Waterlogging increases microbial necromass carbon and particulate organic carbon in alpine meadow soils

Hydrological changes induced by climate warming and human activities significantly impact soil organic carbon (SOC) accumulation in grasslands. Microbial necromass carbon (MNC) and particulate organic carbon (POC) plays a pivotal role in forming stable and labile SOC reservoirs. However, the accumulation of MNC and POC, their contribution to SOC, and the mechanisms governing these processes along the alpine grassland-wetland continuum remain unclear. We investigated the effects of a hydrological gradient on MNC and POC accumulation and their contributions to SOC, as well as the underlying mechanisms in topsoil and subsoils on the Tibetan Plateau. Fens had 320% and 280% higher SOC sequestration efficiency in the topsoil than mesic and wet meadows, respectively. Waterlogging increased the total MNC, fungal necromass carbon (FNC), and bacterial necromass carbon (BNC) content in both topsoil and subsoil, while the MNC proportion in SOC remained stable at an average of 32% along the alpine grassland-wetland continuum. Additionally, waterlogging also increased POC content, with a higher proportion of POC in SOC observed in anaerobic fens compared to mesic meadows. Lower FNC/BNC ratios and a higher POC proportion in SOC indicate reduced SOC stability in waterlogged fens. Correlation analysis and structural equation modelling showed that microbial biomass and pH were key determinants of MNC in the topsoil, while microbial biomass, iron and aluminum oxides primarily determined MNC in the subsoil. We conclude that waterlogging increases total MNC and POC content in soil. While the MNC contribution to SOC remains stable, the POC contribution to SOC increases significantly. Thus, SOC in fens exhibits reduced stability, forewarning of potential C loss risks associated with the decline of wetland areas in alpine meadows.