Effects of pH adjustments after base hydrolysis: implications for understanding organic matter in soils

The composition of soil organic matter (SOM) released during a treatment with base, i.e. a 1 M KOH in 96% methanol solution, from an acidic forest soil was studied using gas chromatography and gas chromatography-mass spectrometry. To provide detailed insight into how pH adjustments of the extract obtained can affect the organic matter released, our approach involved: (I) extraction with DCM of the methanol/water extract obtained at pH values ranging from 13 to 1 after direct pH adjustments, (II) sequential acidification of the methanol/water extract obtained and extraction with DCM at pH values ranging from 11 to 1, and (III) acidification after base treatment of the reaction mixture, i.e. soil and extract, to pH 3 followed by a second alkali hydrolysis of the residue and adjustment of the reaction mixture to pH 7. Results obtained after direct pH adjustment of the extract (experiment I) indicated that the relative amounts of identified compounds are, as expected, pH dependent, i.e. based on pKa values and hydrophobicity. This phenomenon, which affected significantly the final results, may cause an under or over estimation of certain compound classes and a biased view on ester-related moieties in soils. Direct acidification of base extract in the presence of soil residue (experiment III) indicated that hydrophobic long-chain aliphatic moieties may become associated with the organic matrix left after base treatment, form new, insoluble, organic (macro) molecules or become associated with the inorganic matrix upon acidification of a base extract in the presence of a soil residue. An association with the inorganic soil matrix is discussed as another possible mode of occurrence of ester-related moieties in soil in addition to common known modes of occurrence of ester-related moieties in soils, e.g. the biopolyesters cutin and suberin. A possible contribution of ester-related acid and hydroxy substituted aliphatic monomers, in addition to aliphatic biopolymers, to the precipitate formed at the DCM-water/methanol interface is discussed as well.