Abstract: Chemoautotrophic carbon fixation (CCF) and iron-bound organic carbon (Fe-OC) in estuarine wetlands are important processes of carbon sequestration and carbon sink enhancement. We investigated changes in CCF rates and Fe-OC concentrations and their correlations with carbon fractions in Chongming Dongtan wetland in Yangtze Estuary, with the responses of CCF and Fe-OC to changes in salinity and nitrogen concentration being assessed in an incubation experiment of salinity gradient (2‰, 5‰, 10‰, and 15‰) and nitrate concentrations (20, 50, and 100 μmol NO₃^-·L^-1). The results showed that CCF rates decreased with increasing salinity by 6.72 ng C·g^-1·d^-1. Fe-OC concentrations decreased with increasing salinity, with the proportion (Fe-OC) of Fe-OC in the total organic carbon concentration being decreased from 30% to 22%. CCF rates and Fe-OC concentrations increased with increasing nitrate concentrations, with CCF rate of 19.3 ng C·g^-1·d^-1 and Fe-OC concentration of 3.77 mg·g^-1, respectively. CCF was significantly positively correlated with Fe-OC, indicating that chemoautotrophic process can favor Fe-OC formation. The enhanced salinity and nitrogen concentrations facilitated organic carbon decomposition, further decreasing the concentrations and stability of organic carbon. Our results suggested that changes in salinity and nitrogen concentrations in estuarine wetlands have great effects on carbon sequestration and carbon sink enhancement process and organic matter composition, further altering the stability of organic carbon.

Key words: chemoautotrophic carbon fixation, iron-bound organic carbon, salinity, nitrogen, Yangtze Estuary