Litter Inputs Promoted Soil Organic Carbon Formation by Increasing Particulate Organic Carbon on the Eastern Edge of the Tibetan Plateau

Forest soil organic carbon (SOC) accumulation is regulated by plant-derived carbon inputs and soil environmental conditions, but the relative roles of litter composition and soil physicochemical properties in regulating SOC fractions remain unclear in high-elevation forest ecosystems. This study aimed to determine whether variation in SOC among different forest types was mainly associated with particulate organic carbon (POC) or mineral-associated organic carbon (MAOC), and to identify the relative roles of litter characteristics, soil physicochemical properties, microbial biomass, and enzyme activities in regulating SOC fractions. Four forest types on the eastern edge of the Tibetan Plateau were investigated: broadleaved poplar forest (PLF), larch forest (LXF), seabuckthorn forest (SBF), and Dasiphora shrubland (DS). PLF had the highest SOC and POC contents (75.8 and 48.5 g kg−1, respectively), whereas MAOC did not differ significantly among forest types. SOC was strongly positively correlated with POC (R2 = 0.74, p < 0.001), but not with MAOC, indicating that SOC variation was mainly associated with POC accumulation. PLF litter contained higher labile and recalcitrant carbon components, including soluble sugar (19.9 g kg−1), starch (30.1 g kg−1), lignin (94.6 g kg−1), and litter carbon (404 g kg−1). Partial least squares path modeling showed that soil physicochemical properties had the strongest direct path relationship with SOC variation (p < 0.001), while litter composition was positively associated with microbial biomass and POC formation (p < 0.01). These findings suggest that POC formation was the main fraction-level feature associated with SOC accumulation, while soil properties and litter composition were related to SOC through different pathways.