Soil physico-chemical properties have a greater effect on soil fungi than host species in Mediterranean pure and mixed pine forests

Soil fungi are fundamental drivers of forest ecosystem processes. Soil physico-chemical parameters and vegetation features such as host type or stand structure can affect soil fungal communities. However, there is a lack of comprehensive studies describing the relative importance of niche processes (soil physico-chemistry and forest structural drivers) versus neutral processes (geographical distance) driving soil fungal community assemblages, especially in less-studied drought-prone ecosystems such as Mediterranean forests. In this study, we performed Pacific Biosciences sequencing of internal transcribed spacer 2 amplicons to characterize the soil fungal community composition and diversity of 42 forests dominated by either pure Pinus nigra, Pinus halepensis or Pinus sylvestris or a P. nigra-P. halepensis or P. nigra-P. sylvestris mixture. Our specific aims were to identify and disentangle the relative importance of the main soil characteristics and the spatial and forest structural factors that accounted for the greatest proportion of fungal community variation along a regional gradient in the Mediterranean Pre-Pyrenees. Soil parameters accounted for the greatest significant proportion of the total variance in the overall fungal community (25%), and in the mycorrhizal (23%) and saprotrophic (22%) communities, while geographical distance accounted for 14% of the variance in the overall fungal community, 7% in the mycorrhizal and 22% in the saprotrophic communities. Conversely, forest structure did not significantly affect the soil fungal community, as fungal composition and diversity did not differ significantly among the pine hosts. Moreover, pH, followed by P and the C:N ratio explained the largest differences in the composition of the overall fungal community and in the mycorrhizal fungal community. By contrast, the largest proportion of differences in saprotrophic composition were explained by geographical distance, closely followed by the C:N ratio and N. Our results show that, in these Mediterranean pine forests, soil parameters are the most important driving forces shaping soil fungal communities at the regional scale given that ectomycorrhizal and saprotrophic fungi were more influenced by soil physico-chemical parameters or geographical distance than by Pinus species or forest structural variables. Finally, P content in soils also emerged as a significant factor driving differences in mycorrhizal communities.

This work was supported by the Spanish Ministry of Science, Innovation and Universities, grant RTI2018-099315-A-I00. I.A. was supported by a Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Cofund grant agreement No. 801596, J.G.A. was supported by the Ramon y Cajal fellowship (RYC-2016-20528) and J.A.B. benefitted from a Serra-Húnter Fellowship provided by the Generalitat of Catalunya.