CROSTAS BIOLÓGICAS DE SAPRÓLITOS DA REGIÃO DO QUADRILÁTERO FERRÍFERO, MG: CICLAGEM BIOGEOQUÍMICA E MICROMORFOLÓGICA

Résumé

The role of biological crusts in nutrient cycling and soil structure genesis in saprolites, in tropicalconditions, is little known. This work reports on the biochemical weathering and structuralreorganization resulting from the biological crusts action in different saprolites of the QuadriláteroFerrífero and neighbor areas, MG. These road exposures were oriented to the south, encompassing adiabase (DB), gneiss (GN), itabirite (IT), schist (XT) and gold-mine spoil-pile from Morro Velho(MV), comprising metasediments of the Rio das Velhas supergroup. There were collected and identifiedcyanobacteria, fungi, lichens and mosses. Based on the biogeochemical study, the exchangeable levelsof K, Mn and Zn were preferably concentrated in the crust. The exchangeable Fe levels were higherin the altered under zone, constituted possibly by secondary Fe-migration and precipitation throughlateral fluxes. There were no clear trend of Ca and Mg distribution, although they were apparentlyconcentrated in the crust in the IT and XT substrates, and in the altered zone in the DB. In MV spoil,the Ca dynamic was affected by sulfide oxidation, following acid drainage. In terms of total levels Mn,Fe, Mg, Cu and Zn were higher in the diabase saprolite, due to its maficity, in a clear contrast with thehost gneiss, represented by the GN saprolite, with low levels of Fe, Mn, Mg and Cu. The richersubstrates (DB, MV e XT) showed a trend of increasing total levels with depth, whilst the nutrientpoor substrates (GN e IT) had higher levels of some elements in the crust, compared with thesaprolito. The micropedological features are notably rich and varied, showed buried micropedogenetichorizons, sapric and histic micro-horizons, micro-stratification, ferruginous zones similar to Fe/Mnmicropans, amongst others. The biological crusts efficiently covered the saprolites, being responsibleto micropedogenetic features and a clear process of biological microstructuration of the exposedsaprolites. This process is capable of altering the moisture balance and erosion dynamic of the slope,with important geomorphological and environmental implications.