Numerous efforts in investigating the global carbon balance have concluded that the global CO₂ budget cannot be balanced unless a ‘‘missing carbon sink’’ is invoked. Until now there have been considerable uncertainties as to the magnitude of the C-sink in different regions and the contributions of aboveground/ underground processes. This study is aimed at presenting a first estimate of the gap between observed soil respiration and its biological components in arid areas that characterize more than 30% of Earth’s total land surface. In the current literature, soil respiration flux is interpreted in the context of an unstated hypothesis that the fluxes were largely determined by underground biological processes. However, the assumption turned out to be incorrect. Negative soil respiration fluxes in unneglectable arid regions implied a gap in our knowledge. The first estimate of the gap between observed soil respiration flux and its biological components in the global arid regions is a beginning at determining the size of the missing C-sink.