An increasing amount of CO2 emitted from human activities globally does not add to the increase in the atmosphere. Taking the ocean sink into acount, the fate of about 3 Gt C annually remains to be explained. This huge amount is calculated as the residual from known fluxes1 We present an `on the spot´ study that is based on systematic soil sampling in different regions and over the years since 2004. The difference between gross heterotrophic respiration (GHR) in the soil, and net heterotrophic respiration (NHR) that is the part of the carbon dioxide leaving the ground surface, was analyzed. The accumulated data indicate a within-soil CO2 sink of the same magnitude as the sink derived from different fluxes1 . Both approaches describe the same sink but our results show that the sink is CO2 uptake from the soil atmosphere, not emitted CO2 that is returned to some unknown area on land. The energy yield needed from nitrification to explain the observed reduction of CO2 to organic material is large, and NH4 + is recycled several times. It was unexpectedly observed that O2 was released in this gross nitrification cycle and this was confirmed with H2 18O incubations in soils2 . The large CO2 sink changes between seasons, between sites and even from a sink to an additional source, which may explain why it has so long been ”missing”.

1 Le Quéré et al. 2015. Global Carbon Budgets 2015. Earth Syst. Sci. Data 7, 349–396

2 Fleischer S. et al. 2013. Dark oxidation of water in soils. Tellus B 65, 20490