Intimate association between O/N-alkyl carbon and iron oxides in clay fractions of forest soils

Iron oxides with high specific surface areas promote the stabilisation of soil organic matter. We compared solid state ¹³C nuclear magnetic resonance (NMR) spectra of clay fractions from A horizons before and after demineralisation with 10% hydrofluoric acid (HF) to study selective associations between Fe oxides and functional groups in soil organic matter. The samples were collected from soils under temperate forest conditions, classified as Cambisols, Leptosols, Luvisols and Phaeozem. After removal of Fe oxides by demineralisation, solid state ¹³C NMR spectra of clay fractions from Leptosols and Luvisols showed an increase in the relative O/N-alkyl C intensity that ranged from 3% to 20%. A variable contact time experiment with a sample from a Luvisol without HF treatment showed that protons in the vicinity of O/N-alkyl C relaxed faster than in the vicinity of other functional groups. Therefore, part of the O/N-alkyl C was invisible unless very short contact times were applied. This demonstrated that Fe oxides and O/N-alkyl C are preferentially associated in soils. The O/N-alkyl C signal loss in the untreated samples was strongly correlated with the ratio of dithionite-extractable iron oxides and the organic carbon (OC) concentration. Our study showed that OC:Fe ratios >1 do not necessarily assure that reasonable solid state ¹³C NMR spectra will be obtained. In fact, clay fractions with OC:Fe ratios between 2.2 and 3.8 were affected by association between Fe oxides and O/N-alkyl C and had to be treated with HF to circumvent a selective signal loss of O/N-alkyl C.