Soil podzolization induced by reforestation as shown by sequential and kinetic extractions of Fe and Al

The impact of 60‐year‐old reforestation on soil evolution was studied by following the evolution of Fe and Al distribution as an indicator of early podzolization. We determined the distribution of Al and Fe in soils by combining sequential and kinetic extractions. We extracted the soil sequentially with: 1 mol litre^?1 sodium acetate at pH 5.5 (stage 1); twice with 0.1 mol litre^?1 sodium pyrophosphate at pH 10 (stage 2 and 2b); and with 1 mol litre^?1 hydroxylamine in 4.3 mol litre^?1 acetic acid solution at 90°C (stage 3). For each, we drew a kinetic‐extraction curve. We show that no re‐adsorption, precipitation or complexation with dissolved organic matter occurred for the reaction times recommended in this procedure, that is 6 hours for stage 1, 1.5 hours for stage 2 and 2b and 3 hours for stage 3. For a given extraction, the kinetic curves have the same shape for all samples, that is their behaviour is similar for a given reagent. Sequential extraction can therefore be used to compare the speciation of metals in different samples taken from the same soil. In addition, 60 years after reforestation by beech, a decrease of 12 and 17% is noted in the total Fe and Al contents, respectively, in the 0–10 cm layer, accompanied by a re‐distribution of these elements within the different soil compartments: a decrease of the Fe and Al bound to oxy‐hydroxide in favour of organic complexes and more soluble forms. Sequential extraction is therefore a good indicator of the first pedogenetic modifications induced by human intervention through reforestation.