Mineral surface-reactive metabolites secreted during fungal decomposition contribute to the formation of soil organic matter.

Soil organic matter (SOM) constitutes the largest terrestrial C pool. An emerging, untested, view is that oxidation and depolymerization of SOM by microorganisms promote the formation of SOM-mineral associations that is critical for SOM stabilization. To test this hypothesis, we performed laboratory-scale experiments involving one ectomycorrhizal and one saprotrophic fungus that represent the two major functional groups of microbial decomposers in the boreal forest soils. Fungal decomposition enhanced the retention of SOM on goethite, partly because of oxidative modifications of organic matter (OM) by the fungi. Moreover, both fungi secreted substantial amounts (> 10% new biomass C) of aromatic metabolites that also contributed to an enhanced mineral retention of OM. Our study demonstrates that soil fungi can form mineral-stabilized SOM not only by oxidative conversion of the SOM but also by synthesizing mineral surface-reactive metabolites. Metabolites produced by fungal decomposers can play a yet overlooked role in the formation and stabilization of SOM.