Age distribution, extractability, and stability of mineral-bound organic carbon in central European soils
Authors: Schrumpf Marion , Kaiser Klaus , Mayer Allegra , Hempel Günter , Trumbore Susan 
Published: February 2021

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−, 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 H2O2. We selected MOC samples
(> 1.6 g cm3) 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 14C contents, and further chemically characterized
by cross-polarization magic angle spinning 13C-nuclear
magnetic resonance (CPMAS-13C-NMR). We expected that NaF–NaOH extraction would remove
less and younger MOC than H2O2 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 Δ14C 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 Δ14C values
than 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 correlated
and proportional to Δ14C values of bulk MOC but were not dependent
on mineralogy. Neither MOC extractability nor differences in Δ14C values between extracts and residues changed with depth along soil
profiles, where declining Δ14C values might indicate slower OC
turnover in deeper soils. Thus, the 14C depth gradients in the studied
soils were not explained by increasing stability of organic–mineral
associations 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 with
NaF–NaOH (−51 ± 122 ‰), but oxidation residues
(−345 ± 227 ‰) were much more depleted in 14C
than residues of the NaF–NaOH extraction (−130 ± 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 H2O2 resistance did
not coincide with more negative Δ14C values of MOC nor its
oxidation residues. Therefore, none of the applied chemical fractionation
schemes were able to explain site-specific differences in Δ14C
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 Δ14C values of bulk
MOC and depth profiles are driven by the accumulation and exchange rates of
OC at mineral surfaces.