The Relative Importance And Mechanism Of Soil Dissimilatory Nitratereduction To Ammonium And Denitrification Under The Change Of Land Use

Coastal wetland has become a research hotspot in recent years,under the influence of global change and human activity acceleration.Denitrification（Den）and Dissimilatory nitrate reduction to ammonium（DNRA）process are all in the process of nitrate dissimilation and reduction.The former reduces the NO₃^-second reduction to gaseous compound,which is beneficial to the removal of excessive nitrogen,but its intermediate products include typical greenhouse gas N₂O,etc.The latter mainly converts NO₃^-into NH₄⁺.In the past three decades,the coastal urbanization process has been accelerating,and the land use types and modes of coastal wetlands have undergone profound changes.Therefore,it is necessary to study the influence and mechanism of land use change on the relative importance of soil Den and DNRA.In this paper,the study samples GuangTan,flower rice grass,reed,metasequoia,camphor,rape,wheat and rice under different land use/cover of undisturbed soil column in a typical coastal wetland chongming dongtan,using ¹⁵N stable isotope dilution method,different season or soil Den growth duration,intensity of DNRA and the characteristics of the relative importance of synchronous analysis of soil physical and chemical properties and the change of the microbial community structure and diversity,to reveal the land use change on soil Den and DNRA relative importance effect and its mechanism,aimed at the N₂O emissions of greenhouse gases in the coastal wetlands provide theoretical basis for evaluation and regulation research.The main research results and conclusions obtained in this thesis are as follows:（1）The Spartina alterniflora Den and DNRA intensity was significantly higher in different seasons than reed（P<0.05）.Among them,the change of soil Den in the reeds was 9 to 50μg/（kg·h）,and after conversion to farmland（rapeseed,wheat and rice）,Den intensity was increased to 518-535,37-146,11-412μg/（kg·h）.This indicates that the invasion or transformation of wetland to farmland will significantly accelerate the denitrification process of soil NO₃^-in coastal wetland and increase the potential of N₂O emission.（2）Natural cover type of soil DNRA/Den>1.This indicates that the natural wetland soil NO₃^-alienation reduction process is dominated by DNRA.In comparison,the change of wetland for land use makes DNRA/Den<1 and>1,and the relative importance of Den and DNRA is significantly increased.（3）The Spartina alterniflora soil under 200-2000μm particle content,EC,especially the content of Cl^-,significantly greater than the reeds,but the reed soil under cover 63-200μm pa-rticle content,NO₃^-content is greater than the qingjiao rice straw（P<0.05）.This indicates that the relative importance of both soil densities and DNRA may be influenced by particle size,EC,NO₃^-and so on.In comparison,soil pH and Cl^-content decreased significantly after the wetland was converted into farmland（P<0.05）.Therefore,the change of artificial land use to soil pH and Cl-content may significantly affect the relative importance of Den and DNRA.（4）The chongming dongtan under 7 kinds of cover/land use（GuangTan,flower rice grass,reed,metasequoia,camphor,rape,wheat,etc.）soil microbial total of OTUs accounted for less than 32%,but the gate level advantage community are supplied by the Actinobacteria,and Acidobacteria,Bacteroidetes,Chloroflexi,Proteobacateria,relative abundance is more than 78%.（5）Soil microbial communities under different land use were dominated by gram-negative bacteria（G^-）,and the ratio of G^-/G⁺（gram-positive bacteria）varied from 1.5-4.5（phylum level）.However,the relative abundance of G⁺was significantly increased（P<0.05）,although the ratio was still G^->G⁺.（6）The relative importance of soil Den and DNRA was significantly correlated with the relative abundance of Actinobacteria,Bacteroidetes and Bacteria_unclassified（P<0.05）.Among them,the relative importance of Den and DNRA was signific-antly negatively correlated with the relative abundance of G^-and was significantly positively correlated with the relative abundance of G⁺（P<0.01）.