Sodium hypochlorite oxidation reduces soil organic matter concentrations without affecting inorganic soil constituents A. S IREGAR *, M. K LEBER , R. M IKUTTA & R. J AHN Institut fu ¨r Bodenkunde und Pflanzenerna ¨hrung, Martin-Luther-Universita ¨t Halle-Wittenberg, Weidenplan 14, 06108 Halle, Germany Summary 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. Introduction Organic matter (OM) removal from soil samples is conducted for many purposes, such as pretreatment for particle-size ana- lysis and mineralogical analysis; to measure the amounts of metals bound to OM; to investigate the influence of mineral phase variables (specific surface area (SSA), Fe oxides, cation exchange capacity (CEC)) on the contents and composition of OM, and on soil P adsorption and desorption; and to investi- gate the influence of OM removal on B sorption. The choice of the destruction procedure for OM has become an issue of increasing importance in studies concerned with mechanisms of soil OM stabilization. Recent studies suggest that the reactivity of mineral surfaces for stabilization of OM is a function of SSA, hydrous oxide content and the density of reactive binding sites with which organic materials can interact (Baldock & Skjemstad, 2000). Organic matter destruction can be performed by thermal oxidation (Mayer & Xing, 2001) or wet oxidation, which uses a variety of protocols and oxidizing agents, such as hydrogen peroxide (H 2 O 2 ), sodium hypochlor- ite (NaOCl), potassium permanganate (KMnO 4 ), or disodium peroxodisulphate (Na 2 S 2 O 8 ). Because removal of OM by com- bustion at 350  C can alter the mineral and sesquioxide phases, wet oxidation procedures are more commonly used, especially H 2 O 2 . However, peroxide oxidation has been reported to les- sen the amounts of pedogenic oxides (Lavkulich & Wiens, 1970) and to cause partial crystal exfoliation and decomposi- tion of smectites (Theng et al., 1999). Meier & Menegatti (1997) proposed Na 2 S 2 O 8 as a reactant of superior efficiency in C removal from phyllosilicates before further mineralogical analyses. It requires a large soil:solution ratio of 1:40 and, more importantly, heating to 80  C, which is enough to change the crystallinity of goethite (Schwertmann, 1984), ferrihydrite and hydrous-Al phases (Schwertmann & Cornell, 1991), and might also change the BET-N 2 surface area of smectitic samples (Menegatti et al., 1999). In an attempt to circumvent the limitations of conventional oxidation protocols, Kaiser et al. (2002) used a modified version of the NaOCl method as originally proposed by Anderson (1963), who used replicate extractions of 15 minutes duration at 100  C with 6% NaOCl (pH 9.5) at a soil:solution ratio of 1:2. This method extracts more OM with less destruction of oxides Correspondence: M. Kleber. E-mail: kleber@landw.uni-halle.de *Present address: Department of Soil Science, Faculty of Agriculture, Pattimura University, Ambon, Indonesia. Received 2 December 2003; revised version accepted 19 August 2004 European Journal of Soil Science, August 2005, 56, 481–490 doi: 10.1111/j.1365-2389.2004.00680.x # 2004 British Society of Soil Science 481