Mathematical modelling of methane transport by Phragmites: the potential for diffusion within the roots and rhizosphere

The release of methane into the atmosphere by Phragmites australis (Cav.) Trin. ex Steud. can be considered as a two-stage process. The first, a mainly diffusive movement through the rhizosphere from the anaerobic source regions of the soil and into and along the roots to the root–rhizome junction. The second, the removal of the gas from the root–rhizome junction to the atmosphere through the rhizome–culm system, a process often dominated by convective (pressurised) gas flow. This article addresses the first of these stages and is presented in isolation because of its perceived commonality to wetland plants in general.The model treats the root and its oxygenated rhizosphere as a series of concentric cylinders: two non-(or low) porosity stelar cylinders, a highly porous cortex, a non-porous epidermal/hypodermal cylinder and the rhizosphere itself. The methane source lies at the edge of the oxygenated rhizosphere the dimensions of which are determined by the integrated effects of oxygen consumption in root and rhizosphere (the latter including a methanotrophic element) and the diffusive impedances throughout the system.The results demonstrate something of the complexity of root-methane–oxygen relations. Methane entry from the rhizosphere is shown to vary along the length of any individual root and, as expected, methane oxidation within the rhizosphere is found to reduce the potential for methane loss to the atmosphere. Situations are also revealed: (i) where the methane concentration falls to zero within the rhizosphere because of aerobic microbial consumption supported by radial oxygen loss from the root, and (ii) where methane may enter the root at one point and escape to the rhizosphere at some other. In this latter case, methane concentration minima are possible within the rhizosphere supplied by methane fluxes from both the root and the bulk soil.Predictions of the quantities of methane which might be released via Phragmites roots to the atmosphere accord with examples of those previously reported from field data.