| Nitrogen compounds released from sediment are the endogenous origin,which elevate aquatic nitrogen concentrations.When the exogenous source of nitrogen was under control,the endogenous origin of nitrogen became the dominated source of aquatic nitrogen.N2 produced by the coupling of ammonium oxidation and denitrification was the most important nitrogen loss pathway in the aquatic system.At present in China,the water and sediment of urban rivers especially in old areas were under high loading of nitrogen contaminants due to the uncompleted sewage collection system.In this study,physicochemical characteristics and the abundances of functional genes involved in nitrogen cycling were identified from urban river sediments to distinguish the dominating nitrogen transformation pathways and its most impacted factors.At last,an efficient and stable in situ treatment technique of nitrogen contaminants were developed based on improvement of aquatic habit in urban rivers.The main conclusions of this paper is as follows:(1)Nitrogen distribution and transformation pathway in urban rivers.The nitrogen concentrations of overlying water and sediment samples from urban rivers in cities(Nanjing city,Suzhou city etc.)were analyzed.The total nitrogen(TN),NH4+and NO3-concentration in the overlying water were ranged from 2.57 mg·L-1 to 10.30 mg·L-1,0.36 mg·L-1 to 10.04 mg·L-1,0.47 mg·L-1 to 3.02 mg·L-1,respectively,and ammonium was the main component.N4+and NO3-concentration in the pore water were ranged from 0.73 mg·L-1 to 63.00 mg·L-1,0.25 mg·L-1 to 0.98 mg·L-1,respectively.The ammonium in porewater was higher than in the overlying waters,the difference ranged from 16.48 mg·L-1 to 61.90 mg·L-1.The TN,NH4+ and NO3-contents in sediments were ranged from 642.71 to 2688.54 mg·kg-1,33.60 to 327.60 mg·kg-1,7.70 to 25.90 mg·kg-1,respectively,and dominated by organic nitrogen.The abundance of functional genes(amoA,hzsB?hzo,nirS,nirK,norB,nosZ,nrfA)were measured by real time quantitative PCR(qPCR).The aerobic ammonium oxidation bacteria(AOB),archaea(AOA)and anaerobic ammonium oxidation bacteria(anammox)coexisted in the sediment.Their abundance were ranged from 6.45×104 to 1.08×106 copies·g-1,1.73×105 to 7.41×106 copies·g-1,and 7.17×104 to 5.03×105 copies·g-1,respectively.The abundance of AOA bacteria were the most abundant.For denitrifiers,the nirS,nirK and nrfA gene were all detected,and ranged from 1.04×107 to 2.32×107 copies·g-1,9.43×106 to 6.66×107 copies·g01,1.14×107 to 1.76×107 copies·g-1,respectively,and the nrfA gene was the most abundant.The abundances of these genes reveled that,in sediment,the nitrification-denitrification was the domain process in the nitrogen cycle.The ratio?AOA-amoA+AOB-amoA)/(nirS+nirK+nrfA)were lower than 0.09 in all samples,which implied that the denitrification potential was higher than the nitrification potential,and the ammonium oxidation process was crucial for nitrogen cycling.The(nirS+nirK)lnrfA ratio ranged from 2.52 to 3.94,which indicated that denitrification was the main nitrate reduction pathway in the presence of DNRA.In addition,the norB/nosZ ratio(6.23-32.84)implied high N2O emission potential from these urban rivers.Pearson correlation analysis showed that,the NH4+ in porewater and sediment and TN in sediment increased with the abundances of AOA-amoA gene,nirS gene,and nosZ gene.The ammonium oxidizing rate limit and DNRA was resulted in nitrogen accumulation.(2)The impact mechanism of environmental factors on nitrogen cycle.Pearson correlation analysis showed that,the abundance of domain AOA-amoA gene has positive relationships with overlying water DO,and porwater SO42-.Laboratory tests were carried out to study the nitrogen transformation under different DO levels(anoxic,DO<0.5 mg·L-1;oxic,2 mg·L-1<DO<4 mg·L-1;hyperoxia,DO>8 mg·L-1;natural,0.05mg·L-1<DO<5.76 mg·L-1).The results showed that,the nitrogen removal ratios and rates were accelerating under higher DO concentrations.The removal ratios raised from 12.96%to 99.84%with higher DO concentration.With the increasing of DO concentration,the abundances of AOA-amoA,AOB-amoA increased exponentially,and the ammonium oxidizing rate and integrated amount were higher(from 3.34 mg N to 3.34 mg N).The elevated ammonium-oxidizing rate produced more NO2-and NO3-,which caused the nirK and nirS gene abundance increased by an order,then the total N2 production amount raised from 6.12 mg N to 35.44 mg N.The multivariate linear regression equations of nitrogen transformation rates and functional gene abundances and nitrogen species concentrations are established.The ammonium oxidizing rate R2=-3.2802+0.0009log(AOA-amoA)-0.006471og(AOB-amoA)+0.2345log(nirS)+0.1546log(nirK):the denitrification rate R4==0.2425+0.0698NO3--0.0079log(nirS)+0.04281og(nirK);the anammox rate R5=-5.0817-0.2151NO3-+ 0.0360log(AOA-amoA)-0.0015log(AOB-amoA)+0.4058log(nirS)+0.2468log(nirK),their correlation coefficients(r2)were 0.5128,0.6811 and 0.6709,respectively.(3)Development of in situ treatment of nitrogen contaminants technique and Demonstration in urban riversIn order to remove the nitrogen contaminants from urban rivers,the in situ capping technique were developed in consideration of the oxygen demand and nitrogen release.This technique combined two materials,the ammonium adsorption material and oxygen release material.Batch test showed that,adding of oxygen release material raised the DO concentration of the profile,enhanced the abundance of amoA gene at least one magnitude order,and the adsorption material could adsorption NH4+efficiently.The results of a pilot scale test showed that,2 cm capping with the ratio of 1:10 of two functional materials was the optimal combination.This combination of two materials was applied to treat the nitrogen-contaminated sediments of an urban river with a length of 200 meter.Monitoring data showed that,in the sediment,the abundance of amoA increased significantly,the ammonium in pore water and sediment organic matter content were decrease by 61.3%and 45%compared with the control area,respectively. |
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