Distribution and mobilization of Al, Fe and Si in three podzolic soil profiles in relation to the humus layer

The mobilization of Al, Fe and Si in podzols is often associated with the w eathering of silicate minerals in the E horizon, downward migration, and th eir accumulation lower in the soil profile. This study investigated Al, Fe, and Si concentrations in soil water (centrifugation samples) and estimated leaching losses from the humus layer in comparison with those in the miner al E and B horizon. Concentrations of total soluble Al and Fe in soil water were higher in the O and E horizons than in B horizon samples, but not sig nificantly different between the two uppermost horizons. Si concentrations were higher in the E than in the O horizon. The amount of Al and Fe leachin g from the E horizon (i.e. input to the B horizon) compared to amount leach ing from the O horizon (i.e. the input to the E horizon) ranged from 92% to 163%. Calculated leaching losses from the lower B horizon were less than 3 % of the input to the B horizon. The results suggest that a large part of t he illuviated Al and Fe in the Bs horizon is derived from the O horizon. Si milar values for the Si ranged from 56% to 61% (comparing E horizon output with E horizon input). Budget estimates available for one of the investigat ed podzols indicated that biocycling via above-ground litter explained < 12 4E of the estimated annual input of Al and Fe to the forest floor. Inputs o f Al and Fe due to upward flow of capillary water accounted for about 26% o f the Al and Fe in the O layer. The results show that there is a considerab le pool of Si Al, Fe in the humus layer. This pool plays an important part in the present day biogeochemical cycling of these elements in podzolic soi ls. Several mechanisms possibly involved in the transfer of Al, Fe and Si f rom the mineral soil to the humus layer are discussed. (C) 2000 Elsevier Sc ience B.V. All rights reserved.