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.

中文翻译：

---

欧洲中部土壤中矿物结合的有机碳的年龄分布，可提取性和稳定性

在土壤中总有机碳（OC）的最大份额与矿物质有关。但是，对于决定矿物相关碳（MOC）的慢循环和快循环成分的数量和周转率的因素，人们仍然知之甚少。认为MOC的生物利用度受解吸作用的调节，而解吸和竞争离子的迁移可促进解吸作用。但是，MOC的稳定性通常由暴露于化学氧化作用来确定，化学氧化作用涉及有机化合物的化学稳定性，而不是OC-矿物键的结合强度。我们使用氢氧化钠，解吸强烈剂的溶液由于高pH和NaF，加入˚F ^-，一种强吸附性阴离子，可以替代矿物表面上的阴离子有机分子，以测量潜在的最大可解吸MOC。为了进行比较，我们使用加热的H ₂ O ₂测量了MOC的最大电势氧化。我们从中欧五种典型土壤的三种土壤深度增量（0–5、10–20和30–40 cm）中，用一定范围的黏土从样品的密度分级中选择了MOC样品（>  1.6 g cm ³）以及成岩氧化物含量，以及在不同的生态系统类型（一种针叶林，两种落叶林，一种草原和一种农田）下。分析提取物和残留物的OC和¹⁴C含量，并进一步通过交叉极化魔角旋转¹³ C-核磁共振（CPMAS - ¹³ C-NMR）进行化学表征。我们预计，NaF–NaOH萃取会去除比H ₂ O ₂氧化更少且更年轻的MOC ，并且在高土壤成因氧化物含量的土壤和土壤中，MOC的NaF–NaOH萃取能力会降低。结果显示，出乎意料的一致比例是58  ± NaF–NaOH提取的土壤中MOC的含量为11％（标准偏差），与深度，矿物组合或土地使用条件无关。NMR光谱显示，萃取的有机物具有高度相似性，O / N（氧和/或氮）烷基和烷基C区域中的OC含量超过80％。总MOC量与整个站点上的成岩氧化物含量相关，而与总粘土的变化无关，萃取残留物中的OC也是如此。因此，MOC的均匀可萃取性可以通过所有研究地点的OC和成岩氧化物之间的显性相互作用来解释。虽然Δ ¹⁴大量MOC的C值表明位点之间的OC转化率存在差异，这些与MOC可萃取性的差异无关。如所预期的，残基的OC内容有更负的Δ ¹⁴ C值大于提取物（提取物和78个残基之间的平均差 ±  36‰），表明不可萃取OC是老年人。 Δ ¹⁴提取物和残基的C值显着相关，并正比于Δ ¹⁴散装MOC的C值但没有依赖矿物学。既不MOC萃取性也不差Δ ^14个提取物和残基之间C值随深度变化沿土壤剖面，其中下降Δ ¹⁴C值可能表明较深土壤中的OC转换较慢。因此，研究土壤中¹⁴ C深度的梯度不能通过增加有机-矿物之间的联系随土壤深度的稳定性来解释。虽然ħ ₂ ö ₂除去90  ±  8％的MOC中，Δ ¹⁴氧化OC的C值（平均- 50  ±  110‰）的那些相似OC与NAF-NaOH萃取（- 51  ±  122‰），但是 在¹⁴ C中，氧化残留物（− 345  ± 227‰）比NaF–NaOH提取物残留物（− 130  ± 121‰）。因此，两种化学处理均从相同的连续物中去除了OC，并且由于去除了更多的OC，氧化残留物比萃取残留物更老。与此相反的NAF-的NaOH提取，成土氧化物的含量较高稍微增加MOC的耐氧化性，但该较高的ħ ₂ ö ₂电阻与多个负没有重合Δ ¹⁴ MOC也不其氧化残基的C值。因此，没有应用的化学分离方案能够在解释特定地点的差异Δ ¹⁴C值。我们的结果表明，总的MOC取决于OC与成岩氧化物而不是粘土矿物的OC相互作用，因为我们发现富粘土和贫粘土位点之间的粘结强度没有差异。这表明在该位点特异性差异Δ ^14个的散装MOC和深度剖面C值是由在矿物表面OC的积累和汇率驱动。