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

Abstract

We examined the consequences of soil erosion processes following deforestation and long-term pasture establishment for soil chemical and physical properties and Fe isotope compositions within two distinct areas in the Eastern and Central parts of Amazonia (near Rio Capim, Para, Brazil and Lake Balbina/Rio Uatuma, Amazonas, Brazil).For each area studied, soils selected under forest cover and pasture or after recent slash-and-burn practices were investigated. In both forest ecosystems studied, pedogenesis along slopes reflects the evolution of lateritic crust bearing soils (Ferralsol) into iron depleted soils after leaching (Acrisol) and redox processes (Gleysol). Measurements performed by plasma source mass spectrometry show a large range of iron isotope signature within the soil profiles studied at the bottom of the slopes (i.e. foot slope and/or valley soils), under both forest and pasture. Iron isotopic signature in foot slope 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 (δ57FeIRMM-14 ~ +0.1‰). This enrichment in heavy isotopes is attributed to the preferential mobilisation and loss of light iron during pedogenesis that involves redox processes in areas periodically flooded at the foot slope and valley soils. Under forest, light iron depletion (where ~90% of the iron was leached in the first metre of the soil) has greater influence on the soil isotopes signature, particularly in foot slope sand-rich soils close to the river system with a remaining Fe having δ57Fe IRMM-14 ~ +0.6‰. Under pasture where soils experienced erosion processes, iron depletion in foot slope and riverside soils is less important (where only a little more than half of the iron was leached in the first metre of the soil) and the resulting δ57FeIRMM-14 ~ +0.4‰ suggests an input of light iron isotopes via colluvic material deposition. This difference indicates that geomorphological changes due to erosion processes following deforestation (i.e. colluvic processes) lead to an “isotopic rejuvenation” of the soils 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. Erosion can remove isotopically lighter (i.e. close to the continental crust value of ~0.1‰ in δ57Fe) iron bearing soil material from the top of the hill. When such colluvial soil material accumulates in the bottom of the hills on soils that were initially depleted in light iron isotopes, again due to podzolisation as described in the previous site, it induces an “isotopic rejuvenation” (tending again towards a lighter, continental crust-like δ57Fe signature) of the foot slope and riverside soils, besides changes in landscape morphology, water drainage and soil functioning (i.e. hydromorphic processes). Hence, this study illustrates that Fe isotopes can be used to identify erosion within a deforestation context through “isotope rejuvenation”, even when soil textures make it challenging to unravel.