Unique species interactions determine the decomposition activity of fungal communities

1. Fungal diversity plays a critical role in decomposition, a key process in carbon (C) cycling and ecosystem functioning. However, diversity–function relationships in microbial systems remain context-dependent and challenging to predict. 2. We investigated how fungal community diversity influences decomposition across substrates of differing complexity, testing whether functional complementarity versus competition drives community-level decomposition rates. 3. Using a factorial design, we assembled synthetic fungal communities of varying richness (1, 2, 4 and 6 species) and measured mineralization processes on two C sources: heterogeneous leaf litter and homogeneous cellulose filters. Fungal species were selected based on broad phylogenetic coverage and diverging litter mineralization capacities. 4. We found that communities composed of four species had the highest decomposition rates, with no further gains at a diversity level of six. Resource partitioning and facilitation enhanced decomposition in heterogeneous leaf litter, whereas competition suppressed decomposition in more diverse communities on homogeneous cellulose filters. Species interaction outcomes were only weakly predicted by individual functional traits, demonstrating unique community interaction traits. 5. These findings highlight the context dependency of fungal interactions and show that diversity–function relationships are shaped by both community composition and substrate complexity. Predicting decomposition and C cycling under environmental change will require incorporating microbial interactions and resource heterogeneity into ecosystem models.