Study On The Difference And Mechanism Of Nitrogen Absorption And Control Of Farmland Drainage By Different Aquatic Plant Wetlands

The constructed wetlands(CWs)technology that centered on aquatic plants is widely used to treat agricultural non-point source pollution caused by farmland drainage due to its low cost,easy operation,and good decontamination effect.The perennial emergent plants,includingAcorus calamus(A.calamus),Pontederia cordata(P.cordata)and Alisma plantagoaquatica(A.plantagoaquatica)are found commonly in farmland ditches and wetlands.Aforesaid plants have superior pollution tolerance,outstanding absorption and assimilation ability of nitrogen(N)and phosphorus(P).However,there are few reports about the effects of those plants on the denitrification process of farmland drainage.The understanding of the differences in the denitrification effects of those plant wetlands,the main denitrification pathways and mechanisms is still lack.Therefore,this study constructed three plant simulation wetlands,and meanwhile,used the wetlands to simulate farmland drainage.The main objectives of this study are to:(1)explore difference in denitrification effects of the A.calamus,P.cordata and A.plantagoaquatica CWs;(2)decipher the distinct mechanism of N accumulation of those aquatic plants in wetland denitrification process;(3)disclose the driving mechanism of sediment microorganisms in wetland denitrification process.This study would provide a scientific basis for the prevention and control of agricultural non-point source pollution in constructed wetlands.The main results are as follows:1.Difference in denitrification effect of Acorus calamus,Pontederia cordata and Alisma plantagoaquatica constructed wetlandsIn the first 10 days of the test,the total N(TN)removal rate of water in three plant CWs increased by 7.3%～47.5%compared with CK.Among them,the effect of P.cordata was the best,and the A.calamus was the weakest.Compared with the LN condition,TN removal rate of all CWs reduced by 2.4%～35.0%under the HN condition.The TN content of the sediment in all CWs were lower than that before the test,and the denitrification rate of the sediment under the plant treatments were higher than that of CK.Among them,the P.cordata was the highest(9.5%～14.2%),and the A.calamus was the lowest.The relative growth rate(RGR)of P.cordata was the highest,and the A.calamus was the lowest.A.calamus had the highest TN content,however,the TN accumulation of A.plantagoaquatica and P.cordata were higher than that of A.calamus,which due to that the the biomass of A.plantagoaquatica and P.cordata were significantly higher than A.calamus.The N2O emission flux of all CWs reached 11.4～5590.6 μg m-2 h-1,and the cumulative N2O emission reached 14.01～148.96 mg m-2.However,the cumulative N2O emission only accounted for 0.11%～2.18%of the total denitrification capacity of the wetland systems.Among the three plants,the cumulative N2O emission and microbial denitrification of P.cordata were the highest under both of LN and HN conditions.The cumulative N2O emission of A.plantagoaquatica was higher than that of A.calamus under both of LN and HN conditions.The amount of microbial denitrification of A.calamus was higher than that of A.plantagoaquatica under the LN condition,and that of A.calamus was lower than A.plantagoaquatica under the HN condition.According to N mass balance,plant uptake and microbial nitrification-denitrification process were the key N-removal pathways.Under the LN condition,both A.calamus and P.cordata denitrified mainly by microbial action(accounting for 86.4%and 66.0%of the total denitrification effect,respectively),while A.plantagoaquatica showed similar contribution rates of the two pathways(50.0%each).Under the HN condition,the contribution rates of microbial pathway denitrificationin A.calamus and P.cordata were respectively 79.9%and 61.6%,whereas A.plantagoaquatica accounted for 33.0%.Among the three plants,the N-removing effect in the P.cordata CWs was the best,while that in A.calamus CWs was the worst.2.Difference mechanism of N accumulation in three aquatic plants under wetland denitrification processThe relative growth rates(RGRs)of the plant above-ground parts were significantly higher than those of the underground parts,and in particular,the RGRs of the P.cordata and A.plantagoaquatica were significantly higher than that of the A.calamus.The contents of TN and NO3-in the three plants were distributed mainly in the above-ground tissues.The TN content in the roots of the three plants was highest in the A.calamus,and that in the stems and leaves was highest in the P.cordata.The NO3-content in the roots and leaves was highest in the A.calamus,and the A.plantagoaquatica had the highest content in the stems.Compared with A.calamus,the leaves of the P.cordata and the A.plantagoaquatica had higher net photosynthetic rate,stomatal conductance,intercellular CO2 concentration,and transpiration rate.Furthermore,the activities of nitrate reductase(NR),glutamine synthetase(GS)and glutamate synthase(GOGAT)in the leaves of A.calamus displayed the highest in comparison with other two plants.Bacause of the higher biomass of the P.cordata,the overall N assimilation efficiency of the P.cordata was higher than that of A.calamus.Root PM H+-ATPase activity,root activity and bleeding intensity of the P.cordata were significantly higher than those of the A.calamus and A.plantagoaquatica;and meanwhile,the concentrations of NH4+-N and NO3--N in the bleeding fluid of P.cordata were the highest in comparison with other two plants.Compared with the A.calamus and A.plantagoaquatica,the P.cordata had the highest N absorption activity,transport power and metabolic capacity,which promoted it to show higher N accumulation effects in the process of wetland denitrification.3.Driving mechanism of sediment microorganisms in wetland denitrification processThe potential nitrification rate(PNR)and potential denitrification rate(PDR)of the bottom sludge were higher than those of CK treatment.Among the three plants,the PNR and PDR of P.cordata sludge revealed the highest.The PDR of A.plantagoaquatica was significantly higher than that of A.calamus.The PNR of A.calamus was significantly higher than that of A.plantagoaquatica under the LN condition,but lower than that of A.plantagoaquatica under the HN condition.Additionally,plant treatments could increase the abundance of 16S rRNA and functional genes related to nitrification-denitrification reaction of the sediment.Among the three plants,the abundance of 16S rRNA and functional genes in the P.cordata sludge were the highest.Compared with the A.plantagoaquatica,the abundances of 16S rRNA,AOA,AOB and nirS in A.calamus sludge were higher than those of A.plantagoaquatica,while the abundances of nirK and nosZ gene were lower than those of the A.plantagoaquatica.The activities of extracellular enzymes in sediments were significantly higher than those of CK treatment.Among the three plants,the activities of sediment urease(UE),acid protease(ACPT)and leucine aminopeptidase(LAP)in P.cordata wetlands were the highest,while those were the lowest in A.calamus wetlands.The activity of N-acetyl-β-D-glucosidase(NAG)was the highest in A.plantagoaquatica and the lowest in P.cordata.Besides,the plant treatments increased the microbial biomass carbon(MBC)and N(MBN)content of the sludge.Among the three plants,the MBC and MBN content of P.cordata was the highest and those of A.plantagoaquatica was the lowest.The three aquatic plants significantly influenced the physicochemical properties of the sediment.For instance,the TC content and C/N of the sediment under plant treatments were higher than those of the CK,and the TN content and pH were lower than CK.Correlation analysis showed that sediment PNR,PDR,the abundances of AOA,nirK and nosZ,and the contents of MBC,MBN,and the acticies of UE,ACPT,LAP were significantly and negatively correlated with TN content,but those variables were significantly positively correlated with C/N.The activity of NAG was significantly and negatively correlated to the pH.