Comparison of three quantification methods used to detect biochar carbon migration in a tropical soil: A 4.5-year field experiment in Zambia

• Coarse, medium & fine biochar added to upper 7 cm soil layer. • After 4.5 y, biochar carbon (BC) measured in soil profile using three methods. • δ 13 C registered the most BC in the upper layers but less in lower layers. • Chemothermal oxidation detected less fine particle BC, but correlated with δ 13 C-BC. • Benzene polycarboxylic acid method detected relatively more BC in the lower layers. Understanding the stability and movement of biochar in soil is pivotal for its effective use in soil improvement and carbon sequestration projects. Building on a previous study that evaluated the migration of three size fractions of maize biochar carbon (BC) after 4.5 years in a Zambian loamy sand soil using δ 13 C isotopes, this study compares the results with those using chemothermal oxidation (CTO) and benzene polycarboxylic acid (BPCA) biomarkers. While the δ 13 C method registered the most BC in the application layer (0–7 cm), it detected less BC in lower layers (7–30 cm, 3.2–7.9 % downward migration), and with a greater variance, than the other two methods. The BPCA method detected relatively more BC in the lower layers (9.1–20.2 % downward migration), particularly for fine-sized biochar. It also detected the most BC in the control soil plot and outside the experimental block, which suggests either its efficiency in fine biochar detection or an issue with false positive detection. The CTO method, though less sensitive in detecting fine biochar particle BC, was strongly correlated with δ 13 C isotope results, thus representing a cost-effective and simpler alternative to the other BC quantification methods. These findings underscore the necessity of methodological consideration in biochar C quantification to ensure accurate assessment of its distribution and stability. This is a pressing need for correct assignment of climate mitigation credits. More field studies should be carried out involving multiple biochar types and quantification methods to refine our understanding of biochar C dynamics in soil.