Researchers from the Spanish National Research Council (CEBAS-CSIC) and the Council for Agricultural Research and Economics (CREA) conducted an eleven-year field trial in an organic olive orchard in Jumilla, Spain, comparing physical soil organic carbon measurements with simulations from the RothC carbon turnover model. The study evaluated long-term carbon accumulation across soils treated with compost, biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More, a biochar-compost blend, and an unamended control. While both empirical data and model simulations ranked pure biochar as the most effective amendment for increasing soil carbon levels, significant quantitative divergence emerged between projected sequestration rates and field observations.

Carbon market frameworks and agricultural certification protocols increasingly rely on predictive models to quantify soil organic carbon storage and issue carbon credits. However, standard models like RothC struggle to accurately depict carbon persistence and physical distribution when non-traditional organic inputs are applied in semiarid environments. Inaccuracies in model calibration or physical sampling techniques create structural risks for voluntary carbon markets, potentially resulting in overcredited or unverified carbon sequestration claims.

To evaluate these dynamics, the research team applied biochar and compost amendments over five application cycles between 2013 and 2024, tracking carbon concentrations in the top 20 centimeters of soil across 17 sampling campaigns. To adapt the RothC model for semiarid conditions and exogenous organic matter, the authors integrated modified soil moisture functions and assigned pool parameters for biochar based on thermal stability and hydrogen-to-carbon molar ratios. While RothC projected that 98.1 percent of biochar carbon remained in the soil after eleven years, empirical field measurements accounted for only 49.3 percent. The researchers attributed this gap to spatial sampling challenges stemming from biochar’s particulate, non-homogeneous distribution in the soil matrix, alongside potential model overestimation of thermal stability-derived decay rates.

The eleven-year trial demonstrated that while biochar achieved the highest final carbon concentration in the field at 3.01 grams of carbon per 100 grams of soil, RothC simulations overestimated overall sequestration rates by approximately 100 percent across amended treatments. Biochar exhibited the highest model error, generating a root mean square error of 19.44 megagrams of carbon per hectare and a coefficient of determination of just 0.10. These findings indicate that relying solely on current predictive carbon models or standard soil sampling techniques introduces substantial uncertainty, highlighting the need for hybrid verification frameworks combining material characterization, modified models, and targeted field sampling in biochar carbon accounting.