Mobilization of aluminium in the rhizosphere soil solution of growing tree roots 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 soil root interface is of particular interest because of its importance for evaluating the risk of rhizotoxicity. In the present study we have used micro suction cups to collect soil solution from the rhizosphere of oak seedlings (Quercus robur L.) in high spatial resolution and capillary electrophoresis for the determination of major ions and Al³⁺. While the concentrations of nutrient cations, especially Ca²⁺ and Mg²⁺, decreased in the vicinity of growing roots the concentrations of Al³⁺ significantly increased. Al³⁺-ions were probably released when root-exuded protons were buffered by the soil. Their occurrence indicates, that the oak roots in our experiments had only limited capabilities to detoxify Al in their rhizosphere. The restriction of this effect to the very small soil compartment close to the roots suggests, that common soil analysis which neglect rhizosphere processes might greatly underestimate the in situ concentration of Al³⁺ near tree roots. Our experiments furthermore indicate, that also suberized roots have a significant influence on rhizosphere soil solution chemistry. This revised version was published online in June 2006 with corrections to the Cover Date.