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Undisturbed natural wetlands are important carbon sinks due to their low soil respiration. When compared with inland alpine wetlands, estuarine wetlands in densely populated areas are subjected to great pressure associated with environmental pollution. However, the effects of water pollution and eutrophication on soil respiration of estuarine and their mechanism have still not been thoroughly investigated. In this study, two representative zones of a tidal wetland located in the upstream and downstream were investigated to determine the effects of water organic pollution and eutrophication on soil respiration of estuarine wetlands and its mechanism. The results showed that eutrophication, which is a result of there being an excess of nutrients including nitrogen and phosphorus, and organic pollutants in the water near Shang shoal located upstream were higher than in downstream Xia shoal. Due to the absorption and interception function of shoals, there to be more nitrogen, phosphorus and organic matter in Shang shoal soil than in Xia shoal. Abundant nitrogen, phosphorus and organic carbon input to soil of Shang shoal promoted reproduction and growth of some highly heterotrophic metabolic microorganisms such as β-Proteobacteria, γ-Proteobacteria and Acidobacteria which is not conducive to carbon sequestration. These results imply that the performance of pollutant interception and purification function of estuarine wetlands may weaken their carbon sequestration function to some extent.

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ab s t r ac tThe soil microbial respiration (SMR) and physicochemical characteristics of Jiuduansha wetland at the Yangtze Estuary were analyzed in order to clarify the variability of SMR under different vegetation succession stages and its influencing factors. The results indicated that SMR of different vegetation succession stages are significantly various (P < 0.05). The SMR of the Spartina alterniflora (S. alterniflora) zone (0.43 mgCO 2 .g -1 .24 h -1 ) was the highest. These findings implied that S. alterniflora could enhance the SMR. Based on both the SMR and input of organic matter from plant decay, the Phragmites australis (P. australis) community likely possesses a higher organic carbon accumulation capability. Considering both SMR and input of organic matter from decayed plant biomass of wetland with different vegetation type, the P. australis community, in theory, has higher organic carbon accumulation capability. Path analysis shows that the main bio-factors influencing on SMR include bacterial diversity and soil microbial biomass (SMB). Soil moisture, inorganic N (IN), salinity and available P (AP) in soil also have significant effects on the mentioned biological factors.