Age distribution, extractability, and stability of mineral-bound organic carbon in central European soils

Abstract. The largest share of total soil organic carbon (OC) is associated with minerals. However, the factors that determine amount and turnover slower- versus faster-cycling components mineral-associated (MOC) are still poorly understood. Bioavailability MOC thought to be regulated by desorption, which can facilitated displacement mobilization competing ions. stability usually determined exposure chemical oxidation, addresses compounds rather than bonding strength OC–mineral bond. We used a solution NaOH, strong agent for desorption due high pH, NaF, adding F?, strongly sorbing anion replace anionic molecules on mineral surfaces, measure maximum potentially desorbable MOC. For comparison, we measured maximal potential oxidation using heated H2O2. selected samples (> 1.6 g cm3) obtained from density fractionation three depth increments (0–5, 10–20, 30–40 cm) five typical soils central Europe, range clay pedogenic oxide contents, under different ecosystem types (one coniferous forest, two deciduous forests, one grassland, cropland). Extracts residues were analysed OC 14C further chemically characterized cross-polarization magic angle spinning 13C-nuclear magnetic resonance (CPMAS-13C-NMR). expected NaF–NaOH extraction would remove less younger H2O2 extractability reduced in subsoils contents. results showed surprisingly consistent proportion 58 ± 11 % (standard deviation) was extracted across soils, independent depth, assemblage, or land use conditions. NMR spectra revealed similarities matter, more 80 O/N (oxygen and/or nitrogen) alkyl C region. Total amounts correlated content oxides sites, variations clay, same true residues. Thus, uniform may explained dominant interactions between all study sites. While ?14C values bulk suggested differences these not linked extractability. As expected, contents had negative extracts (an average difference 78 36 ‰), suggesting non-extractable older. proportional but dependent mineralogy. Neither nor changed along profiles, where declining might indicate slower deeper soils. gradients studied increasing organic–mineral associations depth. Although removed 90 8 MOC, oxidized (on ?50 110 ‰) similar those (?51 122 (?345 227 much depleted (?130 121 ‰). Accordingly, both treatments continuum, older because removed. In contrast extractions, higher slightly increased resistance this did coincide its Therefore, none applied schemes able explain site-specific values. Our dominated minerals, as detected no bond clay-rich clay-poor This suggests profiles driven accumulation exchange rates at surfaces.