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

Citation metadata

From: Biogeosciences(Vol. 18, Issue 3)
Publisher: Copernicus GmbH
Document Type: Article
Length: 741 words

Document controls

Main content

Abstract :

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 F.sup.-, 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 H.sub.2 O.sub.2 . We selected MOC samples ( 1.6 g cm.sup.3) 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 .sup.14 C contents, and further chemically characterized by cross-polarization magic angle spinning .sup.13 C-nuclear magnetic resonance (CPMAS-.sup.13 C-NMR). We expected that NaF-NaOH extraction would remove less and younger MOC than H.sub.2 O.sub.2 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 ± 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 Î.sup.14 C 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 Î.sup.14 C values than extracts (an average difference between extracts and residues of 78 ± 36 0/00), suggesting that non-extractable OC is older. Î.sup.14 C values of extracts and residues were strongly correlated and proportional to Î.sup.14 C values of bulk MOC but were not dependent on mineralogy. Neither MOC extractability nor differences in Î.sup.14 C values between extracts and residues changed with depth along soil profiles, where declining Î.sup.14 C values might indicate slower OC turnover in deeper soils. Thus, the .sup.14 C depth gradients in the studied soils were not explained by increasing stability of organic-mineral associations with soil depth. Although H.sub.2 O.sub.2 removed 90 ± 8 % of the MOC, the Î.sup.14 C values of oxidized OC (on average -50 ± 110 0/00) were similar to those of OC extracted with NaF-NaOH (-51 ± 122 0/00), but oxidation residues (-345 ± 227 0/00) were much more depleted in .sup.14 C than residues of the NaF-NaOH extraction (-130 ± 121 0/00). 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 H.sub.2 O.sub.2 resistance did not coincide with more negative Î.sup.14 C values of MOC nor its oxidation residues. Therefore, none of the applied chemical fractionation schemes were able to explain site-specific differences in Î.sup.14 C 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 Î.sup.14 C values of bulk MOC and depth profiles are driven by the accumulation and exchange rates of OC at mineral surfaces.

Source Citation

Source Citation   

Gale Document Number: GALE|A651980835