Effects Of Plant Diversity On Productivity And Inorganic Nitrogen In Constructed Wetlands

The relationship between biodiversity and ecosystem function is one of the most important scientific problems in the ecological researches. Substrate nutrient level will affect the relationship between plant diversity and ecosystem function. The studies between plant species richness and ecosystem functions under high N supply condition have been conducted in recently two years. We don't know much about how the plant diversity affected ecosystem functioning (such as productivity, substrate N, substrate solution N, plant N pool sizes and so on) when plants grow in high N condition and among different high N levels.This subsurface vertical-flow constructed wetland (SVFCW) experiment was focused on how the plant diversity affected the productivity and N concentrations in high N condition. We conducted two full-scale SVFCWs in Dongyang and Zhoushan city, Zhejiang Province in Southeastern China, with high N supply. The planted plots in Dongyang study were transplanted with different species (1,2,3 or 4 species) and were designed four different nitrogen levels (146,122,99 or 48 g N m-2 yr-1). The aim of this study was how the plant diversity affected the plant productivity and substrate N, substrate solution ("effluent") N concentrations in high N condition and different N levels. Also, the planted plots in Zhou Shan study were transplanted with species at different plant diversity (SR=1,2,4,8 or 16 species, FR=1,2,3,4), which were mainly native to the subtropical region, representative of mesophytic species found under subtropical monsoonal climate. The goal of this study was how the plant diversity affected the plant above- and belowground biomass and substrate N concentrations and plant N pool and biomass:N ratio (N use efficiency, NUE) in high N condition (409 g N m-2 yr-1). The main results show as follow:1. Increasing plant species richness increased plant belowground biomass and belowground N pool, but the aboveground biomass and aboveground N pool was not related to plant species richness in high N condition (409 g N m-2 yr-1). It suggested that belowground plant components played very important roles in biomass and N pool accumulation.Increasing plant species richness decreased substrate inorganic N concentrations in high N condition (409 g N m-2 yr-1). Substrate ammonium decreased fasted than substrate nitrate concentrations, suggesting that increasing plant species richness promote nitrification. The index DT NO3 and DT NH4 with averages notably below zero suggested that plant mixtures could remove more substrate nitrate or ammonium than the plot of monoculture. None of the plant above- and belowground biomass to N ratio (biomass:N ratio) was affected by plant species richness. It suggesting that plant species richness doesn't affect plant NUE in high N condition. This luxury comsumption of N was beneficial for increasing wastewater treatment efficiency in constructed wetlands.Functional group richness doesn't affect plant biomass and N pool and plant NUE in high N condition. However, different functional groups have different effect on the functions of ecosystem. The presence of C3 grasses increased above- and belowground biomass and N pool sizes in high N condition (409 g N m-2 yr-1). In general, C4 grass species had the largest biomass:N ratios in comparison to forbs and C3 grasses. Legumes had the lowest biomass:N ratios. And the presence of Arundo donax and Saccharum arundinaceum increased above- and belowground biomass in high N condition (409 g N m-2 yr-1). The index DMax in all mixed planting plots was lower than zero, i.e., plant biomass in most mixed communities was lower than the highest yielding monoculture of the species contained in the mixture. It also showed that the selection effect may play very important roles in high N condition (409 g N m-2 yr-1).2. We also found that increasing plant species richness increased plant aboveground biomass in high N condition (146 g N m-2 yr-1). The proportional deviation index DMax in all mixed planting plots was lower than zero, i.e., plant biomass in most mixed communities was lower than the highest yielding monoculture of the species contained in the mixture in high N condition (146 g N m-2 yr-1).We also found that increasing plant species richness decreased substrate and substrate solution ("effluent") inorganic N concentrations in high N condition (146 g N m-yr-1). The index DT NO3 and DT NH4 with averages notably below zero suggesting that plant mixtures could remove more substrate nitrate or ammonium than the plot of monoculture.3. The two species composition of "Ca+Ad"(Cyperus alternifolius+ Arundo donax) had the largest plant aboveground biomass and the least substrate and substrate solution ("effluent") inorganic N concentrations, While the composition of "Ca+It" (Cyperus alternifolius+ Iris tectorum) had the least plant aboveground biomass and the largest substrate and substrate solution ("effluent") inorganic N concentrations in high N condition (146 g N m-2 yr-1). The two species composition of "Ca+Ad" (Cyperus alternifolius+ Arundo donax) had the largest plant aboveground biomass and the least substrate and substrate solution inorganic N concentrations, While the composition of "Cl+Ls" (Coix lacryma-jobi+Lythrum salicaria) had the least plant aboveground biomass and the largest substrate and substrate solution inorganic N concentrations in high N condition (99 g N m-2 yr-1). All the two species composition (except of the composition including Iris tectorum) had the larger plant aboveground biomass and the smaller substrate inorganic N concentrations than relevant species monoculture at the same N condition. It suggested that plant mixtures were beneficial for increasing wastewater treatment efficiency than species monoculture.The three species composition of "Ca+Cl+Ad" (Cyperus alternifolius+ Coix lacryma-jobi +Arundo donax) had the largest plant aboveground biomass and the least substrate inorganic N concentrations, While the composition of "Ca+Ls+Ad" (Cyperus alternifolius+Lythrum salicaria +Arundo donax) had the least plant aboveground biomass and the largest substrate inorganic N concentrations in high nitrogen condition (146 g N m-2 yr-1). Substrate solution inorganic N in all the three species compositions has no difference in high N condition (146 gN m-2 yr-1).The four species composition of "Ca+Cl+Ls+Ad" (Cyperus alternifolius+ Coix lacryma-jobi+Lythrum salicaria+Arundo donax) had the largest plant aboveground biomass in high N condition (146 g N m-2 yr-1). However, substrate and substrate solution inorganic N in all the four species compositions has no difference in high N condition (146 g N m-2 yr-1).4. All the two species composition (except of the composition Arundo donax+Iris tectorum) had the larger plant aboveground biomass in high N condition (146 g N m2 yr-1) than that of in N condition (99 g N m2 yr-1). While the substrate and substrate solution inorganic N concentrations had no difference in two N conditions (146 g N m-2 yr-1and 99 g N m-2 yr-1). It suggested that two species composition increased the plant biomass made the effluent inorganic N concentrations had no difference.We also found the same results of many studies in N limitation grasslands when the N supply level was extended to 400 g N N m-2 yr-1, i.e., this study found that increasing plant species richness increased plant biomass, but decreased substrate and "effluent" inorganic N concentrations in high N condition. Increasing plant species richness was beneficial for increasing wastewater treatment efficiency in constructed wetlands.The availability of N in the ecosystem also affects the relationship in high N condition. However, plant species richness doesn't affect plant NUE in high N condition. In a word, the studies would provide new experimental supports for the researches on biodiversity and ecosystem function in high N condition and different high N levels, and had been instructive for plant configurations of constructed wetlands and their managements in the future.