36 Cl, a new tool to assess soil carbon dynamics

Soil organic carbon is one of the largest surface pools of carbon that humans can manage in order to partially mitigate annual anthropogenic CO 2 emissions. A significant element to assess soil sequestration potential is the carbon age, which is evaluated by modelling or experimentally using carbon isotopes. Results, however, are not consistent. The 14 C derived approach seems to overestimate by a factor of 6–10 the average carbon age in soils estimated by modeling and 13 C approaches and thus the sequestration potential. A fully independent method is needed. The cosmogenic chlorine nuclide, 36 Cl, is a potential alternative. 36 Cl is a naturally occurring cosmogenic radionuclide with a production that increased by three orders of magnitude during nuclear bomb tests. Part of this production is retained by soil organic matter in organochloride form and hence acts as a tracer of the fate of soil organic carbon. We here quantify the fraction and the duration of 36 Cl retained in the soil and we show that retention time increases with depth from 20 to 322 years, in agreement with both modelling and 13 C-derived estimates. This work demonstrates that 36 Cl retention duration can be a proxy for the age of soil organic carbon.