Impact of deforestation on soil iron chemistry and isotope signatures in Amazonia

Abstract We examined the consequences of deforestation and long-term pasture establishment for soil chemical and physical properties and Fe isotope compositions within two distinct forested zones in the Eastern and Central parts of Amazonia (i.e., near Rio Capim, Para, Brazil and lake Balbina/Rio Uatuma, Amazonas, Brazil). For each studied zone, soils selected under forest cover and pasture or after recent slash and burn were investigated. In both forest ecosystems studied, pedogenesis along slopes reflects the evolution of lateritic crust bearing soils (Ferralsol) into iron depleted soils (Acrisol). Measurements performed by plasma source mass spectrometry show a large range of iron isotope signature downhill within the studied soil profiles, both under forest and pasture. Iron isotopic signature in soils under forest cover (δ57FeIRMM-14 ~ +0.2 to +0.6‰) and pasture (δ57FeIRMM-14 ~ +0.3‰) are significantly heavier than both the continental crust baseline and the reference ferralitic soils from the top of the hill (δ57Fe IRMM-14 ~ +0.1‰). This enrichment in heavy isotopes is attributed to the preferential mobilization and loss of light iron during pedogenesis that involves redox processes. Under forest, light iron depletion is far more important in lowland zone and sand-rich soils (with a remaining Fe having δ57Fe IRMM-14 ~ +0.6‰) close to the river system. Under pasture, the light iron depletion is less important, with remaining δ57FeIRMM-14 ~ +0.4‰. This difference is explained by geomorphological changes due to erosion processes (i.e., colluvic processes) following deforestation leading to an “isotopic rejuvenation” of the soil in the valley, mostly apparent within the riverside soils. For comparison, we investigated isotopic changes in a recently deforested small watershed, near Lake Balbina, Central Amazonia, focusing on riverside soils. Results confirm that the isotopic rejuvenation effect appears within a few months following the first stage of deforestation as a consequence of erosion processes following slash-and-burn practices.