Adsorption of organic matter on clay minerals in the Dajiuhu peat soil chronosequence, South China

Abstract

Clay minerals play important roles in stabilizing organic matter (OM) in soils. In order to investigate the nature of OM in association with clay minerals in peatland soils, the coarse (>0.3 mm) and clay (<2 μm) fractions of samples from a 250-cm-long core from the Dajiuhu peatland in Hubei Province (central China) were analyzed using gas chromatography–mass spectrometry (GC–MS), solid state 13C nuclear magnetic resonance (NMR) spectroscopy, and Fourier transform infrared spectrophotometry (FTIR) methods. The n-alkanes consist mainly of carbon numbers of C21 to C33 and show odd-over-even predominance. Alkanes associated with clay minerals are better protected from biodegradation in old soils, whereas clay minerals in young soils are less favorable for OM interaction. Fatty acids are mainly saturated straight-chain acids (C12–C30), with minor branched and unsaturated acids, showing even-over-odd predominance. The wide distribution of short-chain fatty acids in the soils indicates intensive microbial activity. NMR results confirmed the occurrence of alkyl C, O-alkyl C, aryl C, and carboxyl C in the soil deposits. Aromatic compounds are prone to adsorb to clay minerals, and their abundances in the clay fraction increase as diagenesis proceeds. Polysaccharides are present mainly in young soils and are observed strongly binding to clay minerals, but these compounds are prone to microbial degradation due to their structural instability. Aromatic compounds are bound mainly on the silicon-basal planes. In general, old soils are characterized by stronger chemical interactions between aromatic compounds and clay minerals, as reflected by their lower band positions. Nevertheless, only small amounts of aromatic compounds were found chemically complexed with clay minerals, while most OM in the samples was physically trapped by clay-mineral aggregates. Preservation of OM in the lower profile of the peat soils is mainly attributed to persistently reducing conditions.