Soil properties mediate ecosystem intrinsic water use efficiency and stomatal conductance via taxonomic diversity and leaf economic spectrum.

The interactions between plants and soils lead to complex feedbacks that regulate intrinsic water use efficiency (iWUE) and stomatal conductance (gs) at ecosystem level and reflect water constraints on plant productivity. However, the relationships among soil properties, biodiversity, and leaf functional traits contributing to the variability in ecosystem iWUE and gs remain largely unknown. To elucidate these relationships, we used principal component analysis to reduce soil properties to a fertility spectrum and a limiting-resource spectrum across grassland, and early-, mid- and late-successional forests in a karst catchment. Leaf functional traits at community level were calculated based on leaf biomass, and were reduced to an economic spectrum and a limiting-resource spectrum. Leaf carbon (δC) and oxygen (δO) stable isotopes at community levels were used as proxies for ecosystem iWUE and g. The effects of soil properties, biodiversity (taxonomic, functional and phylogenetic diversity) and leaf traits on δC and δO were evaluated using structural equation models. Our results showed that variability in ecosystem iWUE and gs was determined overwhelmingly by indirect effects of soil properties via two different pathways: the soil fertility spectrum, determining the number of coexisting species (taxonomic diversity) and turnover of species (leaf economic spectrum), and the soil limiting-resource spectrum, shaping the specific phylogenetic lineages (phylogenic diversity). In addition, δC and δO were constrained by the interactive effects of leaf economic spectrum, and taxonomic and phylogenic diversity; total effects of biodiversity on δC and δO were larger than those of leaf economic spectrum. Our study highlighted the critical role of the evaluating interaction relationships between leaf functional traits, biodiversity metrics and soil properties in understanding the mechanisms of ecosystem function responding to environmental change.