Sodium hypochlorite oxidation reduces soil organic matter concentrations without affecting inorganic soil constituents

Oxidative treatment can isolate a stable organic matter pool in soils for process studies of organic matter stabilization. Wet oxidation methods using hydrogen peroxide are widely used for that purpose, but are said to modify poorly crystalline soil constituents. We investigated the effect of a modified NaOCl oxidation (pH 8) on the mineral composition of 12 subsoils (4.9-38.2 g organic C kg(-1)) containing varying amounts of poorly crystalline mineral phases, i.e. 1.1-20.5 g oxalate-extractable Fe kg(-1), and of different phyllosilicate mineralogy. Post-oxidative changes in mineral composition were estimated by (i) the determination of elements released into the NaOCl solution, (ii) the difference in dithionite- and oxalate-extractable Si, Al and Fe, and (iii) the specific surface areas (SSAs) of the soils. The NaOCl procedure reduced the organic C concentrations by 12-72%. The amounts of elements released into the NaOCl extracts were small (<= 0.14 g kg(-1) for Si, <= 0.13 g kg(-1) for Al, and <= 0.03 g kg(-1) for Fe). The SSA data and the amounts of dithionite- and oxalate-extractable elements suggest that the NaOCl oxidation at pH 8 does not attack pedogenic oxides and hydroxides and only slightly dissolves Al from the poorly crystalline minerals. Therefore, we recommend NaOCl oxidation at pH 8 for the purpose of isolating a stable organic matter pool in soils for process studies of organic matter stabilization.