Sodium hypochlorite separates an older soil organic matter fraction than acid hydrolysis

Different chemical treatments can be used to isolate an old and chemically resistant soil organic matter fraction from soils. Here, we compared the residues after acid hydrolysis (6 N HCl) with those obtained after NaOCl treatment (6 wt%) of the silt + clay fractions from 48 soil samples. The samples were taken at sites differing in climate and land use across Switzerland. To determine the influence of the two treatments on soil organic matter and mineral structures, we examined infrared spectra and isotopic signatures of carbon (14C and δ13C) of the residues. Treatment with NaOCl removed more (63 to 91%) organic carbon (OC) than did treatment with HCl (35 to 66%), and it had no effect on mineral structures, whereas treatment with HCl converted crystalline minerals to more amorphous ones. Increases in specific soil surface area (SSA) did not correlate with the amount of OC removed. The amount of OC removed by each treatment was not (NaOCl) or only weakly (HCl) related to the initial OC content of the silt + clay fraction, suggesting the presence of a relatively constant fraction of chemically resistant OC. 14C activities of NaOCl-resistant residues were lower than those of HCl residues, indicating that soil organic matter residues isolated by NaOCl treatment were older than the residues obtained by acid hydrolysis. It is concluded that oxidation with NaOCl is the better way than hydrolysis with HCl to obtain an operationally-defined stable organic matter fraction from soils.