Effect of water level and sediment temperature on rates of production and isotopic composition of carbon dioxide and methane from an urban marsh.

摘要

Wetlands play a crucial role in the global carbon cycle. They are sinks for carbon dioxide through photosynthesis and long-term storage of carbon owing to low decomposition rates, and can be significant sources of methane and carbon dioxide under various moisture and temperature conditions. In this study, in situ microcosms were used to monitor emission rates and delta13C values of methane and carbon dioxide evolved under varying water levels and sediment temperatures from the Little Cataraqui Marsh, an urban mineral wetland in Kingston, Ontario.; Methane emission rates ranged from 2.2x10-3 mg CH 4/m2hr to 110 mg CH4/m2hr and delta 13C values ranged from -63‰ to -33‰. Carbon dioxide emission rates ranged from 4.4 mg CO2/m2hr to 800 mg CO2/m2hr and delta13C values ranged from -27‰ to -18‰. At times of high water level (15 cm below ground surface) methane is produced via the acetate fermentation pathway, with higher emission rates and lower delta13C values. When the water level is low (>15 cm below ground surface), methane produced at greater depth is oxidized when it migrates to the surface, resulting in lower emission rates and higher delta13C values. Three processes affect carbon dioxide production. At times of high water level, carbon dioxide may be partly derived from the dissolution of carbonate resulting in higher emission rates and delta13C values. The reduction of carbon dioxide at times of high water level may also result in higher delta 13C values, however, emission rates are lower. When water levels are low, carbon dioxide emission rates and delta13C values are lower, with production dominated by organic matter decomposition and root respiration. Sediment temperature was found to increase emission rates of both gases at times of high water levels only. No relationship between sediment temperature and delta13C values of either gas was identified.; Natural water levels fluctuated dramatically as a result of differential inputs and outputs of water. Precipitation, overland flow, stream flow and groundwater may all contribute to the water level with high rates of evapotranspiration during the summer months likely causing water levels to fall to nearly 1 m below ground surface. The deltaD and delta18O values of near-surface wetland water vary over the season (June to November 2005), which reflects temperature dependent isotopic fractionation and source moisture.; This research has shown that future changes in the hydrologic and temperature regimes of mineral wetlands may significantly alter emission rates and delta 13C values of methane and carbon dioxide, and thus their role in the global carbon cycle.