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

The dynamics of oxygen concentration, pH value, and organic acids in the rhizosphere of Juncus spp.

A novel type of planar optodes for simultaneous optical analysis of pH and oxygen dynamics in the rhizosphere is introduced. The combination of the optical, non-invasive measurement of these parameters with sterile sampling of rhizosphere solution across and along growing roots by use of a novel type of rhizobox provides a methodical step forward in the investigation of the physicochemical dynamics of the rhizosphere and its underlying matter fluxes between roots and soil. In this study, this rhizobox was used to investigate the effect of oxygen releasing roots of three Juncus species on the amount and distribution of organic acids in reductive, oxygen-deficient soils of different pH (pH 3.9-pH 5.9). Pronounced diurnal variations of oxygen concentration and pH along the roots, particularly along the elongation zone were observed. Long-term records over more than eight weeks revealed considerable spatial and temporal patterns of oxygen over a range of almost 200 mu mol O-2 L-1 and pH dynamics of +/- 1.4 pH units in the rhizosphere. A strong effect of oxidative acidification due to oxygen release by the plant roots was clearly visible for Juncus effusus, whereas the roots of Juncus articulatus alkalinized the rhizosphere. In contrast, roots of Juncus inflexus induced no effects on rhizospheric pH. Only four different organic acids (oxalate, acetate, formate and lactate) were detectable in all soil solutions. Maximal concentration of all organic acids occurred at pH 3.9, whereas the lowest concentration of each organic acid was found at pH 5.9. Hence, considering the pH-dependence of the redox potential, the acid soil provided increased reductive conditions leading to slower anaerobic degradation of organic acids to CO2 or methane (CH4). The concentration of organic acids decreased by up to 58% within a distance of only 4 mm from the bulk soil to the root surface, i.e. reciprocal to the pronounced O-2-gradient. The decreasing presence of organic acids toward the oxygen releasing roots is possibly due to a change in the composition of the microbial community from anaerobic to aerobic conditions. The present study highlights the dynamic interplay between O-2 concentration, pH and organic acids as key parameters of the physicochemical environment of the rhizosphere, particularly for wetland plants growing in oxygen-deficient waterlogged soils. (C) 2011 Elsevier Ltd. All rights reserved

Analysis of residual stresses and wear mechanism of HF CVD diamond coated cemetend carbide tools

Chemical vapour deposition CVD diamond coated tools have demonstrated their potential for the machining of difficult to machine materials in the last 10 years. The lack of adequate coating adhesion in many cases remains an issue and often leads to spontaneous coating delamination and sudden tool failure however. The work described in this paper was undertaken with the aim of understanding the influence of residual stresses and coating quality on the coating adhesion during the machining of aluminium alloys. Residual stresses were determined using Raman spectroscopy as well as X ray diffraction analysis and a comparison of the results obtained using the two methods is given. Raman spec troscopy was also implemented to analyse the coating quality. Furthermore, the failure mechanism of CVD dia mond coated tools was studied using cross sections pre pared by focused ion beam sputterin