Dennis Naafs

Occurrence and distribution of ester-bound lipids in Dutch coastal dune soils along a pH gradient

The base-hydrolysable fraction of roots and soil organic matter (SOM) in mineral subsoils from oak forests (coastal dunes, The Netherlands) with a soil pH varying from pH (CaCl2) 6.9 to 3.5 were analysed both quantitatively and qualitatively. Comparison of thermally assisted hydrolysis and methylation (THM) using tetramethylammonium hydroxide (TMAH) with base hydrolysis data confirmed that base hydrolysis is an efficient way of analysing ester-linked compounds in soils. The compounds released from SOM upon base hydrolysis comprised largely suberin-derived lipids, which were clearly related to the oak vegetation and, in particular, to oak roots. Compared with fresh (fine) roots, which are largely responsible for the input of SOM in the subsoils, unsaturated and epoxy compounds decreased in soil, probably due to oxidation. The amounts of the saponifiable fraction increased upon acidification, whilst hardly any changes were observed in their composition. This accumulation, without significant compositional alteration of ester-bound moieties in acidic soils, indicates that these chemically labile compounds can be preserved in soils.

A qualitative study on the chemical composition of ester-bound moieties in an acidic andosolic forest soil

Abstract Solvent insoluble ester-bound moieties, released after base treatment from an acidic andosolic forest soil A-horizon from Madeira, were sequentially extracted and analysed using gas chromatography and gas chromatography–mass spectrometry. The separation of all moieties released into several pH “subfractions” allowed the various products released to be studied in detail. The distributions obtained for 2-alkanols, diols, n-alkanols, n-alkanoic acids, ω-hydroxy acids, trihydroxy acids, dihydroxy diacids and phenolic acids indicate a suberin dominated origin of ester-related moieties in this andosolic soil A-horizon. However, a small contribution from plant cutins can not be ruled out completely. Besides plant biopolyesters, the detection of iso and anteiso alkanoic acids, together with short-chain di and ω-hydroxy acids suggests an additional bacterial-derived contribution. In addition to being attached to intact biopolyesters or molecules trapped in the insoluble organic macromolecular network present in soils, we suggest that hydroxy, n-alkanoic, aromatic and diacid moieties released upon base treatment of andosolic soils may occur as Al/Fe organic complexes.

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

Abstract 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.