Methane and carbon dioxide emissions from oil-contaminated wetland and pathways of CH4 production with stable carbon isotopes
Estuarine wetlands are simultaneously some of the most fragile and most economically important ecosystems in the world. Since there are many important petroleum production regions in estuarine wetlands, wetlands in these areas can be severely affected by long-term crude oil exploitation and petroleum hydrocarbon pollution. Multiple factors—including habitat integrity, vegetation primary production, physical and chemical characteristic of soil—are affected by oil contamination. Together these factors regulate biogeochemical processes that release methane (CH4) and carbon dioxide (CO2) from wetland soil and are the important part of global model predictions of future CO2 and CH4 emissions. However, the characteristics of CO2 and CH4 emission and methanogenic process in oil polluted wetland are less known. Stable isotopic analyses have been widely used to study the hydrocarbon biodegradation processes as they specifically affect the 12C/13C ratio of CO2 and CH4. In this paper, we (i) measured CH4 and CO2 emissions with different petroleum hydrocarbon contents in oilfields with 5-, 10-, and 30-year-old oil wells and compared them with uncontaminated wetland in the Yellow River Delta, China; (ii) presented isotopic data of soil gas (CO2 and CH4) in oilfields to detect the pathways of CH4 production.
We found significant reduction of CO2 and increment of CH4 in oil contaminated areas. The CO2 concentrations in oil contaminated areas were significantly lower than those in uncontaminated wetland (P>0.05). The lowest CO2 concentrations were observed near 30-year-old oil wells with 449.50±17.35 ppm, which were about 64.14% to the uncontaminated wetland. The highest CH4 fluxes were observed near 5-year-old oil wells with 1512.43±288.21 μg·m-2h-1, which were as 43.24 times as the values in uncontaminated wetland. The great amount of CH4 emissions indicated that the oil contaminated areas were important anthropogenic methane emission source in the Yellow River delta. The isotopic data of CH4 and CO2 were used to estimate the methanogeneic pathways during crude oil anaerobic biodegradation. The average δ13C values of CO2 in oil contaminated areas ranged from -15 to -11‰, which were significantly higher than those in uncontaminated wetland (p < 0.05). Conversely, the soil gas of oil contaminated areas had lower δ13C-CH4 than uncontaminated wetland. Significantly enriched CO2 and depleted CH4 carbon isotopic signatures from oil contaminated areas showed that hydrogenotrophic methanogenesis was most likely to occur during crude oil biodegradation. The methane could be an important terminal oxidation product of the degradation of petroleum pollutant in estuarine wetland.