Plant-Sediment Interactions in Salt Marshes – An Optode Imaging Study of O2, pH, and CO2 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.

Shallow Salt Marsh Tidal Ponds–An Environment With Extreme Oxygen Dynamics

“…Below the top 1 cm, the sediment was anoxic on the marsh platform (Figure 9). This is also consistent with previous studies demonstrating that even though the Spartina-plants on marsh platform are capable of translocating oxygen and generating oxic root zones (Maricle and Lee, 2002;Koop-Jakobsen et al, 2018), it has limited impact on the bulk rhizosphere sediment that remain anoxic in the summertime (Koop-Jakobsen et al, 2017). In the permanently inundated sediment of the tidal river, the oxygen penetration was very shallow reaching <2 mm into the sediment, and there was no increased oxygen production at the sediment-water interface.…”

Intraspecific Root Trait Variability Along Environmental Gradients Affects Salt Marsh Resistance to Lateral Erosion

“…This can probably be explained by the metabolic costs associated with mechanisms to cope with low redox (Armstrong, 1979), reviewed in Colmer (2003) and Nishiuchi et al (2012). While release of oxygen from plant roots (Pezeshki, 2001) may have contributed to the observed relationships, we assume the direction of causality to flow from the abiotic environment to RD given previous experimental evidence in salt marsh plants that: (i) waterlogging can directly reduce growth of salt marsh plants (Cooper, 1982;Bouma et al, 2001a); and (ii) the impact of oxygen release from roots on sediment oxygenation is limited (Koop-Jakobsen et al, 2018). Therefore, factors that influence sediment redox potential, including bioturbation, tidal inundation (and sea-level rise) and livestock grazing, may indirectly affect the stability of salt marsh sediments by altering RD.…”

Oxygen Consumption and Sulfate Reduction in Vegetated Coastal Habitats: Effects of Physical Disturbance

“…Many wetland plants, including seagrasses, salt marsh and mangrove plants, release DOC (Moriarty et al, 1986;Pollard and Moriarty, 1991) and O 2 into the rhizosphere from roots and the rhizome (Pedersen et al, 1998;Koren et al, 2015;Brodersen et al, 2015bBrodersen et al, , 2016Koop-Jakobsen et al, 2018). Exudate DOC can be easily degraded by microbes both within the oxic and anoxic sediments and thus lead to local stimulation of the microbial community (Isaksen and Finster, 1996;Hansen et al, 2000;Nielsen et al, 2001;Alongi, 2005;Lovell, 2005), which results in high microbial respiration rates in the surface sediment (and plant rhizosphere) and thus high sediment O 2 demand.…”