Litter-mediated shifts in microbial communities along an altitudinal gradient enhance humification-driven soil organic carbon accumulation

Abstract

Humus constitutes the primary component of soil organic matter (SOM), while forest litter serves as a critical source of humus, driving the formation of soil organic carbon (SOC). However, the mechanisms through which litter inputs modulate humification and SOC sequestration remain elusive. In this study, amplicon sequencing of litter- and soil-associated bacteria and fungi was combined with biogeochemical analyses across an elevational gradient in the Zhongtiao Mountain forest ecosystem (Shanxi Province, China) to identify the drivers of humification and SOC accumulation. Results showed that while litter/soil nutrients increased with elevation, humus content declined. Microbial communities (including rare and abundant taxa) shifted with altitude, with pH, water content, total phosphorus, and microbial biomass nitrogen as key determinants. The Correlation analyses and Mantel tests further demonstrated that humus and SOC fractions were influenced not only by soil properties but also by the chemical properties of litter and the diversity and composition of microbial communities in litter. Notably, rare microbial taxa in the litter had greater impacts on humus and SOC accumulation compared to abundant taxa. Mixed-effects modeling validated that humus content was a significant predictor of SOC accumulation. In addition, structural equation modeling (SEM) indicated that early-stage litter-colonizing microorganisms directly enhanced humus and SOC storage and indirectly promoted humification by modifying soil microbial community structure. This study elucidates the mechanisms by which altitudinal variation in litter inputs regulates humification and SOC sequestration, quantifying the functional contributions of microbial communities to these processes. These findings advance understanding of forest SOM dynamics and provide scientific a basis for predicting soil carbon stabilization under environmental change.