Impacts of mycorrhizal types, tree diversity, and species identity on the soil microbial genomic functional potential...

Soil microbial genomic functional potential refers to the community's collective genetic capacity to mediate nutrient cycling processes involving carbon (C), nitrogen (N), and phosphorus (P). While the drivers of microbial community composition have been extensively studied, particularly in forest ecosystems, the effects of tree species identity, mycorrhizal type, and their mixture along tree diversity gradients on microbial genomic potential remain poorly understood. We predicted microbial genomic functional potential for carbon (C), nitrogen (N), and phosphorus (P) cycling genes, along with their ratios (C:N, C:P, N:P), using fungal ITS2 and bacterial 16S rRNA metabarcoding data from the rooting zone soil samples of eight tree species growing in the MyDiv experiment. We assessed the effects of a tree species identity, plot mycorrhizal type (mono: AM or ECM and mixed: AE = AM + EcM), and tree diversity levels (one, two, four) on the microbial C, N, and P cycling genomic potential and their ratios. We also tested the relationship of microbial diversity, microbial beta-diversity, and soil C:N:P stoichiometry with microbial C, N, and P cycling genomic potential and their ratios. Two-way ANOVAs showed that mycorrhizal type mixture consistently influenced the fungal and bacterial C, N, and P cycling genomic potential and their ratios, while tree diversity significantly affected fungal C and P genomic potential, as well as bacterial C and C:P ratio genomic potentials. Tree species identity impacted fungal genomic potential, particularly in AM and AE types, whereas bacterial functional potentials were significantly linked to tree diversity across AM, AE, and EcM plots. Fungal richness decreased significantly with C and P genomic potential in EcM plots but increased with N genomic potential in AM plots, showing contrasting effects on genomic potential ratios, while bacterial richness showed no significant relationship. Fungal and bacterial beta-diversity were both positively associated with C and P cycling genomic potentials in EcM and AE plots. The C:N and C:P genomic potential ratios increased, and N:P ratios decreased with increasing fungal beta-diversity in EcM and AE plots. Similar patterns were observed for bacterial genomic potential across all AM, EcM, and AE plots. Overall, soil C:N:P stoichiometry showed weak and non-significant correlations with microbial C, N, and P cycling genomic potential. Our findings highlight that plot mycorrhizal type mixture, tree species identity, and tree diversity shape the microbial C, N, and P cycling genomic functional potential. These results also underscore that community turnover, taxonomic replacement, and compositional restructuring of taxa, rather than merely overall richness, are the primary drivers of functional diversification in carbon, nitrogen, and phosphorus cycling among microbial communities.IMPORTANCEOur study suggests that forest management strategies fostering microbial community diversity through mixing tree species with different mycorrhiza types and increasing tree diversity could enhance soil microbial functions, increase nutrient cycling, and improve forest ecosystem resilience and productivity.