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Land Use Effects on Sample Size Requirements for Soil Organic Carbon Stock Estimations

Soil organic carbon (SOC) stock (in metric tons of carbon per hectare) is calculated from SOC concentration (in grams per kilogram) and soil bulk density (&rgr;b; in grams per cubic centimeter). Temporal changes in SOC stock are used to calculate terrestrial carbon sequestration rates used in global climate change models. The inherent variability in soil properties like SOC and &rgr;b means that larger sample sizes may be needed to accurately determine SOC stocks. Our objective was to calculate the minimum sample size required to detect changes in &rgr;b, SOC, and SOC stock for two land uses. Surface soils (0-5 cm) from two reclaimed mine soils and two managed hay fields in northern West Virginia were intensively sampled (60-74 samples each). Mean SOC and SOC stock values were larger in the hay fields (40 g/kg, 29 Mg ha−1) than in the mine soils (20 g/kg, 20 Mg ha−1), but &rgr;b was larger in reclaimed mine soils (1.4 g cm−3) than in hay field soils (1.2 g cm−3). The &rgr;b variance was larger in mine soils than that in hay field soils, but field variances for a given land use were similar (0.09 and 0.11 [g cm−3]2 in mine soils; 0.02 and 0.03 [g cm−3]2 in hay field soils). The variances in SOC concentration and SOC stock were not related to land use and were not similar within a land use. As a result, the minimum number of samples required to detect a change in &rgr;b, SOC, and SOC stock was a site-specific property and cannot be assumed a priori.