Resolving the Relationship Between Microbial Growth and Soil SOC via a Principle of Energy Conservation and Maintenance Priority

Microbial carbon use efficiency (CUE) is often regarded as a key determinant of soil organic carbon (SOC) accumulation, yet this view faces a paradox: SOC continues to accumulate even as rising microbial biomass and associated respiration reduce CUE. We propose a new model (IGMM), grounded in microbial metabolic growth principles, which reveals a power-law relationship between SOC and the ratio of maximum microbial biomass ( MIC _max ) to baseline decomposition rate (BDR). This single ratio explains most global SOC variation (R ²  = 0.82). Notably, MIC _max and BDR show a "triangular boundary pattern": BDR rises with MIC _max initially, declines when MIC _max hits ~ 0.4 g C/kg, and SOC then accumulates stably. Unlike the CUE-centric view, which is indirect, MIC _max provides an explicit and quantitative basis for SOC dynamics. By establishing MIC _max as a unified mechanistic axis, the IGMM opens up a holistic theoretical analysis pathway rooted in the metabolic growth mechanism for understanding soil carbon stabilization processes.