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 (rho(b)). the mass of soil per unit volume. Here, we review methods used for calculating rho(b) and assess their prevalence. We show how treatment of coarse fragments (particles >2 mm diameter) influences rho(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 rho(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 rho(b) calculations. A large portion of the world's soils, particularly in drylands, are fine enough to allow rho(b) determination with cores, but contain coarse fragments that substantially impact SOC mass estimates if not explicitly considered. (C) 2011 Elsevier Ltd. All rights reserved.