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Water is a key element in building and maintaining regional ecosystems functions, particularly in arid and semi-arid regions, in which ecosystem productivity, surface energy balance, and water availability are closely interconnected. However, the effects of differences in soil moisture levels at different depths and humidity index involved MAT and MAP on plant diversity have received only scant attention. To determine effects of soil moisture and humidity index on both plant productivity and diversity of native grasslands on the Loess Plateau, China, a gradient analysis of soil moisture and above- and below-ground properties of plant communities showed that vegetation cover and above- and below-ground biomass were significantly and positively correlated to the levels of soil moisture. More specifically, the below ground biomass was significantly and positively correlated to moisture in the top 10 cm layer of soil, whereas plant height, litter biomass, and root-to-shoot ratio were unrelated to the level of soil moisture. Plant diversity (richness index and the Shannon-Wiener diversity index) was significantly and positively correlated to a greater degree with moisture in the topsoil (0-5 cm layer) than in the subsoil, and was also closely correlated to the humidity index. Overall, changes in plant diversity in arid and semi-arid grassland ecosystems are closely determined by soil moisture and atmospheric humidity, and that higher moisture content in soil and higher atmospheric humidity favour greater plant diversity. (C) 2016 Elsevier B.V. All rights reserved.