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Plant community assembly in wetlands usually changes with elevation gradients, which may be due to the direct effect of flooding and indirect effects such as changes in soil properties and competition. However, the respective importance of each factor remains to be investigated.

We investigated patterns of plant diversity, community biomass and soil properties along an elevation gradient of a lakeshore meadow at Poyang Lake, China.

(i) With increasing elevation, species richness and Simpson diversity index decreased. Both aboveground biomass (AGB) and belowground biomass (BGB) increased with elevation, however, the BGB/AGB ratio also increased, which suggests a significant effect of belowground competition. (ii) Soil N content and soil N:P ratio increased, whereas soil pH decreased with elevation. Other soil properties showed no significant response. (iii) Structural equation modeling showed that variation of plant diversity was mainly explained by BGB. Thus, intensified belowground competition seems to be the primary mechanism causing lower plant diversity at higher elevations. (iv) These findings were further supported by the observed greater response ratio of N and P storage in plant communities than the response ratio of soil N and P content to elevation, suggesting that soil nutrient limitation and belowground nutrient competition increased with elevation. Our study has important implications to wetland management and biodiversity conservation under environmental change (e.g. changes in flooding regimes, eutrophication).