Influence of carbonate rocks on soil properties in the humid tropical climate of atlantic forest, Rio de Janeiro –Brazil

Abstract

Soil carbonate accumulation plays an important role in C sequestration, accounting for approximately 40% of the C stored in soils worldwide. Under humid tropical climate these carbonates are rapidly dissolved, intensifying CO2 emissions. Despite this, little is known about the weathering of carbonates in areas with complex geology. This study aimed to evaluate the influence of the parent material on soil attributes and weathering processes under humid tropical climate conditions along toposequences formed by dolomitic marbles and calc-silicate rocks interspersed with garnet-biotite gneiss. Samples and soils profiles were collected on sites of both parental materials and at different points of a toposequence, namely: upper third, middle third and lower third of a slope. The profiles were analyzed for their morphological, physical, chemical and mineralogical attributes. The attributes of soils formed from metamorphic sedimentary rocks were more influenced by geological processes than pedological ones. It was found that the nature of the parent material was the main responsible for the attributes of the profiles, especially the strongly alkaline pH, being greater than 8.0 in most soil horizons, and the high contents of exchangeable Ca2+ and CaCO3 equivalent, with maximum values of 40.6 cmolc kg−1 and 763 g kg−1, respectively. As for mineralogical composition, it was verified the occurrence of minerals such as talc, vermiculite and Fe and Ti oxides, with metamorphic processes and the addition of hydrothermal fluids being the main responsible for the presence of these minerals. The supply of high contents of basic cations, with maximum values of 61.4 cmolc kg−1, high Si/Al ratio (reaching values close to 5.0) and the high contents of CaCO3 conditioning the strongly alkaline pH hinder the dissolution of silicates, even those with low resistance to weathering, despite the hot and humid climate conditions. The action of weathering processes in humid tropical climate is more marked in acidic materials, being responsible for the in situ formation of kaolinite from the alteration of mica and feldspar.