Abstract: Microbial necromass carbon (MNC) contributes largely to soil organic C (SOC) pool in terrestrial ecosystems. However, the pattern and underlying mechanisms of MNC and their contribution to SOC along elevational gradients are controversial due to montane ecosystems subject to environmental change. Here, in this study, we investigated the seasonal variation of MNC, its contribution to SOC, the necromass accumulation coefficient and the influencing factors across different elevations in the mountain forests ecosystem of Southwest China. Soil microbial biomass rather than MNC showed seasonal variations, this decoupling pattern was mainly attributed to higher soil extracellular enzymes (i.e. N-hydrolyzing enzyme) and C:N ratio, which accelerated the decomposition of MNC especially bacteria necromass C (BNC) during the humid and warm wet season. In contrast, the drought and cold conditions in dry season inhibited microbial activities and conversion to MNC. During the dry season, the MNC and MNC/SOC exhibited hump-shaped pattern along elevational gradients. The fungal necromass C (FNC) was positive with fungal biomass, indicating that living biomass may have a greater influence on the accumulation of FNC than BNC. On average, MNC constituted about 15% of SOC, with the contribution from FNC (11.9%) surpassing that from BNC (3.1%). The joint effects of soil pH and clay composition significantly influenced MNC dynamics along elevational gradients. These findings demonstrate that the rapid decomposition of BNC is the main way of MNC loss in wet season in the mountain forests ecosystem and further highlight the importance of microbial traits in MNC accumulation.

Key words: microbial necromass, montane ecosystems, soil organic carbon dynamics, enzyme activities, alpine forest