Sources,Forms And Decomposition Of Soil Organic Carbon Along A Tidal Flat Transect Of The Jiulong River Estuary

Estuarine tidal flats play an important role in the global soil organic carbon cycle.In order to further understand the processes of soil organic carbon cycle in estuarine tidal flats,we investigated the sources,fractions and decomposition rates of soil organic carbon along a cross transect in the tidal flat of the Jiulong River Estuary,southeast China,where the vegetation varied from the Kandelia obovate mangrove forest,the ecotone of K.obovate and Spartina alterniflora marsh,S.alterniflora marsh and no-vegetation covered tidal flat.The main research results show that:（i）From land to sea along the tidal flat,the soil physical and chemical properties changed significantly,soil total organic carbon（TOC）gradually decreased by 54%,total nitrogen（TN）,carbon/nitrogen ratio（C/N）andδ¹⁵N values gradually decreased,while soilδ¹³C values increased first and then decreased,and were the highest in spartina alterniflora habitat.According to the mixed end-member model,the terrigenous contribution rate of soil organic carbon from land to sea decreased by 51%along tidal flat.Based on the analysis of terrigenous organic carbon,the soil organic carbon mainly came from overlying vegetation in Kandelia obovate mangrove forest,the ecotone of K.obovate and Spartina alterniflora marsh,S.alterniflora marsh.However,in the no-vegetation covered tidal flat,the contribution rate of terrigenous organic carbon is low,and the marine organic carbon represented by phytoplankton is the main input,accounting for about 57%.（ii）From land to sea along the tidal flat,The contents of dissolved organic carbon（DOC）,microbial biomass carbon（MBC）and liable organic carbon（LOC）gradually decreased.The contents of particulate organic carbon（POC）and mineral-associated organic carbon（MAOC）gradually decreased.In MAOC,the content of Fe-Al bonded organic carbon[Fe（Al）-SOC]decreased gradually,while the content of Ca bonded organic carbon（Ca-SOC）increased gradually,and the content of residual mineral-associated organic carbon（residual MAOC）decreased first and then increased.The Carbon stability index（CSI）increased from 1.76 to 8.21,indicating that soil organic carbon stability was gradually enhanced.The contents of DOC,MBC and LOC were significantly affected by soil EC,p H and Cl^–concentration.The contents of POC,MAOC,Fe（Al）-SOC and residual MAOC were negatively correlated with p H,and positively correlated with clay content.The content of Ca-SOC were positively correlated with soil EC and Cl^–concentration.（iii）From land to sea along the tidal flat,soil Fungal abundance,Bacterial abundance and F/B gradually decreased.The activities ofβ-1,4-Glucosidase（BG）,β-D-Cellobiosidase（CBH）,β-1,4-N acetlglucosamine（NAG）,Acid phosphatase（AP）,Phenol Oxidase（PHO）and Peroxidase（PEO）decreased.CMR and MPR decreased by62%and 93%accordingly.The results showed that the soil organic carbon cycling process changed significantly along tidal flat from land to sea.Along the tidal flat from land to sea,terrigenous organic carbon decreased,soil organic carbon storage decreased,different fractions of organic carbon content decreased,soil microbial abundance decreased,extracellular enzyme activity decreased,mineralization rate decreased,while soil organic carbon stability gradually enhanced.