Fungal decomposition and transformation of molecular and colloidal fractions of dissolved organic matter extracted from boreal forest soil

Dissolved organic matter (DOM) plays a central role in soil carbon (C) dynamics, serving as both a substrate for microbial decomposers and a source of material stabilised via physical protection in molecular aggregates and associations with mineral particles. It is well established that soil microorganisms play a key role in mineral-associated C aggregates; however, their impacts on molecular aggregates is not clearly understood. Here, we examined the ability of an ectomycorrhizal fungus (Paxillus involutus) and a saprotrophic fungus (a strain of Gloeophyllum), two major functional groups of fungal decomposers in forest ecosystems, to decompose and process the molecular and colloidal size fractions of DOM. DOM was extracted by water from boreal forest soil, and the chemical composition and colloidal properties were followed over 11 days using nuclear magnetic resonance (NMR) spectroscopy and small-angle light and X-ray scattering techniques. Both fungi decompose various organic compounds into their molecular fractions in the presence of an energy source (i.e. glucose). The decomposition rate was significantly higher for Gloeophyllum than for P. involutus. When glucose was depleted, Gloeophyllum continued to decompose more complex carbohydrates, whereas the decomposition activity of P. involutus almost stopped. A large proportion of the C in the DOM was found in organic colloids. At later stages, Gloeophyllum but not P. involutus, significantly affected the colloids by promoting the formation of larger aggregates. Thus, saprotrophic fungi activity can significantly influence the colloidal properties of DOM. Our results support the view that ectomycorrhizal fungi decompose some of the soil organic C however, their overall capacity for DOM decomposition and transformation is significantly lower than that of saprotrophic fungi.