California Sea Grant College Program

Ecosystems are the largest source of atmospheric methane, an important greenhouse gas whose atmospheric concentration is increasing 1% annually. My dissertation research on the environmental controls over methane flux suggests that there is the potential for significant feedback between climatic change and methane flux from ecosystems.

From 1991 to 1993, I conducted a variety of field experiments in Alaskan tundra, California annual grasslands, and Colorado montane meadow, measuring methane flux with static chambers. Emission rates from the Alaskan wet tundra averaged 52 mg CH4 m-2 d-1 and from California pond margin averaged 38 mg CH4 m-2 d-1. Methane uptake in the Colorado montane meadow averaged -1.2 mg CH4 m-2 d-1 and in the California grassland averaged -0.7 mg CH4 m-2 d-1.

Over the annual cycle and across landscape gradients, methane flux was positively correlated with soil moisture and temperature at all three sites. Moisture was the dominant factor, since unsaturated soils took up methane (small negative flux) while saturated soils emitted methane (large positive flux). The response of flux to changes in water level was rapid and direct, rather than occurring via moisture-induced changes in vegetation. Plants did provide an important pathway for methane emissions, however, by allowing methane to bypass the zone of methane oxidation at the soil or water surface. In the grassland, soil carbon and nitrogen explained 97% of the spatial variation in annually-averaged uptake rates, independent of sheep-grazing.

My research showed that emissions and uptake respond differently to changes in soil micro-climate. An increase in moisture increased emissions but decreased uptake (with an important exception in very dry soils, when further drying decreased uptake rates). Emissions increased exponentially with temperature while uptake showed a weak response to temperature.

A model that takes into account both the differential effect of temperature on emissions and uptake, and the fact that net emissions are the balance of production and consumption, predicts three times more change in global ecosystem emissions in response to a 10°C warming than does a model that ignores these two features and includes only net methane release. Therefore, the magnitude of the eedback generated between methane flux from ecosystems and climate is probably much greater than would be estimated by traditional methane-cycle models, which ignore these two features.