Abstract: Aims Interspecific competition is a key process regulating plant community assembly and productivity, and nitrogen and phosphorus availability are major factors influencing interspecific competitive interactions among wetland plants. However, systematic understanding of the differences between aboveground and belowground competition, especially under nitrogen and phosphorus enrichment conditions, remains limited, hindering predictions of wetland community dynamics. Methods We conducted a greenhouse experiment with two plant species, coexisting and habitat-similar dominant perennial species in the Poyang Lake, Carex cinerascens and Phalaris arundinacea. Experimental treatments included four competition treatments (no competition, aboveground competition [AGC], belowground competition [BGC], and both competition) and four nutrient conditions (control, nitrogen addition [N], phosphorus addition [P], and combined nitrogen–phosphorus addition [NP]). Aboveground and belowground biomass of both species were measured under each treatment to evaluate biomass allocation strategies. Relative interaction index (RII) under different nutrient conditions were calculated to assess the effects of nutrient inputs on interspecific competition. Important findings 1) BGC was the primary driver of growth and biomass allocation, with C. cinerascens showing a strong BGC advantage and maintained overall dominance by allocating more biomass to aboveground parts, whereas P. arundinacea was suppressed by BGC and allocated more biomass to belowground parts to alleviate this suppression; 2) N addition reduced the BGC advantage of C. cinerascens, P addition alleviated the suppressive effects of BGC to P. arundinacea, and NP addition enhanced its competitive advantage while reducing inhibition of P. arundinacea, resulting in increased total biomass; 3) The effects of nutrient inputs on interspecific competition were primarily driven by interactive effects of AGC and P addition on belowground biomass, and by interactive effects of BGC and N addition on aboveground biomass. Conclusions Nutrient addition can alter interspecific competitive relationships between plant species, but only synchronous changes in nitrogen and phosphorus availability can enhance total biomass while maintaining table biomass proportions between the two species . Therefore, under future nutrient-enrichment scenarios, coordinated regulation of nitrogen and phosphorus is critical for sustaining productivity and diversity in wetland plant communities.

Key words: Poyang Lake wetland, above-belowground competition of species, nitrogen and phosphorus addition, aboveground and belowground biomass allocation, wetland plant community