Determining soil carbon stock changes: Simple bulk density corrections fail

Several methods are used to correct total soil carbon data in response to land use or management changes inherently coupled with concomitant alteration to bulk density (BD). However, a rigorous evaluation of correction methods has not been conducted. We compared original, maximum, and minimum equivalent soil mass (ESM) corrections to the fixed depth (FD) method and direct C concentrations. In a simulation exercise of a tillage event that decreased BD without change in total C concentration to a depth of 0.3 m, the original and maximum ESM methods estimated changes in total C storage of −0.34 to 0.54 Mg C ha−1, well within the range of field soil C variability. In contrast, the minimum ESM method estimated changes ranging from −1.19 to 1.01 Mg C ha−1. In a field experiment on reduced and intensive tillage, soil C changes (0–0.18 m) were measured from May to August 2006. The maximum ESM method generally overestimated soil C changes by −0.16 to 0.60 Mg C ha−1 and the minimum ESM method underestimated them by −2.67 to 0.23 Mg C ha−1 compared to the original ESM method. Field-scale soil C changes (0–0.15 m) were also measured from August 2003 to June 2005 and decreased by an unrealistic 6.64 Mg C ha−1 over the first 6 months after tillage when the FD method was used. In contrast, the effect of tillage on soil C could be reasonably estimated by directly comparing changes in C concentration. In a compacted agricultural soil, we found more errors in simulated C differences when using the maximum than the minimum ESM method. Regardless of the direction of BD changes, the minimum ESM method was a better choice than the maximum ESM method in native and restored grassland systems where soil C concentrations decreased through the soil profile. We conclude that (1) the FD method is often not suitable and might be less accurate than direct C concentration measurements, and (2) the maximum/minimum ESM method can be accurate depending on the conditions (e.g., increasing or decreasing BD, systems conversion type), but (3) that the original ESM method is optimal for detecting soil C changes due to land use changes or management effects.