Bioturbation in very deep tropical Ferralsols: A micromorphological study of biomantles

Complex soil systems such as Ferralsols have paleoenvironmentally significant characteristics, such as biogenic features, which are part of the concept of soil memory. In Ferralsols, bioturbation and ferralitization are the main long-term pedogenetic processes contributing to soil formation. While the ferralitization process and its micromorphological features are well-documented, the bioturbation process and its paleoenvironmental significance in deep Ferralsols remains poorly understood. In order to improve the understanding of the bioturbation process in Ferralsols, this study analyzed fifteen complete soil profiles from the topsoil to the saprolite zone (BC/CR horizon) in three different Brazilian regions with variations in environmental conditions. The specific biogenic features showed that the occurrence and activity of roots, earthworms, termites, ants, acari and enchytraeids in the soil profiles occurred at great depths down to 950 cm. Leaf-cutting ants and termites are the main ecosystem engineers responsible for the biogenic features identified in the fifteen studied Ferralsols. Greater complexity and reworking of biogenic features were observed in the upper part of the soil profiles, due to the greater biological activity in this area of the complex biomantle. The occurrence of a high percentage of biogenic features indicates different zones of bioturbation at depth associated with paleoclimatic variations since at least the Late Pleistocene (∼14,422 cal yrs BP). In addition, the very deep biogenic features were formed in different paleoclimatic periods and remained with few modifications until the present time. The detailed micromorphological and micromorphometric analysis from the topsoil to the weathering front enabled a hierarchy of bioturbation interpretation on a continuum and demonstrated spatial variation and zones of preferential bioturbation in soils mainly below 200 cm deep.