Unravelling the facilitation-competition continuum among ectomycorrhizal and saprotrophic fungi

Soil fungal inter-guild interactions may impact ecosystem processes significantly. In particular, competition between ectomycorrhizal and saprotrophic fungi could reduce organic matter decomposition through the “Gadgil effect”. Whether fungal facilitative and competitive interactions predictably shift under moderate environmental stress, as hypothesised by the stress-gradient hypothesis (SGH), is still uncertain, particularly across multiple environmental resource gradients. Here, we quantified reciprocal interactions among fungal guilds in root tips and soil mycelia in 84 temperate forests of various tree compositions comprising a natural gradient of soil fertility and root carbon resources. The two resource gradients were negatively related. In keeping with SGH, we found that the typical interactions between fungal guilds were symmetrically positive at the lowest end of both gradients. These findings corroborate enhanced decomposition, indicating a facilitative effect generated by the ectomycorrhizal and saprotrophic fungal positive interactions. Inter-guild interactions varied with the spatial habitat and resource type gradient, with root carbon resources more strongly influencing root tip than soil mycelium communities. When both gradients were integrated, SGH held for the dominant gradient in the system. The premises of the “Gadgil effect” became apparent in the more fertile soils, but under higher C/N ratios, certain ectomycorrhizal groups, including taxa capable of mobilising nitrogen from complex organic substrates, exerted negative effects on saprotrophic fungi. Under lower soil pH and in drier, warmer climates resembling global change scenarios, soil fungal guilds positively influence each other. These interactions potentially aid in the preservation of soil biodiversity and the support of forest ecosystem function.