Mobilization of aluminium in the rhizosphere soil solution of growing treeroots in an acidic soil

Chemical conditions in the rhizosphere in many respects are different from the bulk soil. Especially in acid forest soils aluminium chemistry at the s oil root interface is of particular interest because of its importance for evaluating the risk of rhizotoxicity. In the present study we have used mic ro suction cups to collect soil solution from the rhizosphere of oak seedli ngs (Quercus robur L.) in high spatial resolution and capillary electrophor esis for the determination of major ions and Al3+. While the concentrations of nutrient cations, especially Ca2+ and Mg2+, decreased in the vicinity o f growing roots the concentrations of Al3+ significantly increased. Al3+-io ns were probably released when root-exuded protons were buffered by the soi l. Their occurrence indicates, that the oak roots in our experiments had on ly limited capabilities to detoxify Al in their rhizosphere. The restrictio n of this effect to the very small soil compartment close to the roots sugg ests, that common soil analysis which neglect rhizosphere processes might g reatly underestimate the in situ concentration of Al3+ near tree roots. Our experiments furthermore indicate, that also suberized roots have a signifi cant influence on rhizosphere soil solution chemistry.