Diffusive gas transport through flooded rice systems

A fully mechanistic model based on diffusion equations for gas transport in a flooded rice system is presented. The model has transport descriptions f or various compartments in the water-saturated soil and within the plant. P lant parameters were estimated from published data and experiments independ ent of the validation experiment. An independent experiment is described in which the diffusion coefficient of sulfurhexafluoride (SF6) in water-satur ated soil was determined. The model was validated by experiments in which t ransport of SF6 through soil and plant was monitored continuously by photoa coustics. The independent default settings could reasonably predict gas rel ease dynamics in the soil-plant system. Calculated transmissivities and con centration gradients at the default settings show that transport within the soil was the most limiting step in this system, which explains why most ga ses are released via plant-mediated transport. The root-shoot interface rep resents the major resistance for gas transport within the plant. A sensitiv ity analysis of the model showed that gas transport in such a system is hig hly sensitive to the estimation of the diffusion coefficient of SF6, which helps to understand diel patterns found for greenhouse gas emissions, and t o the root distribution with depth. This can be understood from the calcula ted transmissivities. The model is less sensitive to changes in the resista nce at the root-shoot interface and in the root fraction active in gas exch ange. The model thus provides an understanding of limiting steps in gas tra nsport, but quantitative predictions of in situ gas transport rates will be difficult given the plasticity of root distribution.