Ectomycorrhizal Decomposition Responses to Nutrient Addition in a Mature Temperate Deciduous Forest

Rising reactive nitrogen (N) deposition and corresponding shifting phosphorus (P) availability can alter plant carbon (C) allocation belowground and modify soil fungal communities, with uncertain consequences for decomposition of soil organic matter (SOM) and hence C storage. Ectomycorrhizal (ECM) fungi are a major pathway for plant C to enter soils and can regulate SOM decomposition through opposing mechanisms, either stimulating free-living microbial activity (Priming effect) or suppressing decay via competition with saprotrophs (Gadgil effect). However, how N and P addition shifts the balance between these pathways remains unresolved, particularly in mature forests that are likely nutrient limited.

We investigated hyphal-mediated controls on decomposition under a factorial N and P addition experiment (control, +N, +P, +N+P) in a mature temperate oak forest, using nested in-growth bags containing either oak leaf litter (litter) or tongue depressor fragments (wood). Two outer mesh sizes manipulated hyphal ingrowth, with a 41-µm mesh allowing fungal entry and a 1-µm mesh largely excluding fungi and roots. Because ECM hyphae can forage over large areas and proliferate through nutrient-poor substrates, we expected this manipulation to mainly affect ECM contributions. Microbial respiration, C-based mass loss, hyphal biomass, potentials enzyme activities of peroxidase (PEROX) and phenol oxidase (PHENOX), and substrate-induced respiration were quantified, and fungal communities of wood were profiled by ITS1 amplicon sequencing.

The mesh treatments generated clear differences in hyphal biomass (p = 0.002) without altering bag moisture or pH (p > 0.05). By the second sampling, linear mixed models showed substrate- and nutrient- specific ECM effects on decomposition. For litter, fungal inclusion increased mass loss by 15.83% compared with exclusion bags (p = 0.012) and substrate-induced respiration by 31.82% (p = 0.019), whereas N enrichment decreased microbial respiration by 16.45% (p = 0.030). In contrast, fungal inclusion did not significantly affect wood mass loss (p = 0.562). Instead, N fertilisation reduced mass loss by 37.84% compared with controls (p = 0.048) and was associated with lower oxidative enzyme potentials (PHENOX: p = 0.085, PEROX: p = 0.15). Similarly, PERMANOVA analysis on fungal communities of wood reflect significant effects of nutrient addition (p = 0.002) and ANCOM-BC analysis shows that N addition significantly altered many fungal classes in wood.

These findings suggest that ECM controls on decomposition are substrate-dependent, and that nutrient supply can redirect fungal foraging and competition with saprotrophs. In mature forests, fertilisation may decouple carbon inputs to the mycorrhizal association from decay responses by reducing plant investment belowground and by reshaping fungal communities. This context dependence helps explain why fertilisation experiments often yield inconsistent soil carbon outcomes. Therefore, to improve projections of forest carbon cycling under rising N deposition and shifting P availability, mycorrhizal effects on decomposition should be linked with nutrient availability and substrate quality.