Mind the gap: non‐biological processes contributing to soil CO₂ efflux

Abstract

Widespread recognition of the importance of soil CO2 efflux as a major source of CO2 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 CO2 efflux focusing on processes of soil CO2 production and transport that have not received enough attention in the current soil respiration literature. It has mostly been assumed that soil CO2 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 CO2 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 CO2 transport in the soil, but changes in atmospheric pressure and thermal convection may also be important mechanisms driving soil CO2 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 CO2 efflux. Traditionally soil CO2 efflux has been measured with accumulation chambers assuming that the main transport mechanism is diffusion. New techniques are available such as improved automated chambers, CO2 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 CO2 sources to quantify this flux on a global scale. Biogeochemical models should include biological and non‐biological CO2 production processes before we can predict the response of soil CO2 efflux to climate change. Improving our understanding of the processes involved in soil CO2 efflux should be a research priority given the importance of this flux in the global carbon budget.

Contribution of soil respiration to the global carbon equation

“…The chemical production or absorption of CO 2 in soil refers to pedochemical and geological processes, such as geothermal and volcanic CO 2 degassing (Werner and Brantley, 2003), photochemical degradation of litter (Austin and Vivanco, 2006), and CO 2 absorption by alkaline soils (Xie et al, 2009). Under normal conditions, the nonbiological production of CO 2 is negligible, but it could be significant in some special ecosystems (Rey, 2015).…”

CO<sub>2</sub> efflux from soils with seasonal water repellency

“…Soil is the most important reservoir of terrestrial carbon (C), storing 4 times more C than plant biomass (IPCC, 2013), but large amounts of C are released back to atmosphere mainly as carbon dioxide (CO 2 ) formed by microbial decomposition of soil organic matter (SOM) as well as biological activity of roots and microfauna (Bond-Lamberty and Thomson, 2010;Rey, 2015). Soil moisture is one of the most important environmental factors regulating the production and transport of CO 2 in terrestrial ecosystems (Maier et al, 2011;Moyano et al, 2012).…”

Approaching the Truth of the Missing Carbon Sink

“…Although widespread recognition of the significance of carbon dynamics in terrestrial ecosystems has been well researched [21,23], there remains considerable uncertainties as to the magnitude of the sink in different regions and the contribution of different processes [5,6]. Combining with theory and analyses in the previous publications, the gap between the biologically produced CO 2 "real soil respiration" (solely interpreted as biological components, autotrophic and heterotrophic respiration) and actual measured F c [24,25] can further highlight a gap in our knowledge.…”