When bulk density methods matter: Implications for estimating soil organic carbon pools in rocky soils

Resolving uncertainty in the carbon cycle is paramount to refining climate predictions. Soil organic carbon (SOC) is a major component of terrestrial C pools, and accuracy of SOC estimates are only as good as the measurements and assumptions used to obtain them. Dryland soils account for a substantial portion of global SOC, but the pool dynamics are highly uncertain. One crucial component of accurate estimates of SOC on an areal basis is bulk density (ρ_b), the mass of soil per unit volume. Here, we review methods used for calculating ρ_b and assess their prevalence. We show how treatment of coarse fragments (particles >2 mm diameter) influences ρ_b values and discuss the implications for SOC estimates in drylands. In four dryland examples, methods that varied in their treatment of coarse fragments led to substantial (up to 26%) differences in ρ_b. Calculated SOC pools responded proportionally, with SOC differing by up to 518 g C m^-2. We suggest a revised method for accounting for coarse fractions in ρ_b calculations. A large portion of the world's soils, particularly in drylands, are fine enough to allow ρ_b determination with cores, but contain coarse fragments that substantially impact SOC mass estimates if not explicitly considered.