<b>Unlocking mechanisms for soil organic matter accumulation: </b><b>c</b><b>arbon use efficiency and microbial necromass as </b><b>the keys</b>

Soil microorganisms transform plant-derived C (carbon) into particulate organic C (POC) and mineral-associated C (MAOC) pools. While microbial carbon use efficiency (CUE) is widely recognized in current biogeochemical models as a key predictor of soil organic carbon (SOC) storage, large-scale empirical evidence is limited. In this study, we proposed and experimentally tested two predictors of POC and MAOC pool formation: microbial necromass (using amino sugars as a proxy) and CUE (by 18O-H2O approach). Soil sampling (0-10 and 10-20 cm depth) was conducted along a climatic transect of 900 km on the Loess Plateau, including cropland, grassland, shrubland, and forest ecosystems, to ensure the homogeneous soil parent material.