Effects Of Coastal Wetlands Conversion Into Paddy Fields On The Soil Carbon Pool Composition And Stability In China

Coastal wetland is an important carbon storage reservoir on the earth,and its carbon storage structure,carbon sink function and stability are of great significance for coping with global climate change.Due to the development and utilization of wetland resources,many wetlands have been converted to other land use types.This conversion process gradually caused changes the function of soil carbon sinks and the scale of carbon pools in wetland.However,the current research mainly focuses on single estuary wetland or coastal wetland.Regarding changes in land use on a regional scale,especially the change characteristics of soil organic carbon and its stability after reclamation into paddy fields are still unknown.Based on this,this study selected 11 typical coastal wetlands in eastern China（including Phragmites australis wetlands and mangrove wetlands）as the research objects,and explored the response of soil carbon pools and their stability after the conversion of coastal wetlands into paddy fields.The main results are as follows:（1）Soil SOC and activated carbon conponents content were significant differences after land use conversion.In general,SOC content,SOC storage（SOCS）and soil microbial biomass carbon（MBC）content were significantly（P<0.05）lower after the conversion of Phragmites australis wetlands into rice fields.On the contrary,soil dissolved organic carbon（DOC）content was higher in the paddy fields,but soil labile organic carbon（LOC）content had no significant effects.In addition,the conversion of mangrove wetlands to paddy fields also significantly（P<0.05）decreased SOC content and SOCS,while soil MBC and DOC contents were lower in the paddy fields.At the same time,compared with before reclamation,soil LOC content was significantly（P<0.05）lower in the paddy fields.（2）Land use conversion affected soil physical and chemical properties and nutrient characteristics,which further affected SOC content.Soil water content,total nitrogen（TN）,ammonium nitrogen（NH₄⁺–N）were positively（P<0.01）correlated with SOC.Besides,soil bulk density,p H,total phosphorus（TP）were negatively（P<0.01）correlated with SOC.（3）In the 0-30 cm soil layer of the Yellow River Estuary,Jiulong River Estuary,and Dongzhai Port,the size of soil aggregates are mainly 0.053-0.25 mm.Among them,the proportion of large aggregates（a size of>0.25 mm）,MWD,GMD and DR_0.25 were lower after land use conversion in the Yellow River Estuary and Jiulong River Estuary,while the the fractal dimension（D）was higher in the paddy fields.On the contrary,the proportion of large aggregates（a size of>0.25 mm）,MWD,GMD and DR_0.25 were higher after the wetland reclamation of the Dongzhai Port wetland,while the the fractal dimension（D）was lower in the paddy fields.In addition,after coastal conversion into paddy fields,the carbon content of soil aggregates at each sampling point were mainly concentrated in the size of0.053-0.25 mm and<0.053 mm.Moreover,the carbon contribution rate of large aggregates（>0.25 mm）were significantly lower,while the carbon contribution rate of large aggregates（<0.25 mm）were significantly higher after land reclamation.（4）After land use conversion,Ca–SOC and Fe（Al）–SOC contents were significantly decreased,while Ca–SOC and Fe（Al）–SOC content in the 15-30 cm soil layer was higher than the 0-15 cm soil layer in the Yellow River Estuary.In addition,soil residue–SOC content of was lower after the reclamation in the Yellow River Estuary,while it was higher after land use conversion in the Jiulong River Estuary and Dongzhai Port.（5）The number of bacteria was significantly（P<0.05）higher in the paddy fields of the Yellow River Estuary and Dongzhai Port,while the number of fungi and the fungi/bacteria ratio were significantly（P<0.05）lower.The number of bacteria and fungi was opposite to the other two sampling points after reclamation in the Jiulong River Estuary.（6）Wetland reclamation changed the soil dominant bacteria and fungi genera.Among them,the relative abundance of Stenotrophomonas was higher,while the relative abundance of Geomyces,Mortierella and Trichoderma was significantly（P<0.05）lower after the wetland reclamation of the Yellow River Estuary and Jiulong River Estuary.Moreover,after the conversion of Dongzhai Port wetland,the relative abundance of Candidatus＿Solibacter,Candidatus＿Koribacter was significantly（P<0.05）higher,while the relative abundance of Sulfurimonas and Trichoderma was significantly（P<0.05）lower.Such changes in microbial community structure and composition were further reflected by changes in soil organic carbon,and affected the sequestration and stability of organic carbon.