Are chemical oxidation methods relevant to isolate a soil pool of centennial carbon?
Résumé
Despite its relevance to long term carbon sequestration in soils, there is to date no successful experimental way to isolate all the stable pool of soil organic carbon (SOC), i.e. SOC that has a residence time of centuries to millennia. Long term bare fallows (LTBF) offer a unique opportunity to study stable SOC, as without carbon inputs and with continuing biodegradation and mineralization, SOC becomes progressively enriched in its most stable components. Here, we took advantage of the 42 plots LTBF experiment of Versailles (France), where C inputs stopped 80 years ago, to test the relevance of chemical oxidation methods to isolate a pool of carbon stable at the centennial timescale. To do so, we studied the effect of two oxidizing reagents -hydrogen peroxide, H2O2, and sodium hypochlorite, NaOCl on soil total organic carbon (TOC) content after 0 and 79 years of LTBF. If these methods can isolate centennial stable C, then chemical oxidation resistant-C from 0 and 79 years of LTBF should equate, in amount, with TOC after 79 years of LTBF. Results showed that chemical oxidation strongly decreased TOC in both soils. Oxidation-resistant OC accounted for ca.1 mgC g(-1) soil, which is five times smaller than TOC in the untreated LTBF79 soils, after 79 years of biological oxidation. Moreover, the amount of oxidation-resistant OC was significantly lower in LTBF79 compared to LTBF0. We conclude that neither centennial stable carbon nor older stable carbon can be successfully quantified by these chemical oxidation methods.