Meitan xuebao (Jul 2023)
Dissolved carbon exchange flux at sediment-water interface in coal mine subsidence wetland based on response surface method
Abstract
The most important thing for wetlands as terrestrial ecosystems is carbon, which is an important way to slow down the rise of dihydride carbon content in the global atmosphere and global warming. At present, most studies focus on carbon storage and carbon density in natural wetlands. However, there are relatively few studies on carbon density and its changes in coal mining collapse wetlands. Based on the investigation of carbon content and influencing factors in water bodies and sediments of coal mining subsidence wetlands, this paper studies the carbon exchange fluxes of different forms at the sediment-water interface of coal mining subsidence wetlands, evaluates the source/sink characteristics of sediments for carbon in overlying water, and discusses the changes of the exchange fluxes of various forms of carbon at the sediment-water interface under the single and interactive interaction of different environmental factors. The sediment and overlying water were collected in December 2020, April 2021 and August 2021, respectively, to analyze the occurrence characteristics of carbon in the sediment and water and the key factors affecting carbon content in the coal mining collapse wetland of Jiuli Lake. The dissolved carbon exchange fluxes at the sediment-water interface were studied through laboratory simulation experiments, and the effects of major environmental factors on dissolved carbon exchange fluxes were discussed. Finally, a regression model of dissolved carbon exchange flux at the sediment-water interface was constructed based on the response surface method to predict the minimum release conditions of dissolved carbon in coal mining subsidence wetlands and estimate the total amount of dissolved carbon release in coal mining subsidence wetlands. The results are as follows: temperature increase leads to the increase of dissolved carbon exchange flux, while pH has little effect on it; The exchange fluxes of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) were relatively high under high oxygen (9.0 mg/L) and low oxygen (3.0 mg/L) conditions, and relatively small under medium oxygen (6.0 mg/L) conditions. The optimal conditions under the interaction of environmental factors are obtained. When the temperature is 5.69 ℃, the pH is 7.18, and the DO is 6.38 mg/L the minimum exchange flux region of DOC and DIC is 2.32 and 3.54 mg/(m2·h), respectively. It was estimated that the total amount of DOC and DIC released in wetland sediments in August was the highest, which was 1.75 t and 5.76 t respectively. The total release of DOC and DIC was the lowest in December, which were 0.98 t and 2.01 t, respectively. The results showed that the environmental factors had different degrees of influence on the exchange fluxes of DOC and DIC, and the exchange fluxes of DOC and DIC increased with the increase of temperature by changing the adsorption capacity and biological activity. pH affects the solubility of carbonate in the sediment, so DIC does not increase significantly in alkaline environment, but has little effect on DOC exchange flux. The variation law of total release of DOC and DIC is consistent. The high temperature in summer and strong microbial activity in sediments promote the release of DOC and DIC in the upward overlying water, while the low temperature in winter significantly reduces the release effect. Only temperature, pH and DO are considered for dissolved carbon exchange fluxes at the sediment-water interface. However, factors affecting the migration and transformation of substances at the sediment-water interface include the characteristics of sediments themselves, bioturbation and heavy metals, etc. For wetlands formed by coal mining, the release of dissolved carbon by metal ions in sediments should be paid more attention. In the following research, microorganisms can be used as the starting point to explore the critical conditions for the conversion of sediment carbon “source” and “sink”.
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