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. The largest share of total soil organic carbon (OC) isassociated with minerals. However, the factors that determine the amount andturnover of slower- versus faster-cycling components of mineral-associatedcarbon (MOC) are still poorly understood. Bioavailability of MOC is thoughtto be regulated by desorption, which can be facilitated by displacement andmobilization by competing ions. However, MOC stability is usually determinedby exposure to chemical oxidation, which addresses the chemical stability ofthe organic compounds rather than the bonding strength of the OC–mineralbond. We used a solution of NaOH, a strong agent for desorption due to highpH, and NaF, adding F−, a strongly sorbing anion that can replaceanionic organic molecules on mineral surfaces, to measure the maximumpotentially desorbable MOC. For comparison, we measured maximal potentialoxidation of MOC using heated H2O2. We selected MOC samples(&gt; 1.6 g cm3) obtained from density fractionation of samplesfrom three soil depth increments (0–5, 10–20, and 30–40 cm) of fivetypical soils of central Europe, with a range of clay and pedogenic oxidecontents, and under different ecosystem types (one coniferous forest, twodeciduous forests, one grassland, and one cropland). Extracts and residues wereanalysed for OC and 14C contents, and further chemically characterizedby cross-polarization magic angle spinning 13C-nuclearmagnetic resonance (CPMAS-13C-NMR). We expected that NaF–NaOH extraction would removeless and younger MOC than H2O2 oxidation and that the NaF–NaOHextractability of MOC is reduced in subsoils and soils with high pedogenic oxide contents. The results showed that a surprisingly consistent proportion of 58 ± 11 % (standard deviation) of MOC was extracted with NaF–NaOH across soils,independent of depth, mineral assemblage, or land use conditions. NMR spectra revealedstrong similarities in the extracted organic matter, with more than 80 %of OC in the O/N (oxygen and/or nitrogen) alkyl and alkyl C region. Total MOC amounts were correlatedwith the content of pedogenic oxides across sites, independent of variationsin total clay, and the same was true for OC in extraction residues. Thus,the uniform extractability of MOC may be explained by dominant interactionsbetween OC and pedogenic oxides across all study sites. While Δ14C values of bulk MOC suggested differences in OC turnover betweensites, these were not linked to differences in MOC extractability. Asexpected, OC contents of residues had more negative Δ14C valuesthan extracts (an average difference between extracts and residues of 78 ± 36 ‰), suggesting that non-extractable OC is older.Δ14C values of extracts and residues were strongly correlatedand proportional to Δ14C values of bulk MOC but were not dependenton mineralogy. Neither MOC extractability nor differences in Δ14C values between extracts and residues changed with depth along soilprofiles, where declining Δ14C values might indicate slower OCturnover in deeper soils. Thus, the 14C depth gradients in the studiedsoils were not explained by increasing stability of organic–mineralassociations with soil depth. Although H2O2 removed 90 ± 8 % of the MOC, the Δ14C values of oxidized OC (on average −50 ± 110 ‰) were similar to those of OC extracted withNaF–NaOH (−51 ± 122 ‰), but oxidation residues(−345 ± 227 ‰) were much more depleted in 14Cthan residues of the NaF–NaOH extraction (−130 ± 121 ‰). Accordingly, both chemical treatments removed OCfrom the same continuum, and oxidation residues were older than extractionresidues because more OC was removed. In contrast to the NaF–NaOHextractions, higher contents of pedogenic oxides slightly increased theoxidation resistance of MOC, but this higher H2O2 resistance didnot coincide with more negative Δ14C values of MOC nor itsoxidation residues. Therefore, none of the applied chemical fractionationschemes were able to explain site-specific differences in Δ14Cvalues. Our results indicate that total MOC was dominated by OC interactionswith pedogenic oxides rather than clay minerals, as we detected nodifference in bond strength between clay-rich and clay-poor sites. Thissuggests that site-specific differences in Δ14C values of bulkMOC and depth profiles are driven by the accumulation and exchange rates ofOC at mineral surfaces.

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