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

Abstract

Abstract 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 μmol 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 inflex us 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 CO 2 or methane (CH 4 ). 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.