INVITED REVIEW Mind the gap: non-biological processes contributing to soil CO 2 efflux ANA REY Department of Biogeography and Global Change, National Museum of Natural Sciences (MNCN), Spanish Scientific Council (CSIC), C/Serrano 115, 28006 Madrid, Spain Abstract 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 chal- lenges. 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 stud- ies 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 tech- niques 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 dif- ferent 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. Improv- ing 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. Keywords: atmospheric pumping, carbonates, dissolved inorganic carbon, dissolved organic carbon, geological CO 2 , photodegradation, soil erosion, thermal convection Received 15 April 2014 and accepted 14 November 2014 Introduction Carbon stored in soils represents the largest carbon pool of terrestrial ecosystems with global estimates of organic pools of ca. 2400 Pg (Batjes, 1996; Jobbagy & Jackson, 2000). Soil carbon dynamics is therefore essen- tial to predict the future global carbon balance. Soil CO 2 efflux is the second largest flux of CO 2 from terrestrial ecosystems to the atmosphere (Bond-Lamberty & Thomson, 2010a) with 10% of atmospheric CO 2 cycling through soils annually (Reichstein & Beer, 2008). For this reason, research on this topic has been very active over the last few decades. In the current literature, soil CO 2 efflux has been mostly assumed to be equivalent to soil respiration, that is, to biological activity within soils, but non-biological processes contribute to soil CO 2 efflux in many regions of the world (e.g. Rey et al., 2012a; Roland et al., 2013). Soil respiration is defined as the flux of CO 2 resulting from the biological activity of roots, microfauna and microorganisms within the soil. Although the term soil CO 2 efflux is often used, the flux of CO 2 from soils is not the result of biological activity alone because other abiotic processes release CO 2 as well. Many studies reporting soil CO 2 efflux rates in sev- eral biomes, including temperate, boreal, tropical, Arc- tic and deserts have been published. This intense research has prompted excellent review summarizing methods (Kuzyakov, 2006; Subke et al., 2006; Brugge- mann et al., 2011), processes (Smith et al., 2009; Kuzya- kov & Gavrichkova, 2010; Kim et al., 2012), estimates for different biomes (Bond-Lamberty & Thomson, 2010a) and current challenges (Vargas et al., 2010a). Apart from evidencing great progress as well as limited understanding of the biological processes involved in soil respiration (Vargas et al., 2010a), non-biological Correspondence: Ana Rey, tel. 0034 917452500 ext: 981162, fax 0034 915640800, e-mails: anareysimo@gmail.com; arey@mncn. csic.es 1752 © 2014 John Wiley & Sons Ltd Global Change Biology (2015) 21, 1752–1761, doi: 10.1111/gcb.12821