Comparison of equivalent soil mass methods for soil organic carbon stock calculations

Equivalent soil mass (ESM) calculations are considered the best practice for determining changes to soil organic carbon (SOC) stocks in response to changing land management practices that influence both soil bulk density and SOC concentrations. However, scarce guidance exists on the appropriate choice of reference mass for ESM calculations. Using nine projects from the USDA-NRCS Dynamic Soil Properties for Soil Health (DSP4SH) dataset, we compared management-driven SOC change (ΔSOC) and ΔSOC error between fixed depth (FD) and ESM with the minimum, mean, and maximum observed soil mass. All ESM reference masses resulted in similar estimates of ΔSOC and ΔSOC error. Compared to ESM calculations, FD calculations resulted in significantly different ΔSOC and produced biased estimates when bulk density varied consistently due to management. Furthermore, ΔSOC error was consistently highest with FD calculations and the mean FD ΔSOC error was an order of magnitude greater than mean ESM ΔSOC error in the top 0–5 cm of the soil profile. Regardless of whether ESM or FD calculation methods are applied, it is imperative that the SOC stock calculation method, including reference mass selection, is clearly described for reproducibility and interpretability.