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

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 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
the 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 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.