Fungal communities mediate but do not control leaf litter chemical transformation in a temperate oak forest

Background and Aims: In temperate forests, fungi are the main actors in leaf litter decomposition. Still, we have minimal knowledge of their influence on changes in leaf litter chemistry. Thus, we aimed to determine the main drivers behind leaf litter chemical transformation during decomposition. Methods: We monitored the development of fungal communities, extracellular enzyme activities, and litter chemical properties during a long-term (768 days) transplantation experiment of two chemically-contrasted intraspecific oak leaf litters. Results: Initial differences in substrate chemistry between native and transplanted Quercus petraea litters incubated at the same forest site largely persisted throughout the decomposition process, indicating that initial substrate quality constrained litter chemical transformation. The two litter types also maintained distinct fungal communities despite similar enzyme profiles. This suggests that fungi act more as constrained mediators rather than controllers of chemical changes during litter decay. Further, the litter elemental (i.e., nutrient composition) and organic (i.e., lignin and carbohydrate composition) chemistries tended respectively to diverge and converge over time between the native and transplanted litter types. Conclusion: The results highlight that leaf litter chemical transformation is a dynamic process mediated—but not oriented— by fungal communities. The factors influencing changes in leaf litter's organic and elemental chemical properties may be decoupled, with potentially contrasting consequences on forest carbon stocks and soil fertility.