Are chemical oxidation methods relevant to isolate a stable pool of centenial carbon ? Using soils from a long term bare fallow to answer this question

Several mechanisms are developed to explain carbon stabilization in soils: (i) physical protection, (ii) protection through interaction with the soil matrix and (iii) chemical recalcitrance. To date there is still no successful experimental way to isolate the stable pool of soil organic matter (SOM), i.e., the SOM that has a residence time of centuries to millennia. Moreover, the relevance of the latter mechanism is currently being discussed at this time scale (Schmidt et al., 2011).A recent study showed that long term bare fallows (LTBF) offer a unique opportunity to study the stable compartment of SOM (Barré et al., 2011), as without carbon inputs and with continuing biodegradation and mineralization, SOM becomes progressively enriched in its most stable components. At the INRA 42-plot experiment, initiated in 1928 in Versailles, over 80% of total SOM is stable.Here, we take advantage of the 42-plot LTBF to test the relevance of chemical oxidation methods to isolate a stable pool of carbon (Mikkuta et al., 2005). We selected two reagents -hydrogen peroxide, H2O2, and sodium hypochlorite, NaOCl- and studied their effect on soil carbon content and SOM chemical nature after 0 and 80 years of LTBF. If these methods can isolate the stable C, then chemical oxidation resistant-C from 0 and 80 years of bare fallow should equate, both in amount and chemical nature, TOC after 80 years of bare fallow.Materials and methods. We used 0-20 cm depth soil samples from the 42 plots archive collection. In order to prevent the influence of decomposing plant debris, we fractionated the soil samples, and separated the fraction of particles