Guiding methodologies for soil processing and analyses to support rigorous soil carbon accounting

Abstract

Precise quantification of Soil Organic Carbon (SOC) is an indicator of soil health, and is critical to build confidence in soil carbon crediting programs and carbon markets. Informative and meaningful comparisons across spatial and temporal time scales can only be made with reliable soil carbon measurements and estimates, which begin in the field and in soil testing facilities. We conducted a laboratory comparison across eight laboratories that are frequently involved in SOC quantification for C markets. We assessed the individual effect of several procedures: sieving, fine grinding, and drying as well as quantification methods on total (TC), inorganic (SIC), and organic (SOC) soil C concentration estimates. We analyzed agricultural soils using procedures that varied either in the sieving, fine grinding, drying, or quantification steps. We found that variations in laboratory procedures had a major impact on soil carbon quantification. Based on our findings, we recommend that labs prioritize methodological standardization to achieve reproducibility and improve trust in analyses outcomes. We also recommend that labs standardize methods that are associated with low variation and most accurate results. However, we recognize that there is often a trade-off between accuracy and cost. In those instances, we suggest that it is most critical to avoid directional bias across labs due to procedural variations and that consistency in how soils are processed should be prioritized to improve accuracy.