Empirical evaluation and recommendations for equivalent soil mass adjustment of soil organic carbon stocks

• ESM theoretically enables accurate SOC change estimation. • But ESM accuracy has not been empirically evaluated. • Our measurements enable first empirical evaluation of ESM accuracy. • ESM with multiple layers is found to be most accurate. • Recommendations provided for maximizing accuracy and cost-effectiveness. Measuring changes in fixed depth (FD; e.g. 0–30 cm) soil organic carbon (SOC) stocks is problematic if soils have compacted or expanded, i.e. if bulk density (BD) has changed. In theory, equivalent soil mass (ESM) remedies this. But in practice ESM is an approximation whose accuracy (systematic error) has not been evaluated empirically or compared to the FD baseline. We sampled 72 soil cores (0–75 cm) across six sites (central USA) of two regionally representative soils (Mollisols and Alfisols). SOC concentration and BD were measured at 2.5 cm depthwise intervals (dataset provided) to support this novel empirical evaluation of ESM accuracy. Under moderate BD changes (± 2.5 cm, or about 0.1 g cm −3 ), 0–30 cm FD measurements had systematic errors up to 8% (5 Mg ha −1 ), which would substantially distort SOC stock change estimates. Linear ESM using those single layer measurements improved accuracy somewhat in Mollisols but not Alfisols. Measuring a 30–35 cm “correction layer” substantially reduced systematic error in both soil types using linear and especially spline ESM (< 1.5% or 0.5 Mg ha −1 ). Of practical importance, pooling the correction layer did not sacrifice accuracy while substantially reducing ESM cost. For deeper soils (60 cm), FD was less inaccurate but ESM with multiple layers further improved accuracy. We conclude with recommendations on when and how to use ESM for accurate and cost-effective SOC stock change estimation in the two major soil types of central USA. However, caution is warranted when estimating SOC changes with short timescales, more extreme BD changes (e.g. ± 5 cm), and other soil types.