Abstract: In recent decades, the Sanjiang Plain has been experiencing excessive agricultural land reclamation from wetlands due to the increased demand for food production. Fertilizer applications during agricultural activities have dramatically increased nitrogen (N) and phosphorus (P) inputs to wetlands in these regions, which would markedly affect wetland ecosystem structure and function. In this study, we conducted a 3year nutrient addition experiment to examine the effects of increased N and P availability on plant community composition and structure, and aboveground biomass in a marshy meadow in the Sanjiang Plain, Northeast China. Our results showed that N addition (6 g N·m^-2·a^-1) significantly increased the dominance of grasses and decreased the dominance of sedges, and thus caused a decline in species diversity. Phosphorus addition (1.2 g P·m^-2·a^-1) significantly decreased the dominance of grasses and increased the dominance of sedges, albeit there were no changes in species diversity and aboveground biomass. Additionally, N and P addition did not produce an interactive effect on species diversity and aboveground biomass. Compared with the control treatment, combined N and P addition decreased species diversity, but increased plant aboveground biomass. These results imply that increased N and P availability have inconsistent effects on species diversity and plant productivity in the marshy meadow in the Sanjiang Plain. Moreover, our study suggests that nutrient enrichment would substantially alter species diversity and plant productivity in freshwater wetlands in Northeast China.

Key words: maize, drought stress, photosystemⅡ, chlorophyll a fluorescence (OJIP) transient.