Grassland afforestation more than forestry intensification shapes soil multifunctionality via microbial compositional change under abiotic constraints

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Soil ecosystem multifunctionality (EMF) is driven by the interplay of abiotic and biological factors, yet how these interactions respond to anthropogenic pressures remains poorly understood. Here, we evaluated how grassland afforestation and its intensification shape soil edaphic conditions, microbial diversity, and EMF along a 200 km grassland-eucalypt plantation transect in Argentina. EMF was estimated, accounting for six ecosystem functions related to nutrient provisioning, organic matter cycling, and pathogen control. Microbial diversity was studied through the taxonomic, functional, and phylogenetic dimensions of prokaryotes, mycorrhizae, and fungal saprotrophs. Abiotic and biotic drivers of individual ecosystem functions and EMF were assessed using correlations, linear mixed models, structural equation models, and Multiple Regressions on distance Matrices. Individual ecosystem functions responded differentially to environmental drivers: functions linked to soil physicochemical processes were primarily associated with edaphic conditions, whereas biologically mediated functions were more closely linked to climate and grassland afforestation. Soil multifunctionality, however, was driven by edaphic and climatic conditions, particularly soil sand percentage and precipitation, with no direct association with microbial alpha diversity or afforestation. In contrast, similarity in fungal composition explained similarity in EMF, suggesting a coupling between microbial composition and soil conditions associated with grassland afforestation. Grassland conversion to commercial forest, rather than forestry intensification, altered individual soil functions and microbial functional composition without further reducing EMF. Overall, our findings indicate that afforestation influences soil EMF through changes in microbial composition, but that these effects are constrained by abiotic drivers.

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