Aerenchyma, the large airspaces in aquatic plants, is a rapid gas transport pathway between atmosphere and soil in wetlands. Oxygen transport aerates belowground tissue and oxidizes rhizosphere soil, an important process in wetland biogeochemistry. Most plant O2 transport occurs by diffusion, and the major challenge for its accurate measurement is avoiding disturbing small-scale gradients in O2 concentration and demand in the pathway. Small O2 sensors with rapid response times and high spatial resolution are the most popular methods for quantifying O2 transport and rhizosphere oxidation of individual roots, but older designs such as the cylindrical Pt electrode are still useful. At a larger scale, O2–scavenging reagents have been used to quantify net O2 exchange of entire root systems but need to be used with caution, because they are indirect methods and artifacts can occur due to issues such as stirring of solutions. In some species, pressurized gas flows develop in shoots and rhizomes, and their contribution to gas fluxes can be assessed with pressure transducers and flow meters. Other gases produced in wetlands (e.g., CO2, CH4, and N2O) are also transported in aerenchyma. Their fluxes are usually quantified by extraction of samples from aerenchyma or from headspace analysis in bicompartment chambers, with remote analysis by methods such as gas chromatography or infrared gas analysis. Finally, mathematical modeling based on quantitative assessments of tissue porosities and air-space dimensions accurately describe both O2 and CH4 fluxes in many species at a range of scales from individual roots to the entire vegetation.
Soil Science Society of America Book Series, 2013, p. 177-196