Determination of absolute changes or relative differences in soil organic carbon (SOC) stocks requires a direct comparison of measurements taken across temporal or spatial gradients. While most SOC stock studies measure both SOC concentration and soil bulk density, many do not explicitly account for potential differences in bulk density among time periods or treatments, which may result in different soil masses being sampled within identical depth increments. The equivalent soil mass (ESM) method accounts for differing bulk densities by normalizing to an area-specific soil mass rather than soil depth. Although the ESM method has been advocated for decades, it is still not used ubiquitously among ecological studies. Our objectives were to assess the potential magnitude of SOC stock errors caused by ignoring bulk density differences and to evaluate the efficacy of the ESM method in mitigating those errors. We used deep soil cores taken from a switchgrass stand in Central Illinois, USA to create depth-specific models of soil mass and SOC stocks. We then simulated 2 cm of soil compaction within the surface 10 cm, resulting in a 20% increase in surface bulk density with no change to the SOC stock.
Results/Conclusions
When changes in bulk density were not explicitly accounted for, the apparent SOC concentration decreased by 1.5% in the 0-10 cm because of the greater mass of soil sampled, but the apparent 0-10 cm SOC stock increased by 18.7% due to the higher bulk density. However, the apparent SOC stocks decreased in deeper depth increments, thus resulting in an overall apparent increase in surface SOC stocks with concurrent apparent SOC losses at depth. The cumulative apparent SOC stocks to 50 and 100 cm were overestimated by 2.8 and 1.0%, respectively, when the bulk density difference was ignored. Using the ESM approach for both baseline and altered bulk density scenarios, the 0-10 and 50-100 cm SOC stocks differed between bulk density regimes by only 0.33 and 0.07%, respectively. Under the increased bulk density scenario, the cumulative SOC stock to 50 cm was underestimated by 0.05% while the cumulative SOC stock to 100 cm was overestimated by 0.24% when ESM was used. Overall, the ESM approach provides substantial improvements to both cumulative and depth-specific SOC stock estimation. Using the ESM method when evaluating land-use transitions, such as annual row crops to perennial biomass crops, will help to accurately assess the resulting SOC stock changes.