What influences upland soil chemistry in the Amazon basin, Brazil? Major, minor and trace elements in the upper rhizosphere

Abstract

Increasing land transformation in the Amazon basin, from forest to post-forest usage such as pastureland, agriculture and agroforestry, triggers significant changes in hydrology, soil fertility and regional climatology. However, relatively little is known about Amazon basin soil chemistry in general and about its possible alteration with recent land-use change. We present robust pedogeochemical data for 65 elements and oxides, and evidence for modification due to recent deforestation and post-forest land use on upland soils in Amazonas state, Brazil. Differences emerge in median element concentrations between these two land-cover types, and between central and southern parts of the basin. These new data, a product of the bi-national EcoRespira-Amazon (ERA) project, are based on triplicate sampling under different seasonal conditions at 29 sites, representing ca. 740,000 km2 and average annual meteorological conditions. Mineral soil samples (TOP: 0–20 cm; BOT: 30–50 cm) characterize the active upper rhizosphere. Data were obtained with very tight quality control from sampling to analysis (following GEMAS protocols), using various overlapping analytical methods. Some major, minor and trace element concentrations deviate strongly from established world soil averages, including the recent PEGS2. Geological (lithological) and weathering boundary conditions define the primary soil chemical signal. This is overprinted by biogeochemical forces (ecosystem feedbacks), and recently by human intervention (change of land cover, deforestation). The general assumption of depleted tropical soils is not justified as such – a more differentiated view is needed, since carbon and macronutrients such as nitrogen and phosphorous, albeit not always plant-available, do often occur in relatively high concentrations (median values TOP: 1.9, 0.15 and 0.02 wt%). Calcium, magnesium and potassium are truly depleted (median values TOP: 0.025, 0.095 and 0.065 wt%), albeit with noticeable variance. Trace elements, from silver to zirconium and including REE, show highly differentiated responses. Most are relatively enriched in post-forest soils; a subtle signal that is interpreted as reduced plant-soil interaction. BOT concentrations are generally higher than those in TOP soil, reflecting weathering conditions and biogeochemical cycling – with interesting exceptions (Br, Cd, Rb). © 2019 Elsevier B.V.