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

Schrumpf, Marion; Kaiser, Klaus; Mayer, Allegra; Hempel, Günter; Trumbore, Susan

The largest share of total soil organic carbon (OC) is associated with minerals. However, the factors that determine the amount and turnover of slower- versus faster-cycling components of mineral-associated carbon (MOC) are still poorly understood. Bioavailability of MOC is thought to be regulated by desorption, which can be facilitated by displacement and mobilization by competing ions. However, MOC stability is usually determined by exposure to chemical oxidation, which addresses the chemical stability of the organic compounds rather than the bonding strength of the OC–mineral bond. We used a solution of NaOH, a strong agent for desorption due to high pH, and NaF, adding Finline-formula, a strongly sorbing anion that can replace anionic organic molecules on mineral surfaces, to measure the maximum potentially desorbable MOC. For comparison, we measured maximal potential oxidation of MOC using heated Hinline-formula2Oinline-formula2. We selected MOC samples (inline-formula> 1.6 g cminline-formula3) obtained from density fractionation of samples from three soil depth increments (0–5, 10–20, and 30–40 cm) of five typical soils of central Europe, with a range of clay and pedogenic oxide contents, and under different ecosystem types (one coniferous forest, two deciduous forests, one grassland, and one cropland). Extracts and residues were analysed for OC and inline-formula14C contents, and further chemically characterized by cross-polarization magic angle spinning inline-formula13C-nuclear magnetic resonance (CPMAS-inline-formula13C-NMR). We expected that NaF–NaOH extraction would remove less and younger MOC than Hinline-formula2Oinline-formula2 oxidation and that the NaF–NaOH extractability of MOC is reduced in subsoils and soils with high pedogenic oxide contents.

The results showed that a surprisingly consistent proportion of 58 inline-formula± 11 % (standard deviation) of MOC was extracted with NaF–NaOH across soils, independent of depth, mineral assemblage, or land use conditions. NMR spectra revealed strong 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 correlated with the content of pedogenic oxides across sites, independent of variations in total clay, and the same was true for OC in extraction residues. Thus, the uniform extractability of MOC may be explained by dominant interactions between OC and pedogenic oxides across all study sites. While inline-formulaΔ14C values of bulk MOC suggested differences in OC turnover between sites, these were not linked to differences in MOC extractability. As expected, OC contents of residues had more negative inline-formulaΔ14C values than extracts (an average difference between extracts and residues of 78 inline-formula± 36 ‰), suggesting that non-extractable OC is older. inline-formulaΔ14C values of extracts and residues were strongly correlated and proportional to inline-formulaΔ14C values of bulk MOC but were not dependent on mineralogy. Neither MOC extractability nor differences in inline-formulaΔ14C values between extracts and residues changed with depth along soil profiles, where declining inline-formulaΔ14C values might indicate slower OC turnover in deeper soils. Thus, the inline-formula14C depth gradients in the studied soils were not explained by increasing stability of organic–mineral associations with soil depth.

page1242Although Hinline-formula2Oinline-formula2 removed 90 inline-formula± 8 % of the MOC, the inline-formulaΔ14C values of oxidized OC (on average inline-formula−50 inline-formula± 110 ‰) were similar to those of OC extracted with NaF–NaOH (inline-formula−51 inline-formula± 122 ‰), but oxidation residues (inline-formula−345 inline-formula± 227 ‰) were much more depleted in inline-formula14C than residues of the NaF–NaOH extraction (inline-formula−130 inline-formula± 121 ‰). Accordingly, both chemical treatments removed OC from the same continuum, and oxidation residues were older than extraction residues because more OC was removed. In contrast to the NaF–NaOH extractions, higher contents of pedogenic oxides slightly increased the oxidation resistance of MOC, but this higher Hinline-formula2Oinline-formula2 resistance did not coincide with more negative inline-formulaΔ14C values of MOC nor its oxidation residues. Therefore, none of the applied chemical fractionation schemes were able to explain site-specific differences in inline-formulaΔ14C values. Our results indicate that total MOC was dominated by OC interactions with pedogenic oxides rather than clay minerals, as we detected no difference in bond strength between clay-rich and clay-poor sites. This suggests that site-specific differences in inline-formulaΔ14C values of bulk MOC and depth profiles are driven by the accumulation and exchange rates of OC at mineral surfaces.

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Schrumpf, Marion / Kaiser, Klaus / Mayer, Allegra / et al: Age distribution, extractability, and stability of mineral-bound organic carbon in central European soils. 2021. Copernicus Publications.

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