| Wetlands are among the most productive ecosystems and play important ecological, environmental and hydrological functions on local, regional and global scales and plants represent a vital component in sustaining the healthy functioning of wetland systems. In this thesis, my research focus was the effects of herbaceous plants on the biogeochemistry of wetland sediments.; In my greenhouse microcosm experiments, it was observed that wetland plant transpiration could significantly concentrate dissolved species. Results showed that under normal drainage and transpiration conditions, diurnal variations in plant transpiration alone do not results in comparable variations in the concentrations of dissolved species in the rhizosphere due the "memory" effect. However, significant variations could occur in pores under strong influence of plant roots, as indicated by results obtained with microelectrodes.; Chromium is a widely used toxic heavy metal. Hexavalent chromium could be reduced to trivalent chromium which under normal pH conditions, forms precipitation and is less toxic. I investigated the roles of plants on the reduction and removal of hexavalent chromium. Plant transpiration could enhance the loading of chromium toward the rhizosphere where it could be reduced. Abiotic reduction, possibly driven by organic materials from plants and bacteria, was found to be responsible fir the reduction of hexavalent chromium.; It was also found that both Typha latifolia and Phragmites australis could actively uptake Cl- on Br-, two widely used tracers. The effect of transpiration on the Br concentration profile in sediments, and hence on its uptake kinetics, was highlighted in this research.; Wetlands also represent one of the major sources of atmospheric methane, which is the second most important greenhouse gas. A one-dimensional, process-based methane emission model from rice paddy systems was developed and tested. Rice paddy systems were selected because they represent one of the manageable sources of methane. Effects of rice plants on the production, oxidation and emission of methane was highlighted in this model and it could enable the examination of the consequences of changes in agricultural practices and environmental conditions on methane fluxes and the evaluation of potential mitigation options. |
