Dam Inundation Modulates the Effect of Plant Diversity on Soil Multifunctionality in the Riparian Zone of the Three Gorges Reservoir

Understanding the biodiversity–ecosystem multifunctionality relationship is critical for predicting the consequences of species loss on the sustainable provision of ecosystem services. Both theoretical and empirical studies generally demonstrate a positive biodiversity–ecosystem multifunctionality relationship. However, the underlying mechanisms linking soil multifunctionality (SMF) to plant diversity remain unclear, particularly in dynamic riparian habitats. In this study, we investigated the plant community, 10 soil functions, and their drivers within the riparian zone regulated by the Three Gorges Dam in China. Our results showed that taxonomic, phylogenetic, and functional diversity affect SMF at alpha and beta scales in both positive and negative ways. Notably, most diversity metrics are negatively correlated with SMF, especially at lower elevations and in areas near the dam. Alpha and beta diversity contribute equally to SMF, whereas functional diversity explains SMF better than taxonomic or phylogenetic diversity. Furthermore, abiotic variables explain 24% of the variance in SMF, significantly exceeding the 3% explained by biotic variables. Dam inundation has both direct effects on SMF and indirect effects mediated by soil pH, bulk density, and functional dispersion, all of which are critical variables in elucidating SMF changes. Our findings indicate that dam inundation modulates the effect of plant diversity on SMF and underscore the roles of biotic factors and functional diversity in mediating this effect. This study challenges the prevalent notion that biodiversity universally positively affects ecosystem multifunctionality and broadens our understanding of the linkages between plant diversity and SMF, as well as its drivers under dam-induced hydrological changes.