Effects of flooding duration on wetland plant biomass: The importance of soil nutrients and season

Extensive studies have shown that the relationship between flooding duration and wetland plant biomass is context specific. The underlying mechanisms, however, are uncertain. A likely explanation is that flooding has both direct effects by exerting stress on plants and indirect effects by affecting soil nutrient availability. The net effect would be context-specific and might depend on season. Here, we tested two potential factors mediating the flooding effects on wetland plant biomass: soil nutrient availability and season. We carried out a field investigation in the lakeshore meadows of the Poyang Lake floodplain, China, along a flooding duration gradient during the non-flooding period. To test the interaction between flooding duration and soil nutrients, we carried out a nutrient (N and P) addition experiment in two types of communities with different flooding durations. The above-ground biomass (AGB) decreased with flooding duration in winter but had a quadratic response in spring, suggesting that the direct negative effect of flooding was stronger in winter than in spring. Soil total N content, total P content, and N:P ratio showed quadratic responses to flooding duration. Our results supported the hypotheses that the influence of nutrient limitation decreased with flooding duration and was greater in spring than in winter. The evidence showed that, with longer flooding duration and in winter than in spring, the correlations between AGB and soil nutrient content were weaker, leaf N and P content of Carex cinerascens were stronger, and AGB response ratio to nutrient addition was lower. Meanwhile, the relative importance of N versus P to wetland plant biomass varied with flooding duration. Consequently, indirect effects of flooding duration on AGB via soil nutrient availability were much less important than direct effects in winter or with long flooding duration, but were more important in spring or with short flooding duration. Therefore, the effects of flooding on wetland plant biomass depended on flooding duration, soil nutrient availability, and season.