Gênese de espodossolos no Brasil, com ênfase na Amazônia Central Brasileira

Abstract

Spodosols are pedogenetic systems of global relevance, whose expressions in tropical environments challenge classical concepts derived from boreal and temperate regions. In Brazil, their occurrence spans from the Amazon Basin to coastal strips, across distinct geomorphological and climatic contexts. This thesis integrates three complementary approaches (three chapters): a comprehensive literature review article based on a unique database; a detailed field study in Central Amazonia, aimed at understanding the geochemical and biological controls of these soils' genesis; and an in-depth investigation into the composition and dynamics of organic matter (OM) in these environments. In the first chapter, 352 profiles and 848 spodic horizons were compiled and analyzed from five decades of research. This review aimed to characterize the morphological, physical, and chemical variability of Spodosols in Brazil, propose an integrated model for their genesis and regional evolution, analyze the chronology of the spodic B horizons, and clarify the ecological functions related to carbon stocks. Morphological, physical, and chemical characterization, evaluated through multivariate statistics, revealed that the variability of Brazilian Spodosols is better explained by pedoenvironmental classes than by vegetation types. Radiocarbon and optically stimulated luminescence chronological data demonstrated that spodic horizons formed over time scales ranging from centuries to hundreds of thousands of years, evidencing both recent events and paleoenvironmental processes. This synthesis allowed the integration of dominant formation theories (complexation, hydromorphic, polymorphological, and biogenic) into a unified model for tropical conditions. In addition to their agricultural limitations, the ecological role of Spodosols in deep carbon storage, organic matter flux regulation, supporting biodiversity, and maintaining hydrological dynamics was highlighted. In the second chapter, the aim was to assess how variations in vegetation and parent material control the formation and differentiation of spodic horizons. Eight soil profiles were characterized across four geological formations and four different vegetation types in Central Amazonia, through morphological, chemical, micromorphological, and microanalytical analyses, as well as multivariate statistics. The results highlighted strong acidity, nutrient poverty, and aluminum saturation in the spodic horizons, characteristics associated with intense leaching. PCA analysis revealed that neither vegetation nor geology, in isolation, explain the distribution of spodic B horizons. Instead, genesis results from a hierarchical interaction between the organic input from vegetation and the geochemical signature of the parent material. Profiles developed on the Trombetas Group showed higher levels of illuvial constituents, while those from the Uatumã Group were associated with higher levels of crystalline iron and sand. In campinarana areas, vegetation promoted more extensive podzolization, corroborating the differential role of OM quality. In the third chapter, the goal was to evaluate the contribution of physical carbon fractions to the stabilization of OM in Spodosols and identify the pedogenetic (reactive Al and Fe contents) and biotic (vegetation types) factors controlling the composition and dynamics of these OM fractions in Central Amazonia. To this end, physical (COP, COAM), chemical (HUM, AF, and AH), and molecular signature (FTIR) analyses of the OC were performed. The results showed that organic carbon associated with minerals (COAM) represents the dominant and most stable fraction of SOM, correlating positively with reactive Al and Fe contents. Vegetation strongly controlled the quality of SOM, with campinarana areas promoting greater accumulation of humins and aromatic compounds, while disturbed forests favored more labile fractions. Chemometric analysis (PCA) of FTIR spectra allowed discrimination of SOM according to vegetation type and humification degree, reinforcing the idea that podzolization in tropical environments is mediated by the interaction between organic input, mineralogy, and hydrological conditions.