Equivalent-mass versus fixed-depth as criteria for quantifying soil carbon sequestration: How relevant is the difference?

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Soil organic carbon (SOC) sequestration is often quantified through the comparison of SOC stocks down to a fixed depth, usually 30 cm. This approach gives biased estimates because SOC sequestration changes soil bulk density. To account for these biases, the comparison of SOC stocks on an equivalent mass basis is recommended. How big are the biases due to the use of a fixed-depth approach (either under- or overestimations of SOC sequestration), and whether these biases imply that the current estimates of SOC sequestration worldwide need an overhaul, are the two questions addressed in this work.We collected from literature 140 pairs (initial-final) of plots affected by land use changes: crop to forest, crop to grassland, etc. Peatlands were excluded. The datasets were detailed enough to calculate SOC sequestration by both approaches: fixed-depth (down to 30 cm) and equivalent-mass (amount of fine mineral earth at the top 30 cm of the initial stage). Differences between both approaches were often small, but relevant in many cases: namely, when changes in soil compaction were relevant too. The fixed-depth approach overestimates SOC sequestration when soil compaction occurs, and underestimates it when soil expands. However, the average error is low, because over- and underestimations of sequestered SOC partly compensate: the fixed-depth approach underestimates SOC sequestration by about 0.326 ± 1.016 kg SOC m−2 (overall amount), or 14 g SOC m−2 yr−1 (rate). This suggest that the corrections needed by current estimates of SOC sequestration worldwide—at least, those related to the problem studied in this paper—would be low.From our data, the equivalent-mass approach is compulsory in studies about SOC sequestration. Nevertheless, it seems less adequate than fixed-depth as an approach to compare SOC stocks worldwide. Both approaches should not be considered as opposite views, but as complementary tools for the study of carbon cycle in soils.