Widespread recognition of the importance of soil CO 2 efflux as a major source of CO 2 to the atmosphere has led to active research. A large soil respiration database and recent reviews have compiled data, methods, and current challenges. This study highlights some deficiencies for a proper understanding of soil CO 2 efflux focusing on processes of soil CO 2 production and transport that have not received enough attention in the current soil respiration literature. It has mostly been assumed that soil CO 2 efflux is the result of biological processes (i.e. soil respiration), but recent studies demonstrate that pedochemical and geological processes, such as geothermal and volcanic CO 2 degassing, are potentially important in some areas. Besides the microbial decomposition of litter, solar radiation is responsible for photodegradation or photochemical degradation of litter. Diffusion is considered to be the main mechanism of CO 2 transport in the soil, but changes in atmospheric pressure and thermal convection may also be important mechanisms driving soil CO 2 efflux greater than diffusion under certain conditions. Lateral fluxes of carbon as dissolved organic and inorganic carbon occur and may cause an underestimation of soil CO 2 efflux. Traditionally soil CO 2 efflux has been measured with accumulation chambers assuming that the main transport mechanism is diffusion. New techniques are available such as improved automated chambers, CO 2 concentration profiles and isotopic techniques that may help to elucidate the sources of carbon from soils. We need to develop specific and standardized methods for different CO 2 sources to quantify this flux on a global scale. Biogeochemical models should include biological and non-biological CO 2 production processes before we can predict the response of soil CO 2 efflux to climate change. Improving our understanding of the processes involved in soil CO 2 efflux should be a research priority given the importance of this flux in the global carbon budget.
