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Changes in soil organic carbon (SOC) storage have the potential to affect global climate; hence identifying environments with a high capacity to gain or lose SOC is of broad interest. Many cross-site studies have found that SOC-poor soils tend to gain or retain carbon more readily than SOC-rich soils. While this pattern may partly reflect reality, here we argue that it can also be created by a pair of statistical artifacts. First, soils that appear SOC-poor purely due to random variation will tend to yield more moderate SOC estimates upon resampling and hence will appear to accrue or retain more SOC than SOC-rich soils. This phenomenon is an example of regression to the mean. Second, normalized metrics of SOC change-such as relative rates and response ratios-will by definition show larger changes in SOC at lower initial SOC levels, even when the absolute change in SOC does not depend on initial SOC. These two artifacts create an exaggerated impression that initial SOC stocks are a major control on SOC dynamics. To address this problem, we recommend applying statistical corrections to eliminate the effect of regression to the mean, and avoiding normalized metrics when testing relationships between SOC change and initial SOC. Careful consideration of these issues in future cross-site studies will support clearer scientific inference that can better inform environmental management.

Removal of atmospheric carbon dioxide (CO2) combined with emission reduction is necessary to keep climate warming below the internationally agreed upon 2°C target. Soil organic carbon sequestration through agricultural management has been proposed as a means to lower atmospheric CO2 concentration, but the magnitude needed to meaningfully lower temperature is unknown. We show that sequestration of 0.68 Pg C year-1 for 85 years could lower global temperature by 0.1°C in 2100 when combined with a low emission trajectory [Representative Concentration Pathway (RCP) 2.6]. This value is potentially achievable using existing agricultural management approaches, without decreasing land area for food production. Existing agricultural mitigation approaches could lower global temperature by up to 0.26°C under RCP 2.6 or as much as 25% of remaining warming to 2°C. This declines to 0.14°C under RCP 8.5. Results were sensitive to assumptions regarding the duration of carbon sequestration rates, which is poorly constrained by data. Results provide a framework for the potential role of agricultural soil organic carbon sequestration in climate change mitigation.

The largest share of total soil organic carbon (OC) is associated with minerals. The portions and turnover of stable and faster cycling mineral-associated carbon (MOC) as well as the determining factors across different soils and soil depths are still unknown. Bioavailability of MOC is supposedly regulated by desorption but instead, its stability was so far mostly tested by exposure to chemical oxidation. Therefore, we determined the extractability of MOC into a mixture of 0.1 M NaOH and 0.4 M NaF as a measure for maximal potential desorbability, and compared it with maximal potential oxidation in heated H2O2. We selected samples of three soil depth increments (0-5 cm, 10-20 cm, 30-40 cm) of five typical soils of the mid-latitudes, differing contents of clay and pedogenic oxides, and being under different land use. Extracts and residues were analyzed for OC and 14C contents, and further chemically characterized by CPMAS-13C-NMR. We hypothesized NaF-NaOH extraction to remove less and younger MOC than H2O2 oxidation, and extractable MOC to be less and relatively older in subsoils and soils with high contents of pedogenic oxides. A surprisingly constant portion of 58 ± 11 % (standard deviation) of MOC was extractable across soils, independent of depths, mineral assemblage, or land use. NMR spectra revealed strong similarities of the extracted organic matter, with more than 80 % of OC in the O/N alkyl and alkyl C region. Total MOC amounts were linked to the content of pedogenic oxides across sites, independent of variations in total clay. The uniform MOC desorption could therefore be the result of pedogenic oxides dominating the overall response of MOC to extraction. While bulk MO14C values suggested differences in OC turnover between sites, these were not linked to differences in MOC extractability. As expected, OC contents of residues had smaller 14C contents than extracts, suggesting that non-extractable OC is older. However, 14C contents of extracts and residues were strongly correlated and proportional to bulk MO14C, but not dependent on mineralogy. Also along soil profiles, where increasing MOC ages indicate slower turnover with depth, neither MOC extractability nor differences in 14C between extracts and residues changed. Increasing bonding strength with soil depths did therefore not cause the 14C depth gradients in the studied soils. Although H2O2 removed 90 ± 8 % of the MOC, the 14C content of the OC removed was similar to that of the NaF-NaOH-extracted OC, while oxidation residues were much more 14C-depleted. Different chemical treatments apparently remove OC of the same continuum, leaving increasingly older residues behind the more OC being removed. Different from the extractions, higher contents of pedogenic oxides seemingly slightly increased the oxidation-resistance of MOC, but this higher H2O2-resistance did not coincide with older MOC or oxidation residues. Our results indicate that total MOC was dominated by OC interactions with pedogenic oxides rather than clay minerals, so that no difference in MOC extraction in NaF/NaOH, and thus, bond type or strength between clay-rich and poor sites was detectable. This suggests that site-specific differences in MO14C and their depth declines are driven by the accumulation and exchange rates of OC at mineral surfaces. Accordingly, future research on M14OC should focus on soil and ecosystem properties driving dissolved organic matter formation, composition and transport along soil profiles. [ABSTRACT FROM AUTHOR]