| With the wide use of constructed wetland technology in wastewater purification,low carbon-to-nitrogen ratio(C/N)wastewater in constructed wetlands has been paid more and more attention.The carbon source released by decomposition of plant residues has shown great potential in improving C/N and improving the nitrogen removal efficiency in constructed wetland.Harmonizing the contradiction between plant carbon source addition and harvesting plants residues in constructed wetlands,as well as the purification function of wetland plants and risks of secondary pollutants,has become an important part of the popularization and application of constructed wetlands technology.In constructed wetlands,the contradiction can be coordinated by reasonable configuration of plants in different phenological periods.There have been a lot of reports about the decomposition of aquatic plant residues and the influence on water quality,and improving purification and the mechanism by reasonable configuration of plant in different phenological periods.But there are few studies on the interaction between the simultaneous decomposition of emergent aquatic plant residues in the period of wilting and the purification function of cold-season plants in growing period.These studies are of great significance for solving the risks of secondary pollutants caused by aquatic plants and improving the purification efficiency of wetlands.In this study,cold-season plants Oenanthe javanica as floating-bed plant and emergent aquatic plants Phragmites australis as carbon source,four types of constructed floating wetlands systems were established:constructed floating wetlands without P.australis litter added(CK),constructed floating wetlands with P.australis litter added at one time(EL),constructed floating wetlands with P.australis litter added four times(added every 20 days)(TL),constructed floating wetlands with P.australis litter added 16 times(added every 5 days)(FL).Dynamic monitoring of O.javanica growth,water quality,and the abundance of nitrogen cycle functional gene,and exploring the effect of plant residues decomposition on nitrogen removal in wetland system.The aim is to provide theoretical basis for solving the problem of poor nitrogen purification efficiency of constructed wetlands with low C/N influent and management of wetland plants.The results of this study show that:(1)P.australis residues decomposition is beneficial to the growth of floating-bed plant O.javanica.Adding P.australis l:itter treatment could significantly increase the plant height,biomass and chlorophyll content of O.clecumbens.Compared to the control(CK),the aboveground biomass of O.javanica in EL,TL and FL increased by 98 g/m2,55 g/m2 and 70 g/m2 respectively,while the underground biomass increased by 20 g/m2,11 g/m2 and 16 g/m2 respectively.During the experiment,the plants in the treatments with.australis litter added EL,TL and FL have healthy plants and a well-developed root system.At the same time,different ways of adding P.australis litter also have some effects on the growth of Ojavanica.In the early stage of the experiment,O.javanica was in the seedling stage,adding P.australis litter in batches(every 5 days)was more suitable for the growth of Ojavanica.In the later stage of the experiment,O.javanica grew steadily and environmental temperature rose,O.javanica in EL growed faster.At the end of the experiment,the plant height and biomass of O.javanica in EL were significantly higher than those in TL and FL(p<0.05).In addition,adding plant litter can reduce the root activity of O.javanica,and adding P.australis every 5 days can alleviate the problem.(2)The carbon source released by decomposition of P.australis residues can improve the problem of low C/N in constructed floating wetlands.During the experiment,C/N of EL was significantly higher than CK(p<0.05),and C/N ratio of TL and FL also increased,fluctuating between 4.5-7.At the end of the experiment,the NO3--N concentrations of EL,TL and FL were kept at a low level(less than 0.33mg/L).However,the decomposition of the P.australis litter released too much carbon source,which will cause the COD content of constructed floating wetlands to rise,and the system will be in anoxia for a long time.During the exper:iment,the COD concentration in EL group was 4.5 times higher than the initial value.DO was lower than 0.70 mg/L.The COD content in the TL group and the FL group was lower in the early stage of the experiment,but the COD content was also higher at the end of the experiment.It indicates that although the plant residues decomposition can improve the nitrogen removal rate of the low C/N constructed wetland,measures such as controlling the dosage should be taken to prevent the COD from being too high.(3)The denitrification function genes of microorganisms attached to the plant roots in const:ructed floating wetlands were determined.The results showed that the denitrification function genes(napA+narG)?(nirK+nirS)and nosZ were more abundant in the constructed floating wetlands with.australis litter.And the effect on napA gene abundances was more obvious.The napA gene abundances in CK,EL,TL and FL were(1.50E+06±4.32E+05)copies/g,(3.83E+07±1.69E+07)copies/g,(3.05E+07±1.80E+07)copies/g,(2.75E+07±7.69E+06)copies/g.The results showed that the carbon source released by decomposition of plant residues provided a suitable growth environment for denitrifying bacteria,thus providing a basis for the transformation and degradation of NO3--N.At the same time,the plants in constructed floating wetlands with plant carbon litter had a well-developed root system,which provided the attached surface for denitrifying bacteria.Therefore,adding P.australis litter can promote denitrification by affecting the community distribution of denitrifying bacteria. |
PDF Full Text Request