Seasonal dynamics of Al‐ and Fe‐bearing secondary minerals in an acid forest soil: influence of Norway spruce roots ( <i>Picea abies</i> (L.) Karst.)

Abstract

Aluminium (Al) and iron (Fe) play a key role in pedogenetic processes; however, short‐term changes in Al‐ and Fe‐bearing secondary minerals are little documented. In this study, Al and Fe dynamics in the solid phase have been evaluated in three soil compartments (bulk, ‘rhizosphere outer’, corresponding to soil volume detached from fresh roots, and ‘rhizosphere inner’, corresponding to soil volume detached from dried roots) and during 4 months (November, February, May and August) in an acid soil with podzolic features at its surface. Soil samples were collected from under Norway spruce ( Picea abies (L.) Karst.) at three depths (0–3, 3–10 and 10–23 cm) and were sieved at 200 µm. The Al and Fe forms were extracted with different solutions: potassium chloride (Al KCl and Fe KCl ), oxalate (Al o and Fe o ), tri‐citrate (Al c ) and dithionite‐citrate‐bicarbonate (Fe d ). The influence of spruce roots and seasons on the dynamics of Al and Fe (precipitation and dissolution reactions) was observed. The rhizosphere effect on Fe d varied with sampling dates. Thus, Fe d ‐Fe o decreased between winter and spring in the rhizosphere inner soil only (decrease of 40%). In contrast, Al c was greater in the bulk soil than in both rhizosphere soils for all seasons. The strongest variations in Al were observed at depth, especially in the bulk and the rhizosphere outer soil, where Al c ‐Al o decreased drastically between winter and spring (from 50 to 60%). The seasonal dynamics of the podzolization process are discussed. This study shows that Al‐ and Fe‐bearing secondary minerals can quickly react to environmental changes, suggesting that these minerals could be markers of currently active functioning of soils.