Mechanism Study on the Spatial Distribution Pattern and Vertical Migration of Phosphorus in a Constructed Wetland

Abstract In this work, phosphorus fractionations (PFs) in the surface, overlying and interstitial water, sediments and two macrophytes (reed and cattail) were investigated in a near-nature constructed wetland. The mechanism of vertical migration and transformation, diffusion flux of sediment and overlying water and the risk of phosphorus release from sediments were studied. The results showed that the occurrence of macrophytes influenced the horizontal and vertical distributions of phosphorus. Reed community area resulted in different orders of TP, TDP and SRP concentrations in the surface, overlying and interstitial water as well as higher removal efficiencies than open water area and cattail community area. Phosphorus distributions in reed community area were closely correlated to the environmental factors and phosphorus in the overlying water were significantly correlated to the biomass and morphological characteristics of reeds (p < 0.01). TP in sediments were positively correlated to the OM contents in both macrophytes free and covered area (p < 0.05) and Org-P was negatively correlated to APA in both open water area (p < 0.05) and reed community area (p < 0.01). Phosphorus diffused from sediment into overlying water due to the positive diffusion flux of sediment-overlying water. The degree of phosphorus saturation (DPS) in macrophytes covered area communities was higher while the phosphorus sorption index (PSI) was lower than that in macrophytes free area. The eutrophication risk index (ERI) of wetland sediment showed the risk of phosphorus released from sediment was high, especially in cattail community area. The results of this study provided references for macrophytes management and removal efficiency improvement in constructed wetlands.