Decoupling species richness and interaction frequency reveals how fungal interactions regulate wood decomposition.

Wood decay fungi play a central role in forest carbon cycling, yet the mechanisms linking fungal biodiversity to decomposition remain unclear because species richness and interspecific interactions are rarely separated. Here, we experimentally disentangled these effects using a microcosm with four common wood-decay fungi. By manipulating the spatial arrangements of pre-colonized wood blocks, fungal species richness and the frequency of interspecific interactions were independently varied. Wood mass loss was quantified, and acid-unhydrolyzable residue (AUR, Klason lignin) and carbohydrate analyses were conducted to examine the changes in decay strategies and the potential accumulation of recalcitrant fungal products. Both fungal species richness and interspecific interactions enhanced wood decomposition, but their effects depended on species identity and combinations. Selection effects were observed when competitively dominant species replaced weaker competitors. Several species combinations showed decomposition rates exceeding those of pure cultures, indicating complementarity and facilitation by interspecific interactions. Generalized linear mixed models revealed that interaction frequency and species richness independently influenced decay rates in a species-specific manner. Chemical analyses revealed that interspecific interactions altered the relative loss of AUR and carbohydrates, indicating shifts in the enzymatic allocation and/or production of acid-insoluble fungal metabolites during competition. Our results indicate that competitive interactions among fungal species often accelerate decomposition to offset energetic costs. However, deadlock interactions among basidiomycetes may promote the accumulation of recalcitrant fungal compounds, potentially slowing decomposition over longer timescales. We propose the "accumulated inhibitor hypothesis" to reconcile contrasting fungal diversity-decomposition relationships and highlight the importance of interaction frequency in fungal biodiversity-ecosystem functioning research.