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

• Published in: Journal of arid environments 2012-02, Vol.77, p.66-71
• Main Authors: Throop, H.L., Archer, S.R., Monger, H.C., Waltman, S.
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Carbon accounting; Carbon pools; Core method; Fundamental and applied biological sciences. Psychology; Soil carbon; Synecology; Terrestrial ecosystems
• ISSN: 0140-1963; 1095-922X
• DOI: 10.1016/j.jaridenv.2011.08.020

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. ► Arid soils frequently have high coarse fragment content. ► Methods for measuring bulk density differ in their treatment of coarse fragments. ► Bulk density methods strongly influence soil organic carbon calculations. ► A revised method better accounts for coarse fractions in bulk density calculations.