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

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 (& chi;lf) and high frequency (& chi;hf) in air-dried fine earth (ADFE) and in clay fraction. Higher & chi;lf values (6-9 x 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 x 10-6 m3 kg-1) and NAS (0.27-1.39 x 10-6 m3 kg-1). The correlation of & chi;lf with sand and clay indicates the presence of Mt. and superparamagnetic Mh. The lessivage process is the one that best clarifies & chi;lf 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 in-formation from spectral curves and & chi;lf is useful to support the soils survey in the Amazonia.

Automated summary

The contribution of Fe oxides in Indian Black Earth, Indian Brown Earth and non-anthropogenic soils in the Southern region of Amazonia was evaluated. Antiferromagnetic oxides hematite and goethite were identified using diffuse reflectance spectroscopy. The presence of magnetite and maghemite was inferred through magnetic susceptibility measurements. The combined analysis of spectral curves and magnetic susceptibility is useful for soil survey in the Amazonia.