New insight into the influence of biochar particle size and aging in soil sorption of fluometuron

Authors: LÓPEZ-CABEZA, ROCIO - Instituto De Recursos Naturales Y Agrobiologia De Sevilla (IRNAS-CSIC); VELARDE, P - Instituto De Recursos Naturales Y Agrobiologia De Sevilla (IRNAS-CSIC); Spokas, Kurt; COX, LUCIA - Instituto De Recursos Naturales Y Agrobiologia De Sevilla (IRNAS-CSIC)

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/16/2026
Publication Date: 3/20/2026
Citation: López-Cabeza, R., Velarde, P., Spokas, K.A., Cox, L. 2026. New insight into the influence of biochar particle size and aging in soil sorption of fluometuron. Agronomy. 16(6). Article 656. https://doi.org/10.3390/agronomy16060656.
DOI: https://doi.org/10.3390/agronomy16060656

Interpretive Summary: Biochar use as a tool for removing agrochemicals from the soil has been increasing. However, the long-term impacts of biochar presence in soil are not fully known. This study examined the impact of soil aging of 12 and 30 months on biochar with a particular interest on different biochar particle size classes. The particle size fraction examined were small (63 µm to 1 mm) and large (1-2 mm). We observed that the aging of the biochar in soil had differing impacts on the sorption capacity of the biochar as a function of particle sizes. The smaller size fractions possessed the largest decrease in sorption compared to the larger particle sizes, although the larger particle size had the more extensive mineral coatings as observed with electron microscopy. Additionally, the impact of the biochar addition on the soil sorption capacity also was a function of fresh versus aged biochar, with aged biochar decreasing the sorption capacity of the amended soil and the fresh biochar additions all increased the sorption capacity of the amended soil. The aging impact was more pronounced in the smaller particle size fractions. The sorption-desorption behavior of BC amended soils depended on both the BC particle size as well as the residence (aging) time in the soil. This is an important consideration given the fact that biochar undergoes particle size reduction with tillage and soil management operations. Therefore, the effectiveness of biochar as a remediation agent for agrochemicals in soil may be greatly reduced with time. These results are significant to assist farmers, scientists, and engineers as well as supplying guidance for biochar use as a remediation tool.

Technical Abstract: Application of biochar to soil can be a sustainable strategy to reduce risks associated to pesticide contamination due to its high sorption capacity. The ability of biochar to sorb pesticides and other organic compounds depends on several physicochemical properties, although little attention paid to biochar particle sizes, which affects physical properties such as porosity and surface area. The aim of this study is to investigate the effect of biochar of different particle sizes on sorption the herbicide fluometuron (FM) and how this is affected by biochar (BC) aging in the soil. A commercial BC from oak hardwood produced at a pyrolysis temperature of 500º C was aged in the soil during 12- and 30-months and after this period sieved at two different mesh sizes, 63 µm-1 mm and 1-2 mm, and characterized. FTIR and SEM studies reveal interaction with mineral soil particles with aging specially in 1-2 mm BC particle sizes. In fresh BC, greater sorption is observed for smaller particle sizes than for larger sizes. After aging in the soil, sorption decreased, and this decrease is significantly greater in the case of smaller particle sizes (0.63-1 mm) than in larger. When amended with fresh BC, FM soil sorption increases, and this increase is higher in the case of soils amended with small particle size BC than for soils amended with larger particle sizes. Amendment with BC aged for 12- and 30-months decreased FM soil sorption, with a greater decrease in soils amended with smaller particle sizes of BC. Significant differences between 12- and 30-months aging in the soil are observed for >1 mm particle sizes. FM sorption on unamended soil is highly reversible, whereas in soils amended with fresh BC, desorption dramatically decreases, particularly in the case of soil amended with 1-2 mm fraction of BC, where no FM is released back to solution in 24 hrs. This is also observed when soils are amended with 1-2 mm particle size BC after aging in the soil. On the contrary, in soils amended with BC of low particle sizes desorption increases with aging. The different sorption-desorption of FM in BC amended soils depending on BC particle sized with residence time in the soil emphasizes the importance of considering this parameter since the effectiveness of soil applied herbicides may be compromised.