Planar optodes: promising tools for non-invasive 2D imaging of rhizospheric dynamics of pH, O2 and CO2.
To improve the understanding of soil bioprocesses, especially nutrient flow and carbon allocation between roots and rhizosphere, knowledge of the spatial and temporal dynamics of the physical and chemical conditions of the soil is essential. Especially the physico-chemical parameters pH, redox potential (Eh), oxygen and carbon dioxide partial pressure (pO2; pCO2) are of key importance, because they characterize the environmental conditions for the entire soil biota (plants, fungi, microorganisms, etc.). However, these parameters are neither stable over time, nor are they homogeneously distributed in the soil. Therefore, quantitative high-resolution analyses of radial pH, oxygen and carbon dioxide gradients from the roots towards the bulk soil and axial gradients along the roots are essential for an advanced understanding of plant-mediated effects on soil biochemistry and biology. Furthermore, to avoid any disturbance of the natural conditions of the biogeochemical micro pattern in the soil by the measuring technique itself, such as infiltration of oxygen or increased turbation of the soil substrate by moving e.g. pH or oxygen electrodes, new non-invasive techniques for accurate high-resolution investigations of soil bioprocesses are required.
To overcome these methodical limitations, a novel rhizotrone-based non-invasive 2D imaging system was constructed, which allows high-resolution optical measurements of the spatial and temporal dynamics of pH, oxygen and carbon dioxide in the soil and in the root-soil interface without any disturbance of the biological and physico-chemical conditions caused by the method itself. The optical measurement of pH, O2 and CO2 by so called planar optodes is based on the measurement of the fluorescence decay time of pH, O2 and CO2 -sensitive indicator dyes, which are fixed in a sensor foil (Gansert & Blossfeld 2008).
This novel technique was firstly used to investigate the effect of roots of selected wetland plants on the dynamics of pH patterns in submerged soil, revealing considerable diurnal pH changes of about 1 pH unit along single roots of Juncus effusus L., closely linked to the onset of photosynthesis (Blossfeld & Gansert 2007). Furthermore, so called planar pH-O2 hybrid optodes were effectively used for analyzing rhizospheric pH and O2 dynamics of three different wetland plants (Juncus effusus L., Juncus inflexus L., Juncus articulatus L.), revealing a species specific diurnal pattern of oxygen release (up to almost 200 µmol O2 l-1), which was even detectable for lateral roots (Blossfeld 2008). Further applications were tested successfully with regard to pH dynamics along growing roots in trace metal contaminated soils, revealing strong effects of trace metal tolerant and intolerant plant species on trace metal availability (publication in prep.), as well as for visualizing for the first time the impact of urea hydrolysis on soil pH under non submerged or waterlogged soil conditions (additional contribution to the IPNC XVI; publication in prep.). First tests of application of planar CO2 optodes are in progress.
Therefore, planar optodes show the potential for becoming a powerful tool for non-invasive and quantitative mapping of the key environmental parameters in the root-rhizosphere-soil interface.