High-arctic soil CO2 and CH4 production controlled by temperature, water, freezing and snow

Soil gas production processes, mainly anaerobic or aerobic soil respiration, drive major gas fluxes across the soil-atmosphere interface. Carbon dioxide (CO2) effluxes, an efflux which in most ecosystems is a result of both autotrophic and heterotrophic respiration, in particular have received international attention. The importance of both CO2 and methane (CH4) fluxes are emphasised in the Arctic because of the large amount of soil organic carbon stored in terrestrial ecosystems and changes in uptake and release due to climate changes. This chapter focuses on controls on spatial and temporal trends in subsurface CO2 and CH4 production as well as on transport and release of gases from the soil observed in the valley Zackenbergdalen. A dominance of near-surface temperatures controlling both spatial and seasonal trends is shown based on data obtained using closed chamber and eddy-correlation techniques as well as in manipulated field plots and in controlled incubation experiments. Despite variable temperature sensitivities reported, most data can be fairly well fitted to exponential temperature-dependent equations. The water content (at wet sites linked to the depth to the water table) is a second major factor regulating soil respiration processes, but the effect is quite different in contrasting vegetation types. Dry heath sites are shown to be periodically water limited during the growing season and respond therefore with high respiration rates when watered. In contrast, water saturated conditions during most of the growing season in the fen areas hinder the availability of oxygen, resulting in both CO2 and CH4 production. Thus, water table drawdown results in decreasing CH4 effluxes but increasing CO2 effluxes. Additional controls on gas production are shown to be related to the availability of substrate and plant productivity. Subsurface gas production will produce partial and total pressure gradient causing gas transport, which in well-drained soils is mainly controlled by diffusion, whereas gas advection, bubbles and transport through roots and stems may be important in more saturated soils. Bursts of CO2 gas have been observed during spring thaw and confirmed in controlled soil thawing experiments. Field observations as well as experimental work suggest that such bursts represent partly on-going soil respiration and a physical release of gas produced during the winter. The importance of winter soil respiration is emphasised because of the fact that microbial respiration in Zackenberg samples is noted down to a least -18 degrees C. Hence, the importance of winter respiration and burst events in relation to seasonal and future climate trends requires more than just summer measurements. For example, the autumn period seems important as snowfall prior to low air temperature may insulate the soil, keeping soil temperatures high. This will extend the period of high soil respiration rates and thereby increase the importance of the winter period for the annual carbon balance. Because of the complexity of factors controlling subsurface gas production, we conclude that different parts of the landscape will respond quite differently to the same climate changes as well as that short-term effects are likely to be different from long-term effects.