The amount of secondary Al in two ploughed podzolic forest soils

In ploughing of coniferous forest soils the soil profile down to the illuvial B horizon is inverted on a soil surface. As a result, aluminium once immobilized in the B horizon during podzolization might start to remobilize. In order to test this hypothesis, selective extractions with dithionite-citrate (d), acid ammonium oxalate (o) and sodium pyrophosphate (p) were used to determine amounts of Al in a ploughed podzolic profile and compared with those of the corresponding undisturbed profile in two forest soils tilled 17 (site K1) and 31 (site K2) years ago. Amounts of oxalate and dithionite-citrate extractable Al in the exposed Bs horizons of the tilt and furrow were lower than in the corresponding undisturbed soil at site K1 suggesting that mobilization of Al had occurred. Since amorphous Al (Alo-Alp), in particular, had decreased released Al most probably originated from imogolite-type materials. Concurrently, the amount of total carbon in these exposed horizons had increased whereas soil pH(CaCl2) decreased implying that mobilization of Al was most likely accelerated by ecosystem internal processes such as organic acids excreted by roots and released during decomposition. Increased amounts of amorphous Al deeper in the tilt suggest that released Al had most probably migrated downward until reprecipitated. Migration and subsequent retention to organic matter might also explain the high amounts of organically bound Al (Alp) in the double humus layer buried beneath the tilt. At site K2, the changes in amounts of Al in the tilt were not as obvious as those observed at site K1. The high amounts of organically bound Al in the double humus layer and amorphous Al in the Bs horizon beneath the tilt, however, support that some mobilization of Al has occurred at site K2.