Responses of soil multifunctionality, microbial diversity, and network complexity to tree species mixing in Eucalyptus plantations

Mixed afforestation has the potential to enhance soil biodiversity and sustain multiple ecosystem functions (i.e., multifunctionality). Microbial diversity is essential in supporting soil multifunctionality (SMF) in forest ecosystems. However, the patterns and drivers of SMF and the role of microbial communities in regulating SMF among different stand types remain virtually unknown. Here, an 11-year field experiment was conducted to evaluate the effects of mixtures consisting of either one N2-fixing or one non-N2-fixing tree species on SMF and microbial community across Eucalyptus stands. Mixtures with either N2-fixing (0.54) or non-N2-fixing species (0.21) presented higher SMF values than did the pure stand (-0.75). Bacterial alpha diversity and network complexity significantly increased with tree species mixing. Compared with other stand types, mixing of N2fixing species presented greater fungal alpha diversity. Microbial alpha and beta diversity, Proteobacteria, and network complexity were strongly and positively linked with SMF, whereas Chloroflexi was highly negatively related to SMF. Random forest modeling indicated that Chloroflexi, network complexity, and bacterial beta diversity were the primary predictors of SMF. Variance partitioning analysis verified that microbial diversity was a major contributor to SMF, accounting for 31 % of the variance. Piecewise structural equation modeling further revealed that linkages between soil properties, microbiome, and SMF were strongly affected by forest type. Additionally, microbial diversity indirectly and positively impacted SMF by promoting network complexity. Altogether, these findings indicated that mixed afforestation positively affected SMF by facilitating microbial diversity and network complexity. Hence, this research highlights the positive ecological implications of tree species mixing as well as the importance of microbial network complexity and diversity in maintaining SMF, helping to elucidate the mechanism of microbially driven SMF to maximize the potential of SMF in plantation ecosystems and providing mixed species candidates for optimizing afforestation strategies.