Rates And Pathways Of Organic Carbon Mineralization Of Land-based Shrimp Ponds Sediments Of Three Estuaries,Fujian

To reveal the spatial-temporal variation of the carbon cycling in the sediments of estuary shrimp ponds,the rates of total organic carbon mineralization,together with sulfate reduction,dissimilatory iron reduction,and methanogenesis has been examined and analyzed in eight shrimp ponds of three main estuaries of Fujian Province,China.The results showed that:（ⅰ）The pH and DOC concentrations of pond water in the farming period are higher than those in the non-farming period,while the ORP,Cl^–,NO₃^–concentrations and salinity were lower in the farming period than those in the non-farming period.The colored dissolved organic matter（CDOM）in aquaculture water mainly includes protein-like fluorescence components and humus-like fluorescence components.A significant negative correlation between salinity and each component of CDOM were observed.The humus-like groups in CDOM were suppressed by the DOC concentrations.（ⅱ）Among the different species of reactive iron,Fe（Ⅲ）accounts for the dominant part（71.9–85.8%）.Overlying pond water SO₄^2–and Cl^–affect the distribution of Fe species in the shrimp ponds.Saltier shrimp ponds had higher iron sulfide content,less organic iron content,and less crystalline Fe（Ⅲ）content than fresh ponds.（ⅲ）Compared with the farming period,the rates of total organic carbon mineralization increased 29%higher in the non-farming period.Similar increase was also observed in the dissimilation iron reduction（41%）and sulfate reduction（17%）.However,the methanogenesis rates decreased by 73%in the non-farming period（ⅳ）The increase of salinity reduced the rates of total organic carbon mineralization in the shrimp ponds.The dominant pathways of organic carbon mineralization of sediments changed from the dissimilation iron reduction and methanogenesis to sulfate reduction as salinity increased.The rates of total organic carbon mineralization in the shrimp pond sediments was also significantly affected by NH₄⁺during the farming period,and was also affected by NH₄⁺and alkalinity during the non-farming period.Proteobacteria was the dominant species in the shrimp ponds.The microbial community structure of the low-salinity shrimp ponds changed significantly between the farming and non-farming periods.However,no such differences could be observed in the saltier shrimp ponds.The results showed that salinity and farming periods affected the physicochemical properties of shrimp ponds,which further affected the rates and pathways of organic carbon mineralization and bacterial community structure in shrimp ponds.High-salinity shrimp farming models may reduce carbonic greenhouse gas emissions.A more comprehensive assessment of the contribution of shrimp ponds to greenhouse gas emissions will be needed.