Effects of Nitrogen Addition on Plant Functional Traits in Freshwater Wetland of Sanjiang Plain, Northeast China

To clarify the responses of plant functional traits to nitrogen (N) enrichment, we investigated the whole-plant traits (plant height and aboveground biomass), leaf morphological (specific leaf area (SLA) and leaf dry mass content (LDMC)) and chemical traits (leaf N concentration (LNC) and leaf phosphorus (P) concentration (LPC)) of Deyeuxia angustifolia and Glyceria spiculosa following seven consecutive years of N addition at four rates (0 g N/(m²·yr), 6 g N/(m²·yr), 12 g N/(m²·yr) and 24 g N/(m²·yr)) in a freshwater marsh in the Sanjiang Plain, Northeast China. The results showed that, for both D. angustifolia and G. spiculosa, N addition generally increased plant height, leaf, stem and total aboveground biomass, but did not cause changes in SLA and LDMC. Moreover, increased N availability caused an increase in LNC, and did not affect LPC. Thus, N addition decreased leaf C:N ratio, but caused an increase in leaf N:P ratio, and did not affect leaf C:P ratio. Our results suggest that, in the mid-term, elevated N loading does not alter leaf morphological traits, but causes substantial changes in whole-plant traits and leaf chemical traits in temperate freshwater wetlands. These may help to better understand the effects of N enrichment on plant functional traits and thus ecosystem structure and functioning in freshwater wetlands.