Variations in the measurement of carbon sequestration due to core diameter and carbon density calculation; do they make a difference?

Carbon sequestration, reported as gains in carbon stocks, requires measurement of carbon concentration and bulk density of soil. Nineteen perennial pasture sites were sampled in the Boorowa region, south-eastern NSW to compare: i) the influence of core diameter size (155 mm vs 75 mm vs 40 mm) on bulk density and ii) carbon density (Mg C ha-1 to 0.30 m) calculation (fixed depth vs equivalent soil mass) on total soil carbon stock. Bulk density was significantly different (P <0.05) with core diameter size for all soil layers to 0.30 m with the exception of the 0.05 to 0.10 m soil layer. However, due to the variability in carbon concentration in soil there was no significant difference in carbon stocks calculated using either the fixed depth or equivalent soil mass carbon density values regardless of core diameter size. The mean carbon stock (Mg C ha-1 to 0.30 m) calculated for the fixed depth and equivalent soil mass carbon density values using the 155 mm, 75 mm, 40 mm diameter cores was 50.9 (7.4 sd) vs 49.9 (7.3 sd) 53.8 (8.3 sd) vs 53.1 (8.2 sd), and 50.9 (8.4 sd) and 49.9 (8.0 sd), respectively. Based on these findings, the diameter of the cores used for bulk density measurements for carbon stock calculation should be selected based on operational ease and sampling efficiency rather than notions of precision of carbon stock reporting.