Microbial metabolism decline in degraded grasslands depends on soil organism size

Soil organisms of various sizes respond variably to environmental changes due to their distinct life-history traits, but their impact on soil organic carbon (SOC) through microbial metabolism remains unclear. Degraded grasslands provide an excellent platform to evaluate the links between specific-size organisms and metabolic efficiency in complex environments. Grassland degradation experiments were established across six sites spanning 2000 km to reveal how 34 taxa of various sizes, bacteria, fungi, protozoa, and nematodes, respond to degradation, and their dependence on microbial metabolic efficiency. Degradation reduced microbial carbon use efficiency (CUE) and necromass accumulation coefficient (NAC) by 9.2–27 % and 19–41 %, respectively, though its extent may be mediated by environmental factors across sites. Organism responses to degradation depended on body size: fungal diversity increased by 15–20 %, whereas large-bodied organisms (nematodes) decreased by 27 %, and bacteria and protozoa remained stable. Increases in soil pH, as well as decreases in plant biomass, soil clay, and nutrients, were associated with the reductions in CUE and NAC. Furthermore, smaller-bodied organisms (e.g., Actinobacteria and Firmicutes within bacteria) explained the reductions in CUE and NAC more than larger-bodied organisms (e.g., nematodes and protozoa). We emphasize that grassland degradation-induced deterioration of plants and soil, as well as changes in smaller-bodied organisms, potentially reduces the efficiency of microbes in converting carbon into biomass and necromass, which may be a key mechanism for SOC loss in degraded grasslands. © 2025 Elsevier B.V.