Assessment of isotopically exchangeable Al in soil materials using Al-26 tracer

The solubility of aluminium (Al) in many acidic soils is controlled by complexation reactions with soil organic matter. In such soils, Al solubility is theoretically a function of the pool size of active Al, i.e., the total amount of Al that equilibrates with the sod solution within a defined period...

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Bibliographic Details
Published in:Geochimica et cosmochimica acta 2005-11, Vol.69 (22), p.5263
Main Authors: Kleja, D. B., Standring, W., Oughton, D. H., Gustafsson, Jon Petter, Fifield, K., Fraser, A. R.
Format: Article
Language:English
Subjects:
accelerator mass-spectrometry
acid soils
aluminum solubility mechanisms
aqueous-solution
equilibrium-model
forest soils
humic substances
organic-matter
podzol bs horizon
quickly reacting aluminum
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Summary:The solubility of aluminium (Al) in many acidic soils is controlled by complexation reactions with soil organic matter. In such soils, Al solubility is theoretically a function of the pool size of active Al, i.e., the total amount of Al that equilibrates with the sod solution within a defined period of time. To date, no reliable measurements of active Al in soil materials exist. In this study, we determined the isotopically exchangeable pool of Al (E-A1) as an operationally defined assessment of active Al in acidic mineral soils. The suitability of CuCl2 and pyrophosphate (Na4P2O7) as extractants for active Al was also evaluated. Eleven samples, mostly from spodic B horizons, were spiked with carrier-free Al-26 and equilibrated for different time periods (1-756 h). The size of the Al pool with which the Al-26 tracer exchanged increased with time during the whole experimental period. Thus, contact time between solid and solution phases needs to be defined when assessing the active Al pool. Values of E-A1 obtained after I to 5 d of equilibration were equal to the amount of CuCl2 extractable Al, but considerably smaller than the Na4P2O7-extractable pool. Equilibration times greater than 5 d resulted in CuCl2 extractable Al concentrations that under-estimated the active Al pool. Three of the investigated samples were rich in imogolite-type materials (ITM). In these samples, 30-50 % of the added Al-26 rapidly became associated with soil constituents in forms that could not be extracted by Na4P2O7, indicating that a part of ITM may be in a dynamic state.
ISSN:0016-7037
1872-9533
DOI:10.1016/j.gca.2005.06.010