Changes in soil carbon and nitrogen stocks following degradation of alpine grasslands on the Qinghai-Tibetan Plateau: A meta-analysis

Until now, nearly 90% of alpine grasslands on the Qinghai-Tibetan Plateau (QTP) have been degraded. However, there is currently no consensus on how soil organic carbon (SOC) and soil total nitrogen (STN) stocks vary with the degradation succession of alpine grasslands in this region. Here, a meta-analysis was conducted to quantify the dynamics of SOC and STN stocks in topsoil (0-30 cm) at different degradation stages of alpine grasslands on the QTP. The results showed that grassland degradation led to average losses of 48 and 39% for SOC and STN stocks, respectively. The changes in SOC and STN stocks following grassland degradation did not differ significantly between grassland types, but were significantly affected by grassland degradation stage. The reductions in both SOC and STN stocks increased with the degradation stage, and the highest reductions were all found at extreme degradation stage. The results indicated that the depletion of SOC and STN pools were aggravated with the degradation succession of grassland. The tightly coupled SOC and STN implied that the depletion of SOC stock was closely related to that of STN stock during the process of grassland degradation. Positive relationships were detected between the dynamics of vegetation coverage and that of SOC or STN stock, indicating that vegetation coverage could not only reflect grassland degradation, but might also be a potential indicator of SOC and STN status. The findings suggest that preventing the degradation succession of alpine grasslands is vital to maintain or promote SOC and STN levels on the QTP.