Nutrient availability drives the ecological linkage between microbial functional diversity and soil organic matter molecular complexity during forest restoration.

Forest restoration facilitates soil organic carbon (SOC) preservation, a process that is fundamentally governed by the molecular complexity of soil organic matter (SOMMC) and its dynamic interaction with microorganisms. Yet, the changes in SOMMC and its ecological linkages with microbial communities, and the mechanisms driving these relationships during forest restoration remain poorly understood. To fill this knowledge gap, we collected O- and A-horizon soil samples across two independent restoration chronosequences in tropical forests and applied thermally-assisted hydrolysis and methylation coupled with gas chromatography and mass spectrometry (THM-GC-MS) for molecular-level SOMMC characterization and high-throughput sequencing for microbial community profiling. The results showed that in the O-horizon, the SOC content was 32 ± 1 mg g in the early restoration stage, which was 3.5 and 2.3 times of that in the middle and late restoration stages. However, SOC content exhibited a significant increasing trend in the A-horizon with forest restoration (P < 0.01). The α diversity of SOM molecules presented an increase with forest restoration in both the O- and A-horizons. Microbial functional diversity and biomass significantly explained over 50 % of the variances of the SOMMC and SOC content (P < 0.001), in contrast to microbial species diversity and community stability. In the O-horizon, the increase in microbial functional diversity and the decrease in biomass during forest restoration were associated with enhanced SOMMC but reduced SOC content. Conversely, in the A-horizon, increased microbial biomass and functional diversity, resulted in simultaneous increases in SOC content and SOMMC during forest restoration. The distinct ecological linkages between SOC preservation and microbial communities in the O- and A-horizons were attributed to the nutrient limitations for microorganisms. These findings underscore the role of forest restoration in enhancing SOMMC, highlight the necessity for improved nutrient availability to sustain microbial functional diversity, and facilitate SOC preservation in future forest management practices.