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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 0.3m, the original and maximum ESM methods estimated changes in total C storage of -0.34 to 0.54MgCha⁻¹, well within the range of field soil C variability. In contrast, the minimum ESM method estimated changes ranging from -1.19 to 1.01MgCha⁻¹. In a field experiment on reduced and intensive tillage, soil C changes (0-0.18m) were measured from May to August 2006. The maximum ESM method generally overestimated soil C changes by -0.16 to 0.60MgCha⁻¹ and the minimum ESM method underestimated them by -2.67 to 0.23MgCha⁻¹ compared to the original ESM method. Field-scale soil C changes (0-0.15m) were also measured from August 2003 to June 2005 and decreased by an unrealistic 6.64MgCha⁻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.
