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With rapid changes in climate conditions and grazing regimes, a decline in soil quality has become a main challenge for sustainability of Tibetan rangeland. Increasing evidence highlights the significance of plant biomass, but few studies test the role of realistic loss of plant diversity in impacting soil quality. We hypothesized that loss of species diversity will decrease soil quality, not only through lost standing biomass (i.e., "mass effect") but also directly via the loss of the "diversity effect". Considering the realistic loss of species diversity following overgrazing, grazing exclusion and climate change, we experimentally removed grass, sedge and various forb species from a Tibetan alpine meadow to test this hypothesis. After target plant species had been removed for five years, we examined the effect of the species removal on the plant community and soil quality with Bayesian linear mixed effect models and inferred potentially causal linkages between plants and soil with a Bayesian network. Results showed (i) overall, plant species removal not only significantly decreased plant diversity and standing biomass but also decreased soil organic matter and nutrients; (ii) the standing biomass decreased more pronouncedly in response to the removal of sedge and grass species, but plant diversity and soil nutrients decreased more pronouncedly in response to the removal of various forb species; and (iii) the loss of species diversity rather than standing biomass was directly associated with the decline in the-soil quality, especially a decrease in soil available P. In short, our results indicate that realistic loss of plant species diversity can decrease soil quality in a Tibetan alpine meadow through lost the "diversity effect". The functional significance of plant species diversity in maintaining soil quality supports the expectation of local pastoralists that diverse plants, promoted by traditionally rotational grazing, can ensure the sustainability of alpine soil and Tibetan rangeland.