Effect Of Plant Diversity On Nitrogen Removal In Constructed Wetlands

As a simulated natural ecological engineering technology, constructed wetland was used for sewage treatment; its application has attracted more and more attention. Constructed wetland in sewage treatment is inseparable from the system’s complex interactions between the plant, microbial. matrix and waste water. As the most important component in constructed wetland, plants play an important role in nitrogen removal of the system, including direct and indirect purifying effect. Mechanisms of removal by plants also need in-depth study. and the roles of plant species diversity in constructed wetland’s ecological functioning still have a lot of controversy.In order to test plant diversity effect on the system’s ecological functioning (including nitrogen removal efficiency, productivity, and denitrification intensity), we carried out an experiment with simulated constructed wetland. The experiment consisted of two treatments supplied with nitrate as the sole source of dissolved combined nitrogen, and mixed nitrogen resources of nitrate and ammonium respectively. Each set of nitrogen treatment include sand substrate and hydroponic system. In the above four treatments, we arranged plant species richness1.2.3and4species (and set no plant as control) in the simulated constructed wetlands. The results show that:(1) Plant biomass increased with increasing plant species richness in nitrate nitrogen supply treatments. The performance was different between the sand substrate and hydroponic system, since plant aboveground biomass increased with increasing plant species richness in the sand substrate, but the belowground biomass increased with increasing plant species richness in the hydroponic system (P＜0.05). There was no significant correlation between plant species richness and plant biomass in the mixed nitrogen treatments of nitrate and ammonium. In every treatment, the species composition had significant effects on the plant biomass. the presence of Coix lacryma-jobi can enhance the community productivity, and it also has the strongest competitive advantage in mixed planting systems.(2) The effluent nitrate and total inorganic nitrogen concentration in the treatment of sole nitrate was significantly reduced with the species richness (P＜0.05), significantly improved the nitrogen removal efficiency of the wetland system. Except the mixed nitrogen supply hydroponic treatment, the system’s nitrogen removal efficiency in4species mixture was better than monoculture treatment (P＜0.05). The species composition had significant effects on the effluent nitrogen, and the presence of Coix lacryma-jobi can significantly reduced the effluent nitrate of the svstem.(3) The substrate total inorganic nitrogen retention did not shown response to the plant diversity in different treatments of nitrogen form (P>0.05). but the species composition has a significant influence on the substrate nitrate concentration, system with Phragmites australis reduced the substrate nitrate and ammonium concentration even with lower biomass. Ammonium was the main nitrogen form stored in the sand substrate.(4) The main factors contributed to nitrogen removal in the system are plant uptake, substrate retention, and microbial denitrification. Through the mass balance analysis, piat uptake and microbial denitrification were most important processes in nitrogen removal for this study. Because the plant uptake at different diversity had no significant differences, therefore, we infer that the microbial denitrification plays a very important role in the enhancement of plant diversity in nitrogen removal.(5) On the whole, plant biomass in sand treatment was higher than hydroponic system, but the differences between the two are not significant (P>0.05):nitrogen removal efficiency in sand system was significantly higher than the hydroponic system, that should be the comprehensive contribution results of substrate retention and microbial activity. Substrate retention in the system was very low. so microorganisms in this study might be of a relatively important contribution in nitrogen removal.With comprehensive study of the above, plant diversity has important influence on wetland ecosystem functioning, including productivity, nitrogen removal, and substrate inorganic nitrogen. In this research, we have summarized the various contributing factors in constructed wetland’s nitrogen removal and put forward some optimizing measures for its improvement. We intend to provide experimental support for theoretical research in the relationship between plant biodiversity and nitrogen removal in constructed wetland. Findings from this study gave some suggests on how to achieve higher nitrogen removal by using mixed planting of multiple species with high nutrient removal efficiency in constructed wetlands.