Soil attributes as proxies of anthropic influence in the floodplain of the Amazon Valley, Brazil

Abstract

Recent studies identified the presence of Amazonian Dark Earths (ADE - anthropogenic soils) at the floodplains, similarly to the well-known ADE of the upland. Previously, different parameters have been established for characterizing ADE anthropic horizons, but they are unlikely to be useful in ill-drained environments of the hydromorphic Amazon floodplain. It raises the question of to what extent can the established parameters of ADE be applied to anthropic horizons at floodplains, and what other characteristics or attributes could be used as criteria for identifying and classifying anthropic horizons in those environments. The main hypothesis is that the ADE are contrasting to typical ADE found in the uplands. So, we selected eleven representative soil profiles in the Amazon River floodplain between the cities of Coari and Careiro da Várzea, in the Amazonas State - Brazil. Soils were described, sampled and classified according to the Brazilian Soil Classification and the World Reference Base for Soil Resoucers (WRB) System. Morphological, chemical and physical attributes were described and determined. The results indicated that pH value, the organic C (OC) and Cu concentration can be characteristic for upland ADE, but not for the buried Anthrosols from the floodplain. The concentration of OC required by the IUSS Working Group WRB (2022) as one of the criteria for pretic horizon, was not reached in all soils, and some could not be considered an Anthrosol for having less than 0,6 % OC even though all other criteria were met. We postulate that this criteria should be changed in such a way that these soils classify as Anthrosols. However, other soil attributes normally used as indicators of anthropic influence on upland ADE are also noticed on floodplain soils such as: high available or total P (in Mehlich-1 or total), Ca (Ca²⁺ or CaO), Zn (Mehlich-1 or total). It was further observed that the total concentrations of Ba, Cu and Sr also proved to be good indicators of anthropic horizons of floodplains buried soils. Hence, differential soil attributes can be used as proxies of Anthrosols presence in Amazon floodplain soils, enlarging the extent of these soils in the Amazonian landscape.