Bulk density calculation methods systematically alter estimates of soil organic carbon stocks in United States forests

Soils are the largest terrestrial carbon sink on Earth, yet substantial uncertainty in the size and stability of this pool remains. Much of this uncertainty stems from the characterization of bulk density, which is the mass of a soil sample divided by its volume, a key property in the calculation of soil organic carbon (SOC) stocks. We used data from nearly 2900 plots in the United States (U.S.) Nationwide Forest Inventory to quantify SOC stocks in forests with three common methods of calculating soil bulk density. Mean SOC stocks calculated with these methods varied by up to 13 Mg ha⁻¹, a difference equivalent to more than 70 percent of the 2022 economy-wide carbon dioxide emissions in the U.S. when scaled across all forest area. These differences were primarily driven by inconsistent treatment of coarse materials (i.e. rocks and roots) in soil bulk density calculations, which led to an overestimation of SOC content by 32 percent of the mean SOC stock across all U.S. forests. The largest discrepancies were found in soils with high coarse fragment content, which are more common in ecologically sensitive ecosystems like alpine zones and drylands, and in commercially important softwood forest types. Quantifying the size and stability of SOC in the land sector is essential to understanding how this carbon pool may serve as a nature-based solution to climate change. Consistent and transparent methods are necessary when estimating and reporting SOC content and when comparing SOC dynamics across ecological gradients, with disturbance, and over time.