New approaches to studying chemical and physical changes in the rhizosphere: an overview

The past decade has seen the rapid development of new techniques that have revealed substantial changes in soil physical and chemical properties in the rhizosphere compared to the bulk soil. This brief overview focuses on some examples of recently developed, innovative techniques now available and indicates the technical developments required for the future. The development of non-invasive imaging allied with computed tomography has begun to allow the study of root systems in situ and the measurement of localized uptake of water. Further development is still required to disaggregate the simultaneous changes in bulk density and water content that may occur as roots occupy new soil volumes, but resolution of 0.1 mm is now feasible in scanning times of less than 1 h thereby allowing dynamic processes to be measured. Changes in surface tension and composition of solutions close to roots, and of pH, can now be measured with a variety of techniques. Temporal and spatial changes of pH can be measured with micro-electrodes and dye indicator/agar gel techniques have allowed quantitative estimates of H+ fluxes albeit in artificial systems. Novel micro-sampling techniques are under development to quantify rhizosphere changes. So far these techniques have rarely been applied in soils but innovative sampling and analytical techniques should soon allow such studies.