Micromorphological Features of Carbonate Soils Threatened by Desertification in Northeastern Brazil

Desertification is a major environmental problem in drylands, resulting from the interaction between climatic and anthropogenic factors and involving processes such as vegetation loss and soil erosion. In northeastern Brazil, long-term land-use pressure has intensified the vulnerability of semi-arid landscapes, leading to severe degradation and reduced environmental resilience. Within this context, the Salitre River Basin (Bahia State) represents a particularly relevant area for investigating desertification, as it combines a history of intense human pressure and soil degradation with arid climatic conditions recently recognized through climatological assessments. This study investigates the micromorphological organization, hillslope dynamics, and environmental degradation of carbonate soils of the Salitre river basin, aiming to reconstruct pedogeomorphological evolution and identify indicators of desertification. A toposequence-based approach was applied using ten soil profiles distributed across sectors with contrasting erosion intensity. Soil horizons were characterized through field descriptions, physical and chemical analyses, and micromorphological observations of thin sections. Soils are predominantly eutrophic, carbonate-rich, shallow Calcisols, with very high CaCO3 contents (500–900 g kg−1), alkaline pH, and low total organic carbon. The spatial organization of profiles results from strong lithological and geomorphological controls, with well-developed horizons in stable sectors, whereas truncated and homogeneous profiles occur in areas affected by severe erosion. Micromorphological features, including planar and moldic voids, Fe–Mn nodules, calcite coatings, and needle calcite infillings, indicate active carbonate dissolution/redistribution processes under increasing seasonality and aridity (calcification). The coexistence of inherited dissolution and recent precipitation features suggests polyphasic pedogenesis linked to Holocene climatic oscillations. The preservation of well-developed needle-fiber calcite is consistent with prolonged water-deficit conditions, although its precise chronological significance remains unconstrained in the absence of direct dating. At landscape scale, these processes are associated with hillslope retreat and dense networks of linear erosion, defining a scenario of severe land degradation.