Micromorphology of spodic horizons of the restinga region of São Paulo State

MICROMORPHOLOGY OF SPODIC HORIZONS OF THE RESTINGA REGION OF SAO PAULO STATE The soils, especially the Spodosols, of the "restinga", a coastal plain, are poorly studied and understood in Brazil. In this work, the micromorphology of spodic horizons was investigated at four sites of the coastal plain of the State of Sao Paulo (Bertioga, Ilha de Cananeia, Ilha do Cardoso, and Ilha Comprida). This allowed the characterization of different organic matter forms. These data, together with macromorphology, were used in the analysis of the main processes involved in the genesis of spodic horizons of eight representative soil profiles of the coastal plain of Sao Paulo. The observations of occurrence of organic monomorphic coatings on the surface of coarse grains in most of the studied horizons as well as the total filling of voids between grains in some cemented and friable horizons are evidence of the classic theory of metal-organic complex mobilization, transport and precipitation in the studied soils. However, the polymorphic organic matter and/or plant residues in different decomposition stages were the most important soil features observed in poorly-drained spodic horizons and below the histic horizons. In these poorly-drained spodic horizons, root decomposition by mesofauna and microorganisms in situ are important mechanisms of organic matter accumulation in the deeper B horizons. However, there are other ways in which roots influence the genesis of these horizons: the organic matter features of a cemented horizon, containing root rests, indicated that roots can contribute to organic matter immobilization through selective absorption mechanisms. In this process, the soil solution with high metal-organic complex contents is selectively absorbed by the roots. This leads to the segregation of metal-organic complexes at the surface and in the proximity of roots, as water and nutrients are being absorbed. If this process is continued, it leads to the precipitation of illuviated and segregated organic matter by its dehydration, which is a consequence of the proper root absorption.