An interlaboratory study of biochar reflectance measurements: Methodological implications

Reflectance has been recognized as a key indicator of biochar permanence, yet no internationally standardized biochar-specific methodology exists for its measurement. To evaluate the currently used practices, a round-robin study involving nine experienced petrographers across multiple international laboratories was performed to assess the reproducibility of reflectance values and the influence of biochar particle size on analytical outcomes. Despite shared petrographic expertise, the results revealed noticeable, though anticipated, variability among analysts, primarily stemming from system calibration and subjective differences in selecting measurement spots. Among the three biochars studied (hardwood, kolmar wood, and rice hull), this factor was particularly pronounced in the highly heterogeneous rice hull biochar. Average reflectance values for analyzed hardwood biochars were consistently high, ranging from 4.25% to 5.39% across petrographers and three particle sizes (20, 60, and 230 mesh). In contrast, kolmar wood biochar showed lower and more uniform values of 1.29–1.91%, while rice hull biochar exhibited the lowest reflectance, between 1.05 and 1.52%. Discrepancies in reflectance reached up to ∼1.0% for high-reflectance biochars and 0.4–0.5% for the kolmar and rice hull samples—acceptable for permanence assessment but highlighting the need for standardized measurement protocols. Statistical validation confirmed that most petrographers produced results within accepted limits (h ± 2.23; k < 2.09), demonstrating reliable consistency and reproducibility. Particle size influenced reproducibility, with coarser fractions (20- and 60-mesh) yielding more consistent results than finely ground samples (230-mesh), which introduced greater subjectivity in spot selection. Additionally, no statistically significant differences were detected between manual grid-based and automated frame-based measurement setups, indicating that both approaches provide comparable analytical outcomes. This study highlights the considerable capability of reflected light microscopy (RLM) as a rapid and quantitative approach for assessing biochar permanence. At the same time, it emphasizes the need to consider biochar heterogeneity, calibration range, and the importance of explicit spot-selection criteria in developing biochar-specific protocols. Based on the results obtained, the method proved reliable and reproducible, reinforcing its suitability as a standardized technique for biochar characterization.