Methods of evaluating soil bulk density: Impact on estimating large scale soil organic carbon storage

• Bulk density for soil organic carbon (SOC) storage was estimated by 6 methods.
• Pedo-transfer functions (PTFs) provide the most accurate approach.
• PTFs underestimate SOC storage; mean or median overestimate SOC storage.
• Estimation errors increase with increasing SOC content for all methods.
• Optimized PTFs are effective approaches to improve SOC storage estimates.

Bulk density (BD) is one of the most important parameters used to calculate soil organic carbon (SOC) storage. Differences in the methods available to substitute missing BD data, including mean, median, and pedo-transfer functions (PTFs), are considered the main reason for the high uncertainty in SOC storage estimations at large scales. In this study, we used the measured BD and SOC contents of 1007 soil profiles to evaluate the accuracy of six BD substitution methods (two mean methods, a median method, and three PTFs). The results showed that PTFs underestimated SOC storage by 8% in the 0–20 cm soil layer, while mean and median methods overestimated SOC storage by 45% and 51%, respectively. Furthermore, the accuracy of PTFs estimates decreased with increasing soil depth. Relative to the measured values, and based on mean errors, root mean square errors, and method efficiency, the PFTs estimates were more accurate than those of the mean and median methods. Estimation bias increased with increasing SOC content for all methods, but differed among different soil types. The results of this study demonstrate that PTFs provide more accurate estimates of SOC storage when the extent of missing BD is substantial. Furthermore, the results provide new insights into establishing optimized PTFs, which contain more soil properties and piecewise functions of SOC content, in order to improve SOC storage estimates at large scales.