Weathering and mineral alteration of metamorphic rocks and genesis of Planosols along a rainfall gradient in Borborema Province, Northeast Brazil

Abstract

Soils in Northeast Brazil have a correlation with soils in the Northwest Africa, as Planosols are predominant in these regions. However, the weathering and genesis of these soils are little understood, compromising their use and conservation. The present study aimed to investigate weathering and mineralogical changes as well as genesis of Planosols along a pluviometric gradient, considering their formation from gneiss rocks, and analyze climate influence on soil formation. We collected three profiles in different climatic regions of the Brazilian northeastern region and described and classified the samples morphologically. We also performed the physical, chemical, mineralogical, and micromorphological analyses. The results indicated that external material added to soil surface contributed to the formation of surface horizons rich in coarser fractions. Subsurface horizons however were rich in clay (e.g.: P1–2Bn: 433 g kg−1; P2-2B: 427 g kg−1; P3-2Bn1: 429 g kg−1) and were formed from changes of the parent material and the clay formation in situ. Biotite, plagioclases, K-feldspars, and hornblende are the main primary minerals sources for the formation of clay minerals (mainly smectite and kaolinite) from transformation processes and neoformation. Pedoplasmation, monosiallitization, bisiallitization, redoximorphism, and alkalization contributed to the formation of the Planosols studied. Despite incipient soil evolution, the Planosols displayed variation according to the climatic condition. Nevertheless, the incipient mineral alteration favored important soil attributes that could influence agricultural productivity, even in the region with higher precipitation rates.