Long-term effects of biochar on microbial biomass carbon and glomalin in a tropical soil

The biochar application has been widely investigated as a strategy to improve soil health and promote carbon sequestration in agricultural systems. This study evaluated, over 10 years, the effects of different biochar doses (0, 4, 8, and 16 t ha⁻¹), produced from forest residues, on glomalin and microbial biomass carbon (MBC). The experiment was conducted in the Brazilian Cerrado, in the municipality of Uruçuí, Piauí, on a typical dystrophic Yellow Oxisol. Biochar was applied once at the beginning of the experiment and incorporated into the topsoil layer. Data were submitted to analysis of variance (ANOVA), and treatment means were compared using Tukey's test at a 95% confidence level (p ≤ 0.05). The results indicated that MBC increased significantly with increasing biochar doses at all evaluated depths (0–10, 10–20, and 20–30 cm). In contrast, glomalin content did not show significant differences in the 0–10 cm layer but decreased significantly with increasing biochar rates in the 10–20 and 20–30 cm layers. This reduction may be associated with changes in the arbuscular mycorrhizal fungal community and with reduced plant dependence on mycorrhizal symbiosis under greater nutrient availability, leading to lower glomalin production. Additionally, the high adsorption capacity of biochar may physically retain glomalin on its surface, resulting in lower extractable concentrations. Thus, further studies are needed to elucidate the mechanisms involved and define the effects of biochar application on soil in tropical systems, aiming to maximize its benefits for soil microbiota and carbon stabilization.