Assessment of isotopically exchangeable Al in soil materials using 26Al 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 soil 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 (EAl) 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 26Al and equilibrated for different time periods (1–756 h). The size of the Al pool with which the 26Al 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 EAl obtained after 1 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 26Al 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.