Magnetic and spectral signature of anthropogenic and non-anthropogenic soils to identify pedogenetic processes in Amazonia, Brazil

Abstract

The present study evaluated the contribution of Fe oxides of the clay fraction as an indicator of pedogenetic and anthropogenic processes of Indian Black Earth, Indian Brown Earth and non-anthropogenic soils in the Southern region of Amazonia, Brazil. The A and B horizons of six soil profiles, identified as Indian Black Earth (IBE), Indian Brown Earth (BE), and non-anthropogenic soils (NAS), were studied. Hematite (Hm) and goethite (Gt), both antiferromagnetic oxides, were identified by diffuse reflectance spectroscopy. The Kubelka-Munk function was used to determine their proportions. The presence of minerals magnetite (Mt) and maghemite (Mh) was inferred by mass-specific magnetic susceptibility measurements at low (χlf) and high frequency (χhf) in air-dried fine earth (ADFE) and in clay fraction. Higher χlf values (6–9 × 10−6 m3 kg– 1) in the ADFE for the surface of IBE indicated formation of Mt. and Mh associated with its pedogenesis, and differentiate it from BE (0.49–4 × 10−6 m3 kg−1) and NAS (0.27–1.39 × 10−6 m3 kg−1). The correlation of χlf with sand and clay indicates the presence of Mt. and superparamagnetic Mh. The lessivage process is the one that best clarifies χlf in the subsurface horizons of IBE. Spectral curves detect pedogenetic (melanization and xanthization) in the soil profiles of IBE, BE and NAS and anthropogenic (anthrosolization) processes in the soil profiles of IBE, BE. Melanization causes reduction in the reflectance and absence of concavity in the spectral curve of IBE, separating it from BE and NAS due to the greater spectral reflectance induced by the processes of lessivage and xanthization. Therefore, the combined information from spectral curves and χlf is useful to support the soils survey in the Amazonia.