Journal of Conference Abstracts

Aluminium Solubility in the Spodic B Horizon and its Relationship with Pedogenesis and Acidification

Jon Petter Gustafsson¹ (gustafjp@l.kth.se), Magnus Simonsson² (magnus.simonsson@mv.slu.se), Dan Berggren² (dan.berggren@mv.slu.se) & David G. Lumsdon³ (d.lumsdon@mluri.sari.ac.uk)

¹ Division of Land and Water Resources, Royal Institute of Technology, S-100 44 Stockholm, Sweden.

² Department of Soil Sciences, Swedish Univ. of Agricultural Sciences, Box 7014, S-750 07 Uppsala, Sweden.

³ Soil Science Group, Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB9 2QJ, U.K.

It is still not quite understood which Al phases actually dissolve from acidified spodic B horizons. We have performed extensive batch equilibrations on two distinctly different sets of spodic B horizons; one set consisted of 60 samples from the uppermost 5 cm of the B horizon of Podzols from southern Sweden (Simonsson and Berggren, 1997), while the other set consisted of 15 Bs horizon samples from 5 less acidified Podzols in central/northern Sweden and Finland (Gustafsson et al., 1997). Both sets of extracts were equilibrated at 8°C and "quickly reacting" Al, pH, and ions were determined on the resulting extracts using identical procedures.

In the samples from the uppermost B horizon, most oxalate-extractable solid-phase Al was organically complexed. In more than 30 % of the soils there was, however, some inorganic Al present, mostly as imogolite-type materials (ITM). The Al activities in the equilibrated extracts could often not be explained by any Al(OH)₃- or imogolite-type phase. On the other hand, the Al solubility, defined as log{Al³⁺} + 1.65 pH, was closely related to the molar ratio of aluminium to carbon in the pyrophosphate extracts when this ratio was < 0.1. This indicated that the solubility of Al was determined by organic complexation in these soils.

In the other set of samples, the most reactive solid-phase Al was inorganic and present mainly as ITM. When shaking the soils with 1 mM NaCl, equilibrium with respect to Al(OH)₃ was reached in one to two weeks (log *K_s at 8°C = 9.40), while equilibrium with respect to an imogolite-type phase was slower (>1 month; log *K_s at 8°C = 7.66). The results from initially oversaturated extracts confirmed that both these inorganic Al phases determined the Al solubility in the soil samples; however the most reactive inorganic Al pool was small, as indicated by the observation that equilibrium could generally not be reached when equilibrating with HCl.

There was no single mechanism that determined the solubility of Al in all spodic B horizons. We believe that the different behaviour of the two sets of soils should be seen in the contexts of pedogenesis and acidification. In many cases the uppermost B horizon, and also the whole of strongly acidified B horizons may be seen as zones of eluviation since more Al is leached from such horizons than is supplied from above horizons and from weathering. Since the pool of reactive inorganic Al is small, this pool may soon become depleted. The solubility of Al would therefore be determined by organic complexation in the short term even though some dissolved Al would originate from the weathering of ITM and other minerals. In active zones of illuviation, however, which are present in much of the B horizon in nonacidified soils, hydroxy-Al minerals would precipitate, renewing the reactive inorganic Al pool which is otherwise slowly converted to more crystalline imogolite-type minerals.

In conclusion, we hypothesize that the solubility of Al is determined by Al(OH)₃ and ITM in active zones of illuviation of the spodic B horizon while organic complexation is more likely to determine the Al solubility in zones of Al eluviation.

References

Gustafsson, J.P., Lumsdon, D.G. & Simonsson, M., Clay Miner. Accepted (1997).

Simonsson, M. & Berggren, D. Submitted (1997).