The patterns of species diversity loss upon resource addition in grassland communities can be explained by several competitive hypotheses， including the light competition hypothesis， the niche dimensionality-diversity hypothesis， the nitrogen detriment hypothesis， and the litter hypothesis. Nevertheless， few studies have explored whether the mechanism of species diversity loss can be attributed to the type and number of added resources. To address this question， we conducted a field experiment with different numbers （0， 1， 2， and 3） and types （nitrogen， phosphorus， and potassium） of added resources， with the aim of exploring the four mechanisms of species diversity loss in an alpine meadow of Gannan on the Qinghai-Tibet Plateau. The results yielded the following findings： 1） Compared with the control， nitrogen addition resulted in a notable increase in aboveground net primary productivity， and a reduction in species richness. However， neither phosphorus addition nor potassium addition significantly affected these variables. 2） Both nitrogen addition and the number of added resources had a direct negative impact on species richness. This was related to belowground soil competition that underlies the niche dimension-diversity hypothesis and the nitrogen detriment hypothesis. 3） As plant aboveground net primary productivity increased， light limitation became more intense， and this indirectly led to the loss of species richness， consistent with the aboveground light competition hypothesis. In conclusion， our results indicate that， with the addition of nitrogen or a number of added resources， aboveground and belowground ecological mechanisms act synergistically to drive losses in grassland species diversity， consistent with the multiple competition hypothesis. The findings of this study can serve as a basis for the formulation of an ecological theory that will contribute to the long-term maintenance and conservation of grassland species diversity in alpine ecosystems.
