Soil organic carbon is a key component of soil health, because of its role in soil structure and fertility and in mitigating annual anthropogenic CO2Soil organic carbon is one of the largest terrestrial carbon reservoirs that humans can manage.
An important element in assessing a soil's capacity to store carbon is its age, which can be assessed by modelling or experimentally using carbon isotopes. But the results obtained by the different methods are not consistent, and there are even major differences. For example, the well-known carbon-14 dating method gives soil carbon ages that are a factor of 6 to 10 higher than those estimated by modelling and using stable carbon isotopes.
A totally independent method is therefore required. We propose an alternative and independent method based on the measurement of the 36Cl in soils. The 36Cl is a radionuclide produced naturally in the atmosphere by cosmic radiation, but it is also man-made by the nuclear industry. Its production increased by three orders of magnitude when nuclear bombs were tested in the 50s and 70s. In soils, some chlorine, including 36Cl, is retained by soil organic matter in the form of organochlorine molecules.
We show that the massive arrival of 36Cl in soils during nuclear testing and its storage in soils can be used to assess the age of soil organic carbon. We measured the stocks of 36Cl retained in the different layers of a forest soil sampled at one of the stations of the Observatoire Pérenne de l'Environnement (OPE) workshop site in Meuse/Haute-Marne. Cl and 36Cl were extracted from the soils using a hydropyrolysis protocol developed at CEREGE. Measurements of Cl and 36These data are compared with the inflows (rain, vegetation) and outflows (drainage) in the region. 36Cl (Figure 1) and can be used to determine the retention time of the 36Cl in the soil. Our results show that this time increases with depth, with durations ranging from 20 years at the surface to 322 years at a depth of 60 cm. These retention times for 36Cl in soils are comparable to the average ages estimated by approaches based on modelling or stable carbon isotopes. This work therefore suggests that the retention time of 36Cl in a soil can be used as an indicator of the age of the soil's organic carbon.
Figure 1: Conceptual model used to estimate the retention time of 36Cl in the soil. The different boxes represent the stocks of 36Cl measured in different soil layers. The arrows represent the fluxes between these different layers. Within each layer, a small fraction of the 36Cl is fixed in the organic matter of the layer. This fraction is estimated at 5 % of the surface flux and decreases exponentially with depth. This decrease in quantities retained is matched by an increase in retention time with depth.
Read more
36Cl, a new tool to assess soil carbon dynamics, Scientific Report, https://doi.org/10.1038/s41598-023-41555-x
Cécile Grapeloup, Sophie Cornu, Xavier Giraud, Julie Pupier, Aster Team, Valery Guillou, Philippe Ciffroy, Beatriz Lourino Cabana, Cécile Couegnas, Christine Hatté, Lucilla Benedetti
Contact
Sophie Cornu and Lucilla Benedetti, CEREGE