Removal Mechanism Of Nitrogen And Phosphorus By Salt-tolerant Emergent Macrophyte And The Community Characteristics Of Rhizosphere Ammonia-oxidizing Microorganisms

In recent years, the constructed wetlands has been increasingly used for Tianjin coastal area restoration of eutrophic water and wastewater treatment due to having the advantages of less investment, samller energy consumption, higher ecological benefits. However, in the practical engineering, the constructed wetlands could not achieve the ideal effect due to the higher salt concentration in surface water and sewage. This paper according to the Special Funds for Public Industry Research Projects of the National Ministry of Water Resources “Key techniques of aquatic plants ecological restoration in brachish water body” requirements, Screening the emergent and floating-leaved aquatic plants used for Tianjin coastal constructed wetlands, studying the effect mechanism of salt concentration in water on plant physiological and biochemical characteristics and nitrogen(N) and prosphorus(P) degradation process, exploring rhizosphere ammonia-oxidating microorganisms distribution characteristics, which provided the technical supports and theoretical basis for engineering construction of the saline eutrophic water and wastewater treatment through constructed wetlands in coastal area.Based on literature analysis and field research, salt tolerance of seven types of emergent aquatic plants and two kinds floating-leaved aqutic plants was chose to assess according to The Survival Threshold of plants under hydroponic condition.(Salt concentration was represented as NaCl content in water(%), The Survival Threshold referred to the water salt concentration that triggers the death of half of the plant species grown in water). The results showed that Phragmites australis, Lythrum salicaria and Typha angustifolia were the most salt-tolerant plants, and The Survival Threshold was 1.0% salinity. The Survival Threshold of Scirpus tabernaemontani, Iris pseudacorus, Sparganiaceae Hanin and Nymphoides peltatum was 0.5% salinity, and the survival rates were beyond 70%. However, Pontederia cordata and Nymphaea tetragona could not survive at 0.5% salinity due to poor salt-tolerance. The static hydroponic degradation test showed that the N and P degradation capacity was proportional to the salt-tolerence at higher salt concentration, and the N and P degradation capacity of emergent aquatic plants was better than floating-leaved aquatic plants.Base on the above screening results, three types of emergent aquatic plants(Lythrum salicaria, Typha angustifolia and Iris pseudacorus) were chose to investigate stress response of the plants to different salinity(0.05%、0.5% and 1.0%), and the changes of two main sensitive indicators(proline and malondialdehyde) under salt stress was analyzed and showed that the two indicators were not only closely related to the healthy growth of aquatic plants, but also negatively correlated with the degradation rate of the nutrients at high salinity around 1 week. As a result, the proline and malondialdehyde could act as early warning and monitoring factors of wetlands plants under salt stress. Moreover, the results showed that the superoxide dismutase, catalase and peroxidase activity of three aquatic plants increased to varying degrees with increasing salinity, but the enzyme activity were differential among different plant species.The N and P removal mechanism of Lythrum salicaria, microorganisms in water and rhizosphere microorganisms in aquatic microcosm at different salinity was studied by hydroponic test. The results showed that Lythrum salicaria absorbtion of N was the dominating way to remove N, followed by rhizosphere microbial degradation, and degradation of microorganisms in water was the smallest in freshwater(0.05%); Rhizosphere microbial degradation of P was the dominating way to remove P, followed by Lythrum salicaria absorbtion, and degradation of microorganisms in water was the smallest in freshwater(0.05%). However, in brackish water(0.5%), Rhizosphere microbial degradation of N was the dominating way to remove N, and degradation of microorganisms in water was the dominating way to remove P. The capacity of Lythrum salicaria absorbtion and degradation of microorganisms in water of N and of three kinds of removal action of P in brackish water were less than those in freshwater.Degradation of N and P, aquatic plants growth and variation of the performance of fillter surface in simulated vertical flow constructed wetlands(VFCWs) at different salinity(0.05%、0.5% and 1.0%) had been studied, and the results indicated that the inhibition effect of the increase of the salt concentration on TN removal was not significant. Compared with 0.5% salinity, the 1.0% salinity restrained significantly NH4+-N, NO3--N and TP removal. Moreover, the height and the branching numbers of Lythrum salicaria decreased with increasing salinity during the operation of the VFCWs, and the root surface area, root tips and root forks of Lythrum salicaria were sensitive to salinity. Na~+ content of aboveground parts of Lythrum salicaria increased with increasing salinity, this phenomenon indicated that Lythrum salicaria have the capacity of gather salt. Moreover, N, P and Na~+ content of aboveground parts of Lythrum salicaria was higher than underground parts at different salinities, and N content of Lythrum salicaria and P content of aboveground parts decreased significantly with increasing salinity. The uptake rate of N was higher than P by Lythrum salicaria, and the increase of Na~+ concentration in Lythrum salicaria accompanied with the reduction of N and P removal rate with increasing salinity, which demonstrated that Na~+ accumulation in restrain the uptake of N and P. The weight percentage of Na on ceramsite surface was proportional to the salt concentration of influent, and the salt in water would deposit on the ceramsite surface, which affect the performance of the filler surface, microbial attachment and its purification ability.Molecular biology methods were applied to construct the AOB and AOA amoA gene clone libraries by using amoA gene as a specific marker. In total, 18 amoA gene clone libraries were obtained. From all clone libraries combined, 18 and 47 unique bacterial and archaeal amoA gene OTUs respectively were recovered defined at a 2% distance cut-off. Phylogenetic analyses showed the 18 OTUs of bacteria amoA gene sequences recovered in the VFCWs were distributed into Nitrosomonas, Nitrosospira and undefined bacterial species, and Nitrosomonas was the dominant species. The 47 OTUs of archaeal amoA gene sequences were distributed into Nitrosoarchaeum-like, Nitrososphaera-like and undefined archaeal species. The effect of salt concentration and rhizosphere effect on the OTUs species in AOB amoA gene clones library was smaller. The effect of salinity on AOB and AOA amoA gene copy numbers in rhizosphere sediment was less than in non-rhizosphere sediment, and the AOB and AOA amoA gene copy numbers in non-rhizosphere sediment decreased with the increasing salinity. The rhizosphere AOB and AOA amoA gene copy numbers at 0.5% and 1.0% salinity was higher than that at 0.05% salinity(freshwater), and having the largest number at 0.5% salinity. Moreover, A strong positive relationship was observed between the potential nitrification rate and bacterial and archaeal amoA gene copy numbers.