Dissolved organic matter dynamics and chemistry under fungal activity: A microcosm incubation with litter differentially 13C-labelled

• Trichoderma harzianum mainly consumes DOC during the first weeks of decomposition. • Residual DOC is depleted in carbohydrates, enriched in lipids and lignin moieties are oxidised. • Typical fungal component did not substantially accumulate in DOC. • Abiotic processes release substantial amount of DOC. This work aimed at better documenting the effects of fungal activity on dissolved organic matter (OM) in soils. Dynamics of water-extractable OM (as surrogate for dissolved OM) quantity and chemical quality was monitored during a ca. 6 month microcosm incubation of plant residues in the presence of fungi. Differential 13 C-labelling of metabolites vs structural compounds of the incubated residues further allowed clarifying the balance between fungal mineralisation and production of soluble compounds (through biosynthesis and/or decomposition). The fungus Trichoderma harzianum was mainly active during the first weeks of incubation, substantially mineralizing WEOM, preferentially consuming carbohydrates. The fungus induced chemical modification of WEOM, notably selective preservation of lipids and oxidation of lignin moieties. While T. harzianum probably degraded some insoluble structural molecules and produced biomass, these contributions to bulk WEOM appeared minor (when compared with leaching and mineralization), either because non-significant or entering non-extractable carbon pool. Additionally, characterization of control fungus-free microcosms, highlighted the potential role of abiotic processes on WEOM production, including leaching and depolymerisation by extracellular enzymes, notably of carbohydrate rich (insoluble) macromolecules.