Abstract

Soil microbes play a crucial role in soil organic carbon storage by processing plant-derived carbon and producing microbial carbon, which can accumulate over time and form a substantial fraction of the soil organic carbon stock. Whereas the contribution of microbial cellular residues, the remains of dead microbial cells, to soil carbon storage is well established, the role of extracellular residues remains poorly quantified. Here we develop an 18O-H2O isotope-tracing method to quantify the production of extracellular polymeric substances, mucus-like mixtures of sugars, proteins and other biopolymers released by microbes and apply it across a national-scale survey of cropland soils in China. We find that extracellular polymeric substances constitute a major microbial carbon input to soils, with production rates comparable to those of microbial cellular residues and they exhibit statistical associations with mineral-associated organic carbon across soils. In 13C-labelling microcosm experiments, carbon derived from extracellular polymeric substances is incorporated into the mineral-associated organic carbon pool at rates comparable to or exceeding those of microbial cellular residues. These results identify the extracellular pathway as an important and previously underappreciated mechanism contributing to the mineral-associated carbon pool and provide mechanistic insights into how microbiomes shape the fate of soil carbon under global change.