Study On Water Purification Ability And Decomposition Of Salt-tolerant Wetland Plants

With the development of marine resources and the rapid development of economy and society,the saline and eutrophication problems of water bodies in China are prominent,threatening the safety of water resources,affecting water body aesthetics and resource utilization.In recent years,wetland plants have played an increasingly important role in ecological restoration and water landscape construction.However,when plants enter the decline period,the nutrient elements produced by decomposition of plant residues are liable to cause secondary pollution of water bodies.Selecting wetland plants with good salt tolerance and purification effect and discussing the management scheme of plant residues are helpful to provide theoretical and practical technical guidance for ecological restoration of saline eutrophic water bodies.Based on the investigation of typical wetland plants and their growing environment in the estuary and coastal zone of Eastern Jiangsu Province,laboratory experiments were carried out on Scirpus validus,Thalia dealbata,Iris ensata var.hortensis,Typha orientalis and Lythrum salicaria with good growth potential and good landscape effect.The salt tolerance of five wetland plants under different salinity was studied by hydroponic experiment,and two wetland plants were selected to compare the purification effect of water quality.Subsequently,the changes of water quality and the transformation of nutrient elements in plant residues after decomposition were further explored under different salinity and biomass conditions.The results show that:?1?Under salt stress,all five plants have salt tolerance and can be used for ecological restoration of wetlands with salinity below 3.5‰.In terms of plant growth indicators,Scirpus validus and Lythrum salicaria were more adaptable to salt stress in five wetland plants.The physiological and biochemical indices of Scirpus validus and Lythrum salicaria were further studied.The results showed that the growth adaptability of Scirpus validus under salt stress was better than that of Lythrum salicaria,and its salt tolerance was stronger by comparing the growth indexes?relative growth rate,increment of plant height,etc.?and physiological indexes?relative water content,relative conductivity,MDA content,proline,SOD activity,etc.?.In low salt environment,root is the pioneer organ for withholding Na⁺in Lythrum salicaria,and salt tolerance of Scirpus validus can accumulate salt in the body without being harmed.?2?By comparing the water purifying ability of Scirpus validus and Lythrum salicaria under different salinity,a slight salinity?2.5‰?was conducive to the removal of ammonia nitrogen,total nitrogen and total phosphorus of the two plants.The removal rates of phosphorus and ammonia nitrogen of Lythrum salicaria were higher under low salinity.Under higher salinity,Scirpus validus grew better,whose removal rates of nitrate nitrogen,total nitrogen and phosphorus were 39.5%-55.0%,33.6%-38.5%and 63.9%-90.2%respectively.Under 0‰-5‰salinity,Lythrum salicaria and Scirpus validus could be used as alternative plants to purify water.Scirpus validus was the preferred choice in remedying eutrophic water above 5.0‰salinity.?3?In the decomposition experiment of Scirpus validus,the nutrient elements in water body showed a rapid rise in the early stage and a decrease in fluctuation in the later stage,and the release rate of phosphorus was higher than that of nitrogen.The effects of different salinity conditions on water pH,DO,NO₃^--N,NH₄⁺-N concentration in the decomposition process reached significant levels.When salinity was 7.5,microbial activity would be disturbed and the decomposition process of aquatic plants would be inhibited.High salinity was not conducive to microbial nitrification,nitrogen fixation and phosphorus binding,so the nitrogen and phosphorus content of plant residues were the lowest.Salinity below 5 had no significant effect on the decomposition rate,tannin and nitrogen content of residues.?4?Under different biomass densities,the decomposition process of Scirpus validus had different effects on water quality.When the amount of residue was 0.3g/L,the nitrogen fixation of plant residue microorganisms could be promoted,and the concentrations of TN,NO₃^--N and NH₄⁺-N in water would be reduced to varying degrees.Therefore,the retention of appropriate plant residues in water can effectively reduce the nitrogen load of the water environment to a certain extent.Under high biomass density?0.6-0.9g/L?,the concentration of nitrogen and phosphorus and oxygen consumption were higher,especially the concentration of total phosphorus increased rapidly.Therefore,attention should be paid to timely salvage of wetland litter and control the total amount of litter in wetland environment.