Land degradation and restoration strongly influence terrestrial soil organic carbon (SOC) dynamics. However, the underlying mechanisms are not well understood. Here, based on a meta-analysis of 803 observations from 138 studies worldwide, our data analyses suggest that C-degrading enzymes play a crucial role in regulating SOC dynamics under land degradation and restoration. Our result showed that decreased cellulase activity but unchanged ligninase activity was associated with land degradation, whereas higher increased cellulase activity compared with ligninase activity was associated with land restoration. Consequently, the ligninase-to-cellulase ratios were higher under land degradation and lower under land restoration. Also, the specific enzyme activity (the amount of enzyme produced per unit microbial biomass) was greater under land degradation but lower under land restoration. By comparison with the short-term (<= 30) land degradation, the long-term (> 30 years) land degradation significantly increased the ligninase-to-cellulase ratio. On the contrary, the long-term land restoration exerted a more negative effect on the ligninase-to-cellulase ratio. The increases in the specific enzyme activity and ligninase-to-cellulase ratio were tightly correlated with decreases in SOC content under land degradation. A similar correlation was also found between decreases in specific enzyme activity and ligninase-to-cellulase ratio and increases in SOC content under land restoration. Overall, the decrease of SOC storage under land degradation is not only due to the low plant inputs, but also likely because of the accelerated degradation of recalcitrant C pools. However, the reverse applies for land restoration. The novel insights provided by our results contribute to the understanding of microbial mechanisms underlying the changes in SOC accumulation in response to land-use changes.
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