A considerable fraction of soil-respired CO2 is not emitted directly to the atmosphere

Abstract

Soil CO2 efflux (F-soil) is commonly considered equal to soil CO2 production (R-soil), and both terms are used interchangeably. However, a non-negligible fraction of R-soil can be consumed in the subsurface due to a host of disparate, yet simultaneous processes. The ratio between CO2 efflux/O-2 influx, known as the apparent respiratory quotient (ARQ), enables new insights into CO2 losses from R-soil not previously captured by F-soil. We present the first study using continuous ARQ estimates to evaluate annual CO2 losses of carbon produced from R-soil. We found that up to 1/3 of R-soil was emitted directly to the atmosphere, whereas 2/3 of R-soil was removed by subsurface processes. These subsurface losses are attributable to dissolution in water, biological activities and chemical reactions. Having better estimates of R-soil is key to understanding the true influence of ecosystem production on R-soil, as well as the role of soil CO2 production in other connected processes within the critical zone.