Mineral-associated soil organic matter: characteristics and behavior under diagenesis

Abstract

The main part of soil organic matter (OM) is mineral-associated: 88 ± 11% of С and even more – 93 ± 9% of N. The aims of the given study were: 1 – to demonstrate experimentally the adsorption selectivity of organic compounds towards minerals with different physico-chemical properties (palygorskite vs montmorillonite); 2 – to characterize mineral-associated OM of buried Late Holocene palaeosols and estimate its diagenetic transformations; 3 – to investigate the OM of humin from modern soils of different genesis and Pleistocene and Holocene palaeosols and estimate its diagenetic transformations. The basic soil properties were determined using standard methods. Clay fractions (<2 um) – natural organo-mineral complexes (OMC) were obtained by sedimentation, their mineralogy was studied by XRD. The elemental composition of OM was studied with CNS-analyzer. The structural characteristics of organic matter were determined with the solid-state ¹³C-NMR-spectroscopy and FTIR-spectroscopy, isotopic composition of C and N – by mass-spectrometry. The obtained results show that the characteristics of mineral-associated OM depends on the properties of mineral “filter” as well as the fate of OM under diagenesis: how long, in what quantity and quality it will persist. It was shown that palygorskite adsorbed predominantly O-alkyls, which are chemically strongly bound. As a result, the age of fulvic type humus in palygoskite palaeosols can reach 300 My. From other side humus of smectitic paleosols of the same age is present by deeply transformed aromatic structures (“coal”). Mineral-associated OM of buried under kurgans Holocene palaeosols contains more alkyls and carboxylic groups, is less aromatic in a comparison with OM of the respective soils. The specific feature of mineral-associated OM is its enrichment in N-compounds. The later are present by both vegetal and microbial compounds, and demonstrate the large affinity towards the mineral surfaces. The formation of chemical bounds between them provides the persistence of OM in OMC. E.g. H₂O₂ treatment results in preferential destruction of C-rich compounds and oxidized OM demonstrates larger C/N values. Mineral-associated OM of buried Holocene soils keeps the decreased values of C/N (7–14 vs 14–21 for OM of whole soils). Additionally they are characterized by heavier isotopic composition of δ¹⁵N in a comparison with the respective soils (5–11‰ vs 6–9‰). It could be explained either by the accumulation of microbial N, or increasing of the humification degree – the loss of aliphatic C and increasing of aromaticity. Humin is the considerable part of soil humus. Experimentally shown that OM of humins both of soils and OMC is enriched in O-alkyls and C of acetal groups. OM of humins are not homogeneous, and consists from at least two groups: mineral-associated OM and partly mineralized plant fragments. As a consequence, the content of humin in OMC is smaller in a comparison with respective soils. It is concluded that mineral-associated OM and humin as well as soil humus represent dynamic soil systems.