Soil Carbon Inventories under a Bioenergy Crop (Switchgrass): Measurement Limitations

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Abstract
Approximately 5 yr after planting, coarse root (>2 mm) carbon (C) and soil organic C (SOC) inventories (0–40 cm deep) were compared under different types of plant cover at four switchgrass (Panicum virgatum L.) production field trials in the southeastern USA. There was significantly (p ≤ 0.05) more coarse root C under switchgrass (Alamo variety) and forest cover than under tall fescue (Festuca arundinacea Schreb.), corn (Zea mays L.), or native pastures of mixed grasses. Inventories of SOC under switchgrass were not significantly greater than SOC inventories under other plant covers. At some locations the statistical power associated with ANOVA of SOC inventories was low, which raised questions about whether differences in SOC could be detected statistically. A minimum detectable difference (MDD) for SOC inventories was calculated. The MDD is the smallest detectable difference between treatment means once the variation, significance level, statistical power, and sample size are specified. The analysis indicated that a difference of ≈50 mg SOC/cm2 or 5 Mg SOC/ha, which is ≈10 to 15% of existing SOC, could be detected with reasonable sample sizes (n = 16) and good statistical power (1 − β = 0.90). The smallest difference in SOC inventories that can be detected, and only with exceedingly large sample sizes (n > 100), is ≈2 to 3% (≈10 mg SOC/cm2 or 1 Mg SOC/ha). These measurement limitations have implications for monitoring and verification of proposals to ameliorate increasing global atmospheric CO2 concentrations by sequestering C in soils.

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