformation processes, as well as the impact of seasonal fluctuations in soil temperature and moisture on net nitrogen mineralization. Results indicated that vegetation degradation significantly reduced the net ammonification rate by 22.09%-97.10%, significantly increased the net nitrification rate by 45.38%, and significantly decreased the net mineralization rate by 9.49%-16.25%. Redundancy analysis (RDA) and random forest models revealed the supportive role of soil water content in the soil nitrogen transformation processes, particularly in promoting nitrification, while soil temperature was identified as a positive regulator of N cycling enzyme activity, indirectly affecting net nitrogen mineralization rates.