A microanalytical study of iron, aluminium and organic matter relationships in soils with contrasting hydrological regimes

• Soils of contrasting hydrological class had similar Fe (hydr)oxide/SOM interactions.
• Weakly crystalline Fe (hydr)oxides had sorptive association with amorphous fine SOM.
• Strongly crystalline Fe (hydr)oxide had precipitative association with SOM.
• Strongly crystalline Fe (hydr)oxide nodules encapsulate tissue residues.

It is recognised that interactions between mineral oxides and soil organic matter (SOM) are an important factor in the stabilisation of soil organic carbon (SOC). The nature of these interactions is particularly complex in gleyed soils that experience periodic waterlogging and changeable redox conditions. This study explores the complex patterns of iron (Fe) (hydr)oxides and SOM in three soils with contrasting hydrological regimes (Gleysol, Stagnosol and Cambisol). Micromorphological examination of undisturbed soil thin sections was teamed with SEM–EDS analysis and sequential dissolution of Fe pedofeatures to gain a better understanding of the mechanisms involved in SOM stabilisation by mineral oxides. All soils contained a diverse range of particulate SOM forms and Fe pedofeatures; the degree of impregnation of the Fe pedofeatures was found to increase with depth and a strong correlation between the presence of SOM and Fe pedofeatures was found to exist through all soils. Weakly crystalline Fe (hydr)oxides were found in association with partially degraded tissue residues and amorphous fine organic matter (OM). Strongly crystalline Fe (hydr)oxides were found in all impregnative Fe pedofeatures and high Fe/C ratios suggested precipitative processes rather than sorption dominate SOC sequestration in these features. In addition, at the core of some strongly impregnated Fe nodules, occluded well preserved organic tissues were identified. The study highlights the range of processes and complexity involved in SOC sequestration over mm to cm scales and untangling this complexity is vital to understanding and modelling terrestrial C fluxes. Whilst the methods used here are not without their complications, the value of micro-scale studies of undisturbed soil thin sections is clearly demonstrated.