Rewetting of soil: Revisiting the origin of soil CO2 emissions

Rewetting dry soils is associated with a burst of microbial activity and mineralization, which manifests itself as a pulse in soil CO2 emissions, long-known as the Birch effect. In arid and semi-arid systems, soil CO2 losses upon rewetting at the end of extended dry periods can contribute a significant fraction to the overall carbon (C) budget. Microbial biomass is one of the sources of mineralized C, as was demonstrated over 30 years ago (Kieft et al., 1987). The present paper offers a perspective on how the field has progressed since the 1987 paper was published in Soil Biology & Biochemistry, what it means in terms of current concerns about global climate change, and the needs and potential emphases of future research. Many studies since 1987 have addressed the origin of this CO2 pulse, finding multiple possible C sources involving both biotic and abiotic processes. We propose that the magnitude of the rewetting event (Delta psi) determines the relative contribution from the array of substrates that contribute to the soil CO2 pulse upon rewetting. The magnitude of the CO2 pulse is likely related to soil physical characteristics and to the size of the available C pool, which is partly controlled by plants. Further, the relative contributions of the mechanisms generating soil CO2 pulses upon rewetting are likely to be modified by climate change. To understand and predict the magnitude of soil CO2 pulses upon rewetting, we advocate continued cross-disciplinary research involving soil microbial ecology, soil physics, soil chemistry, as well as increased integration and recognition of the importance of plant-soil interactions in controlling the soil C pools contributing to soil CO2 pulses.