Aggregate-associated carbon contributes to soil organic carbon accumulation along the elevation gradient of Helan Mountains

Physically or biochemically protected soil organic carbon (SOC) can dampen microbial decomposition by reducing substrate availability, enhancing the stability of SOC. However, microbe-mediated changes in the physical and chemical compositions of SOC along an elevation gradient are poorly understood, especially in arid mountain environments. Here, soil samples from depths of 0–20 cm and 20–40 cm along five elevations ranging from 1380 to 2438 m in Helan Mountains were collected. We explored the effect of the soil microbial community and enzyme activities on the physical (via aggregate-associate C) and chemical compositions of SOC. The results showed that the amount of aggregate-associated C, the abundance of microbes and enzyme activities were significantly greater in the 0–20 cm than the 20–40 cm soil layer. The ratio of fungi to bacteria (F/B), aggregate-associated C, and cellobiohydrolase (CBH) activity increased with elevation. However, the highest proportion of aromatic C, total PLFA, β-1,4-glucosidase (BG), N-acetyl-β-glucosaminidase (NAG), leucine aminopeptidase (LAP), and alkaline phosphatase (AP) were found at the mid-elevation (2139 m), with a hump-shaped unimodal distribution pattern. This indicated that soil microbes at mid-elevation had the highest potential to decompose the relatively labile carbon compositions. CBH explained the most variations in the physical and chemical compositions of the SOC and had a significant effect on the macroaggregate- and silt and clay-associated C. Structural equation modeling (SEM) indicated that with an increase in elevation, F/B and the enzyme activities involved in C cycling (BG, CBH) decreased physical protection and recalcitrance of SOC. Collectively, our findings demonstrated that the primary mechanism for SOC accumulation in the Helan Mountains was the decrease in aggregate-protected C with an increase in elevation.