Towards sound comparisons of soil carbon stocks: A proposal based on the cumulative coordinates approach

• Differences in bulk density distort the comparison of SOC stocks.
• The cumulative coordinates approach (CCA) allows to correct for these distortions.
• The SOC stored by the uppermost 300 kg m− 2 of fine mineral earth is the key parameter.
• In contrast with other CCAs, gravel and stones are excluded from the calculations.
• The mathematical treatment of data affects the comparisons of SOC stocks.

The study of changes in soil organic carbon (SOC) stocks as a result of land use or climate change requires sound criteria to compare SOC stocks in different plots. Just comparing SOC stored down to a standard depth (often, 30 cm) is prone to errors, owing to the changes in soil bulk density due to shifts in SOC content or the soil compaction due to heavy machinery. To solve this problem several approaches have been suggested. Jenkinson proposed in 1971 the concept of ‘equivalent soil depth’, the depth to which a soil should be sampled to reach a certain amount of fine mineral earth. Here we present a procedure which combines that concept with the modern cumulative coordinates approach (CCA). Our method involves sampling soils down to a given depth (30 cm) with a volumetric core sampler which maintains the profile structure, and divide the obtained core in depth layers (0–5, 5–15 and 15–30 cm). Each layer is studied separately for its stone and gravel content, fine earth (< 2 mm) and coarse organic fragments. From these data we calculate the cumulative (with depth) amounts of fine earth (< 2 mm), soil organic carbon (SOC), and fine mineral earth (FME), i.e. the fine earth corrected for organic matter. Then, SOC and FME are plotted together and fitted to a double-exponential equation, to obtain the amount of SOC (kg m− 2) accumulated in the uppermost Y kg m− 2 of FME. As an example, we apply this approach to the comparison of SOC stocks in four plots differing in land use (crop, old forest, and recent pine stands), and also as to their stoniness and bulk density. Essentially, instead of comparing SOC stocks on a depth basis — amount of C stored in the uppermost Y cm of soil, for instance 30 cm — we propose to compare SOC stocks on a fine mineral earth basis — amount of C stored by the uppermost Y kg m− 2 of FME. The value we propose is Y = 300 kg m− 2 of FME. Such an approach automatically corrects for the compression or expansion of soil due to heavy machinery, or the changes in bulk density due to the shifts in SOC content, thus making contrasting plots directly comparable. It is less clear, nevertheless, the usefulness of CCAs to compare SOC stocks in plots strongly differing as to their stoniness.