Microbial Carbon Accumulation Efficiency: Assessing Microbial Carbon Pump Efficiency based on 13C-glucose Amendment Experiment

Soil microbes play an important role in stabilizing soil organic carbon (C) as microbial residues, a process known as soil ‘microbial C pump’ (MCP). Accurately assessing MCP efficiency is essential for understanding microbial-mediated soil C sequestration. Conventional assessments based on microbial C use efficiency (CUE) hinge on microbial biomass only and do not include microbial necromass, which may not depict MCP efficiency. Here we propose a relatively simple and rapid approach based on ¹³C-glucose amendment experiment to assess microbial C accumulation efficiency (CAE)  as a novel metric for assessing MCP efficiency. We first validated the approach by showing negligible retention of glucose to soils with a wide range of edaphic properties. Glucose-derived ¹³C may hence be considered to represent microbial C (including biomass and residues) after a few days of addition, given the rapid uptake of glucose by microbes. Microbial CAE may thus be assessed as the recovery of glucose-derived ¹³C in the soil. By further conducting a meta-analysis of literature data involving isotopically labeled glucose amendment experiments, we revealed distinct variation patterns and influencing factors of microbial CAE and CUE across various terrestrial ecosystems. Compared to CUE which is mainly regulated by factors influencing microbial physiological processes (particularly substrate availability), CAE is jointly regulated by factors that influence microbial growth (e.g., biomass and climate) and residue preservation (e.g., clay content). These findings underscore that CAE is decoupled from CUE. Incorporating CAE into soil C models may provide new insights into future SOC dynamics under climate change.