Direct measurement of the in situ decomposition of microbial-derived soil organic matter

Soil organic matter (SOM) is the dominant reservoir of terrestrial organic carbon and nitrogen, and microbial necromass represents a primary input to it. However, knowledge of stabilization mechanisms and direct measurements of the decomposition of microbial-derived SOM are lacking. Here we report a novel 15N isotope pool dilution approach using labeled amino sugars and muropeptides as tracers to quantify the decomposition of proteins and microbial cell walls, which allows to estimate in situ decomposition rates of microbial-derived SOM. Our results demonstrate that microbial cell walls are as recalcitrant as soil protein, exhibiting comparable turnover times across various ecosystems. The bacterial peptidoglycan in soils was primarily decomposed to muropeptides which can be directly utilized by microbes without being further depolymerized to free amino compounds. Moreover, bacterial peptidoglycan decomposition was correlated with soil microbial biomass while fungal chitin and soil protein decomposition were correlated with high soil pH and fine soil texture. This approach thereby provides new insights into the decomposition pathways and stabilization mechanisms of microbial-derived SOM constituents pertaining to SOM persistence.