Plant-Sediment Interactions in Salt Marshes - An Optode Imaging Study of O2, pH, and CO 2 Gradients in the Rhizosphere

Abstract

In many wetland plants, belowground transport of O₂ via aerenchyma tissue and subsequent O₂ loss across root surfaces generates small oxic root zones at depth in the rhizosphere with important consequences for carbon and nutrient cycling. This study demonstrates how roots of the intertidal salt-marsh plant Spartina anglica affect not only O₂, but also pH and CO₂ dynamics, resulting in distinct gradients of O₂, pH, and CO₂ in the rhizosphere. A novel planar optode system (VisiSens TD^®, PreSens GmbH) was used for taking high-resolution 2D-images of the O₂, pH, and CO₂ distribution around roots during alternating light-dark cycles. Belowground sediment oxygenation was detected in the immediate vicinity of the roots, resulting in oxic root zones with a 1.7 mm radius from the root surface. CO₂ accumulated around the roots, reaching a concentration up to threefold higher than the background concentration, and generally affected a larger area within a radius of 12.6 mm from the root surface. This contributed to a lowering of pH by 0.6 units around the roots. The O₂, pH, and CO₂ distribution was recorded on the same individual roots over diurnal light cycles in order to investigate the interlinkage between sediment oxygenation and CO₂ and pH patterns. In the rhizosphere, oxic root zones showed higher oxygen concentrations during illumination of the aboveground biomass. In darkness, intraspecific differences were observed, where some plants maintained oxic root zones in darkness, while others did not. However, the temporal variation in sediment oxygenation was not reflected in the temporal variations of pH and CO₂ around the roots, which were unaffected by changing light conditions at all times. This demonstrates that plant-mediated sediment oxygenation fueling microbial decomposition and chemical oxidation has limited impact on the dynamics of pH and CO₂ in S. anglica rhizospheres, which may in turn be controlled by other processes such as root respiration and root exudation.

Keywords: Spartina; imaging methods; planar optode; plant–soil interactions; roots; salt marsh; sediment oxygenation; soil chemistry.

FIGURE 1

Planar optode set-up for investigation of rhizosphere O₂, pH, and CO₂ in waterlogged wetland plants with illumination of the aboveground biomass. The rhizobox was equipped with an optode foil over the root selected for investigation and placed hanging from the top of an up-side-down cardboard box into a water-filled aquarium. Camera for image acquisition and LED lights for excitation of the optode foil were placed in darkness inside the cardboard box.

FIGURE 2

Spatial variation of O₂, pH, and CO₂ around roots of Spartina anglica. Oxygen release, CO₂ enhancement, and pH decline are clearly visible in the vicinity of the root. (Top panels) Optode images of the spatial distribution of O₂, pH, and CO₂ in a 4 cm × 4 cm area around single roots of S. anglica from three individual plants (#1-3). Images of the roots and their relative position are shown on the left side (position of root may change slightly between optode images due to root growth and movement during exchange of optode foils). The optode images show the largest impact of roots on O₂, pH, and CO₂ in the surrounding sediment observed in these experiments. (Table inlet) Table showing the average radius of the area of influence inflicted by roots on O₂, pH, and CO₂ in the rhizosphere, and the maximum range of O₂ and CO₂ concentrations and pH measured in the profiles. (Bottom panels) Cross-sectional concentration profiles of O₂, pH, and CO₂. Gray bars indicate relative root position and average root width. Location of profiles are shown by a punctuated line in the root images.

FIGURE 3

(Top panels) Temporal variation of O₂, pH, and CO₂ concentrations in the immediate vicinity of S. anglica roots and in the bulk sediment. O₂, pH, and CO₂ were monitored around single roots from three individual plants during two consecutive light (12 h)-dark (12 h) periods (left). Average O₂ and CO₂ concentrations and pH were measured over time within the affected root zone and in a designated area of the bulk sediment. The root zone was selected as the largest area affected by the root, and the bulk sediment was selected in an area unaffected by root presence. Both areas are exemplarily depicted for plant #1 in optode images (right). (Table inlet) Table showing the average difference in the rhizosphere concentrations between light and darkness, and the time it takes the rhizosphere concentration to stabilize at a new level.