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Review of Soil Organic Carbon Measurement Protocols: A US and Brazil Comparison and Recommendation

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Accurate measurements of soil C changes are important for quantifying the impact of current land management practices and identifying opportunities to enhance soil C sequestration and retention. Addressing soil organic carbon (SOC) measurement is an important topic because the global soil carbon pool represents three-to-four times the amount of carbon (C) stored in the atmosphere and in living biomass. Furthermore, most SOC measurement research has focused on productivity effects because of the multiple critical soil properties and processes C influences with regard to meeting increasing global demand for food, feed, fiber, energy and other renewable biomaterials. Quantifying land management effects on SOC conservation and accumulation is also becoming more important as a cost-effective mitigation and adaptation strategy to address climate change. SOC changes can have a major impact on lifecycle assessment (LCA) calculations for globally traded bio-based products. Unfortunately, the broad agreement on the importance of SOC stands in direct contrast with uncertainties associated with how to best measure SOC changes. This paper reviews SOC studies associated with the two primary feedstock crops being used for global ethanol production: corn (Zea mays L.) in the U.S., and sugarcane (Saccharum officinarum L.) in Brazil. We compared SOC measurement protocols and found a lack of consensus on sampling strategies, measurement techniques, and verification methods, even though all three factors can significantly influence reported SOC values. Different sampling designs (i.e., a random sampling in the U.S. versus a systematic design in Brazil), sample collection methods (i.e., excavated pits in Brazil versus core sampling in the U.S.), sampling depths, (e.g., defined depth increments), and analytical procedures (i.e., wet oxidation versus dry combustion) contribute to variability in reported values. We conclude by providing a U.S. – Brazilian research recommendations for improving the quanitly and comparability of SOC measurements in these two countries.