Plant competition is a determinant of plant community formation, while resource partitioning is regarded as a critical factor for maintaining species coexistence under competition. However, how resource partitioning varies with species richness remains elusive; additionally, empirical studies of the dynamic processes involved in plant resource competition are urgently needed, especially for belowground resources. Here, we used the stable isotope approach to study the temporal dynamics of plant hydrological niches from species-poor to species-rich communities (including monocultural plantations, agroforestry systems and more diverse tropical rainforests) in a tropical area of southwestern China. We found that plant species in multispecies communities could be divided into two groups by comparison of their hydrological niches: deep-resource users and shallow-resource users. Resource partitioning was obvious between these two groups. However, resource partitioning was narrower within deep and shallow-resource users. In addition, with the increase in species richness, the general tendency of resource partitioning among all species became not just increasingly narrow, but also uniform and stable. When we combined this information with the analysis of the belowground resource distribution and its correlations with plant resource use, we also found that the greater the species diversity and the smaller the resource use advantages among species, the easier it was for the species to achieve “competitive reversals”. This study confirmed the generality of the seasonal segregation of the plant hydrological niche and demonstrated that variations in both environmental resources and plant resource partitioning weaken the resource use advantages among species, representing an important mechanism to help maintain the coexistence of competing species in a species-rich community. These findings therefore provide novel insights for understanding species combinations and plant belowground interactions in complex communities and will be beneficial for exploring solutions to some important ecological issues, such as reconstruction of the tropical rainforest.