Turnover of organic carbon as a function of soil depth

International audienceThe size of the organic carbon reservoir of the planet's soils is estimated to be between1500 Gt C and 2400 Gt in the first meter. The organic carbon in soils comes from the inputs of vegetation, which has converted atmospheric CO2 into biomass. This carbon remains stored in the soil for varying periods of time, before returning to the atmosphere as CO2 produced by decomposers. This continuous exchange between soil and atmosphere helps to regulate the CO2 atmospheric content. But knowledge is lacking on organic carbon turnover in soil horizons below 30 centimeters, although they contain half of the organic carbon in soils.To fill this gap, we performed a global meta-analysis using natural stable carbon isotopes(13C) on 112 soils that were submitted to a vegetation change (C3 / C4 vegetationconversion) and estimated the depth distribution of the carbon recently transferred fromthe atmosphere to soils and the SOC age distribution over the soil profile.We found, in agreement with previous works, that soil at a depth of 30–100 centimetresbeneath the surface (the subsoil) contains on average 47 per cent of the topmost metre’sSOC stocks. However, we showed that this subsoil accounts for just 19 per cent of the SOC that has been recently incorporated (within the past 50 years) into the topmost metre. Variations in the relative allocation of carbon to deep soil layers are better explained by the aridity index than by mean annual temperature. Land use for crops reduces the incorporation of carbon into the soil surface layer, but not into deeper layers.There is a very strong vertical gradient in the dynamics of organic carbon. In tropicalenvironments, for example, the median carbon age ranges from 7 years at the top of theprofile, to 1250 years at a depth of 1 meter.Our results showing that SOC dynamics is strongly depth dependent called for improvingEarth system models by incorporating multilayer SOC modules