Soil micro- and mesopores studied by N₂ adsorption and ¹²⁹Xe NMR of adsorbed xenon

N₂ adsorption (77 K) was combined with ¹²⁹Xe nuclear magnetic resonance spectroscopy of adsorbed xenon to characterise soil meso- (2-50 nm) and microporosity (<2 nm). Materials from the Alh and Bt horizons of a Luvisol, the Go horizon of a Gleysol and the Bvs horizon of a Podzol were analysed. Additionally, we examined samples obtained by mixing of H ₂O₂-treated soil fractions with organic soil material ("soil + organic matter" samples). N₂- specific surface areas (S_BET) and micropore volumes (V_micro) and areas (S_micro) were markedly affected by the presence of iron oxides in soils. Their removal with dithionite-citrate-bicarbonate (DCB) treatment was accompanied by a significant decrease in S_BET and almost complete disappearance of the micropores. The organic carbon (OC) content decreased by 10-35% after the DCB-procedure showing that a certain proportion of the soil organic matter was extracted together with iron oxides. This may point to a close association between carbon compounds and iron oxides, possibly by incorporation of low molecular weight organic compounds into the phase of iron oxides. Such interactions are expected to contribute to the stabilisation of organic carbon in soils. Indeed, as compared to the top horizon (Alh of Luvisol), a higher proportion of organic matter was co-extracted with iron oxides from the subsurface horizons (Bt of Luvisol, Go of Gleysol) characterised by higher amounts of organic carbon resisting oxidation with H ₂O₂. Examination of the mixed "soil + organic matter" samples supports that after addition, organic molecules occupy micropores (evidenced by N₂ adsorption) and narrower mesopores of the mineral matter (evidenced by ¹²⁹Xe NMR).