In-situ application of stable isotope tracers in the rhizosphere of an oak seedling

In a controlled rhizotrone experiment, stable isotope tracers of Mg, Ca and K were applied directly to the rhizosphere of an oak seedling using a 2D-array of micro ceramic cups. Before starting isotope application the oak root induced a significant reduction of K⁺, Ca²⁺, Mg²⁺ and NO₃^– in the soil solution of the rhizosphere, as well as an increase of Al³⁺. The effect of adding stable isotopes in the soil (soil solution and exchangeable cations) was mainly restricted to a distance of about 1 cm from the point of application. All stable isotopes were taken up by the oak seedling, especially Ca which according to leaf analysis was in the range of insufficiency. As expected, Ca showed low mobility in the phloem, resulting in a low percentage of label in the root tip as compared to other root segments. Our experiment proved, that in-situ application is an easy to handle tool for carrying out tracer studies in real soil.