Moss Cover Redirects Soil Organic Carbon from Active Turnover to Mineral-Associated Stabilization in Subalpine...

Understory mosses modify near-surface soil conditions, but how elevation regulates their influence on active and mineral-associated soil organic carbon (SOC) remains unclear. We compared independently selected moss-covered and non-moss-covered soils across a 3200-3500 m elevational gradient and integrated soil physicochemical measurements, microbial biomass (MB), dissolved organic matter (DOM), microbial necromass carbon (MNC), particulate organic carbon (POC), mineral-associated organic carbon (MAOC), metagenomic profiling, and piecewise structural equation modeling. Moss-covered soils consistently contained higher SOC and MAOC, but lower DOM, MB, and generally lower POC, than non-moss-covered soils. MNC showed an elevation-dependent reversal, with higher values under moss cover at 3200 m but lower values under moss cover at 3300-3500 m. Elevation was not a significant uniform driver of MB, DOM, MNC, POC, or MAOC; instead, its influence was mainly reflected in interactions with surface cover and in elevation-related changes in moss-layer structure, diversity, and hydrothermal conditions. Core carbon-fixation and degradation functions remained broadly stable, whereas specific functional modules shifted within moss-covered soils: acetate and acetyl-CoA metabolism genes (ackA and abfD) were relatively abundant at 3300-3400 m, while the polysaccharide-reprocessing gene SGA1 and oxidative-transformation gene katG increased toward higher elevations, and pmoC/amoC rebounded at 3500 m. Structural equation models linked the microbial functional gene system more strongly to POC, whereas MNC was positively associated with MAOC, and the direct POC-to-MAOC pathway was not significant. These findings indicate that moss cover is associated with contrasting SOC allocation patterns and stronger microbial necromass-MAOC coupling, while elevation modulates these relationships indirectly through changes in moss communities, soil microenvironment, and microbial functional potential.