Active restoration of degraded alpine grassland weakens mineral‐associated soil organic carbon retention

Abstract Active restoration is predominant to reverse grassland degradation and soil organic carbon (SOC) loss. However, early studies focused on the bulk soil carbon (C) dynamics upon grassland degradation and restoration, with limited evidence involving SOC fractions. Here, we explored how the particulate and mineral‐associated OC (POC and MAOC), metal‐bound OC, soil aggregate‐associated OC, SOC molecular structure and microbial‐derived OC respond to grassland degradation and restoration on the Tibetan Plateau, and also studied the effects of active restoration on SOC fractions across grassland ecosystems through a global‐scale meta‐analysis. We found that POC and soil aggregate‐associated OC contents substantially decreased, while MAOC content remained stable after grassland degradation. In contrast, short‐term active restoration induced a significant decrease in MAOC and metal‐bound OC due to lower soil pH, but POC content remained stable, indicating that grassland restoration resulted in a decrease in SOC stability. Moreover, soil aggregate structure and its stability recovered rapidly during short‐term active restoration, but aggregate‐associated OC did not recover or even declined, suggesting asynchrony between them. Nuclear magnetic resonance spectroscopy revealed no significant changes in the SOC molecular composition between intact, degraded and restored grasslands, suggesting that molecular structure alone does not mirror SOC stability. Furthermore, the meta‐analysis revealed that the response of bulk SOC, MAOC and POC in degraded grassland to active restoration occurred sequentially in three phases characterized by vulnerability, retention and re‐loss of SOC, respectively. Synthesis and applications . Combining a field experiment with a global synthesis, we found that grassland restoration induces divergent shifts in SOC fractions, with threshold effects during the grassland restoration chronosequence. Relying solely on overall changes in bulk SOC may misrepresent the true impact of grassland restoration on SOC retention. Therefore, it is essential to pay more attention to the response of the functionally distinct SOC fractions to grassland restoration, which could help us more accurately assess and predict the grassland soil C–climate feedback.