Plant Root Exudates And Their Function On The Nitrogen Removal In Constructed Wetlands

Nitrogen?N?is a pollutant that causes eutrophication in surface water.Non-point source pollution affects a water body from source such as polluted runoff from agricultural areas draining into a river,which with N and carbon?C?pollutants occur at relatively low concentrations(typically less than 10 mg L^-1),and nitrate is the main existence form.Pollution concentration is far lower than the domestic sewage,belong to micro-polluted water,but the pollution load draining into lake is very high,its purification is an important content of eutrophication control.Constructed wetlands?CWs?,have been widely used to treat micro-polluted water.In CWs,biological nitrification followed by denitrification is believed to be the major pathway for N removal.Most denitrification is accomplished by heterotrophic bacteria,the biochemical reaction is strongly dependent on C availability.Generally,C deficiency makes biological treatment more difficult.Plants can convert atmospheric CO₂ into biomass through a number of pathways such as the death and decomposition of plant litter and the secretion of root exudates.Root exudates are composed of lots of chemical constituents,which added in some studies were consider to the major C sources for denitrification,therefore,it is important to elucidate the availability of exudates,and particularly their effects on the activity of denitrifying bacteria.Many research aimed to detect the effects of plant root exudates on microbial activity,biomass,and ecological distribution in field of phytoremediation?eg,complexation or activation of metal ions to promote plant uptake?.However,studies focused on inorganic nutrient?such as NO₃^--N and NH₄⁺-N?are little.In order to add to the pool of information on root exudates effects on microbial denitrification and provide knowledge about the potential N removal mechanism in CW,we analyzed root exudates of macrophyte and their function on denitrifier gene abundance,community structure and activity in micro-polluted CW.The main studies of this paper were as follows:1.The form of inorganic N affects macrophyte growth and their root exudates.Three macrophyte species,Typha augustifolia,Phragmites australis,and Acorus calamus L.were cultivated in hydroponic cultivation systems fed with micro-polluted water to investigate the plant growth and root secretion.The results indicated NH₄⁺-N and NO₃^--N all met the need of the plants growth and higher NH₄⁺-N is conducive to plant biomasss accumulation,meanwhile there exists a linear relationship between plant biomass and TN removal efficiency,with positive significance?r=0.946,p<0.05?.It is also found that NH₄⁺/NO₃^-ratio could influence plant biomass and root exudates significantly.Additionally,organic acids and soluble sugars were the hydrogen donors and sources of energy in denitrification in this study.Succinic acid,malic acid,and tartaric acid were the main organic acids,while arabinose and glucose were the main soluble sugars.2.The effects of root exudates for the denitrification process in micro-polluted water was analysed by testing denitrifying microorganism concentration and the intensity of denitrificaion.The root exudates of Typha augustifolia,Phragmites australis,and Acorus calamus L.were studied for their differences.The average release rates were0.52,0.26 and 0.60 mg g^-1 DM d^-1,respectively.Three kinds of plant root exudates as auxiliary C source or only C source for denitrifying bacteria can promote the growth of denitrifying bacteria,of which Typha augustifolia had the greatest influence,and its composition is more conducive to the growth of denitrifying bacteria and denitrification.A variety of environmental factors have impacts on root exudates,when the C/N ratio was 10:1,plant root exudates had the highest concentration,while at the same C/N ratio,higher COD and TN were more conducive to plant growth and secretion secretion.3.In view of Dengbeiqiao wetland in erhai lake basin,the denitrification process was analyzed in plant root zone.Root exudates can act as endogenous carbon sources for heterotrophic denitrifying bacteria and ultimately determine the microbe distribution patterns in micro-polluted CW;the abundances of nirK and nirS encoding bacteria were significantly influenced by the concentration of sucrose and glucose,and microbial community richness and diversity had response to sucrose and oxalic acid.In theory,each species could fuel a denitrification rate of approximately 156-841 kg NO₃^--N ha^-1year^-1 if all were used by the denitrifying bacteria.4.Mixtures of artificial exudates were added to CW microcosms containing micro-polluted water to explore the promotion mechanism of denitrification.Organic acid and soluble sugar treatments promoted N removal,while amino acids were negative to N removal,when amino acids were added,denitrification depended TOC.The number of denitrifying bacteria was promoted and the microbial community was changed when artificial exudates were added,but there were no significant relationships between exudate compounds and bacterial community composition.Organic acids mainly affects nirK bacteria genera,soluble sugar mainly affects nirS bacteria genera.C source with electronic material?tartaric acid?and the easy to degradation of organic matter?glucose?are more likely to be using denitrifying process.5.Based on the analysis of nitrogen transformation process,an eco-dynamic model based on nitrogen removal mechanism,nitrogen mass balance equations and formulas for each reaction was established by using system dynamics software-Stella in Dengbeiqiao CW.The simulation results showed that the calibrated model could generally present the concentration variations for N pollutants.When TOC parameter was added to the eco-dynamic model,the model efficiency of simulating was improved,NH₄⁺-N?NO₃^--N?ON were 56.4%,61.1%and 53.0%,respectively?...