The Effect Of Cd On The Relative Importance Of Denitrification And Dissimilatory Nitrate Reduction To Ammonium In Soils

Coastal wetland is the most important intermediate place for the exchange of matter and energy in the land-sea interaction process.The study of the accumulation of various pollutants in wetlands,especially heavy metals,and their environmental effects attracts much attention.In recent years,with the global climate change and the intensification of human activities,the influence of heavy metals on nitrogen transformation in wetlands has become a research hotspot.Denitrification（Den）and amination（DNRA）are the two most important simultaneous processes for the reduction of soil nitrate,the relative importance of the two and the removal of nitrate nitrogen and the emission of greenhouse gases N₂O are closely related.However,the study of heavy metals on the relative importance of the features about Den and DNRA is very inadequate.In this paper,we take the Chongming Dongtan which is a typical coastal wetland as an example and collect soil samples including Phragmites australis,Spartina alterniflora and mudflat.The rates and relative importance of Den and DNRA in soils were detected by ¹⁵N isotope tracing technique with different doses of CdCl₂·2.5H₂O.The effects of different forms of Cd were analyzed by the method of three-step continuous extraction（BCR）.Furthermore,the relationship between soil physical and chemical properties and microbial community structure and diversity and Den and DNRA and their relative importance under Cd addition were studied,aiming to provide a theoretical basis for nitrogen reduction and effect assessment of coastal wetland sediments under heavy metal pollution.The main results and conclusions in this research were as following:（1）The process of DNRA is the main of the reduction of nitrate nitrogen in the coastal wetland（DNRA/Den>1）.Compared with the control（without the addition of Cd）,the rate of Den under the vegetation of Phragmites australis and Spartina alterniflora decreased from 9.21μg/kg/h and 4.73μg/kg to 7.61⁸.67μg/kg/h and 3.36⁴.76μg/kg/h when low-dose Cd（0.05-0.5 mg/kg）was added,respectively.Significant reduction in the Den process at low doses of Cd（0.05-0.5 mg/kg）increases the relative importance of the DNRA process.In comparison,the addition of high-dose Cd（0.5-5.0 mg/kg）resulted in a significant decrease in Den and DNRA in the soil under the vegetation of Phragmites australis and Spartina alterniflora,but did not cause significant changes in DNRA/Den.（2）For the vegetation of Phragmites australis,there was a significant negative correlation between exchangeable Cd and the rate of Den and DNRA（P<0.05）,and there was a significant positive correlation between reducible Cd and the rate of Den and DNRA（P<0.05）.In comparison,the rate of Den and DNRA in the soil under the vegetation of Spartina alterniflora were significantly decreased with the increase of oxidizable Cd content（P<0.05）.（3）With Cd addition,the rate of Den in soil under under the vegetation of Phragmites australis was significantly positively correlated with the content of NO₃^--N（P<0.05）,and the rate of DNRA increased significantly with the increase of pH（7.5-7.7）（P<0.05）.For the soil in the vegetation of Spartina alterniflora,the rates of Den and DNRA were positively correlated with NO₃^--N content,however,the rates of Den and DNRA rate were significantly negatively correlated with pH（P<0.05）.（4）The response of the rate of Den to Cd addition in coastal wetland may be closely related to the metabolism of Gemmatimonadetes microorganisms.In comparison,the microbial communities of Actinobacteria and Bacteroidetes may have a significant effect on the response to the rate of DNRA.At the genus level,the relative abundances of the bacterial communities of Gp6 and Gp10 in soil and the rate of Den showed consistency with the response of Cd.To a certain degree,Ilumatobacter and Gp7 affected the rate of DNRA in soils under different vegetations with the response of Cd.