Methane Plant transport
Gas transport through plants is known to operate by molecular diffusion, effusion or active transport due to pressure differences (Whiting, 1992). The rate at which methane is removed from soil by plants depends from the growing status of the plant:
where kplant is a rate constant parameter (CH4_WaterPlantCoef), Eta is the root water uptake, f(PlantEff) is a function describing plant transport efficiency and cCH¤w is the concentration of CH4 in water:
where pCH4_W1 (CH4_WaterPowerCoefCH4 PlantOxShapeCoef)r is a shape coefficient and Eta_limit is the threshold water uptake (CH4_LimitWaterUptake) above which plant transport efficiency begins to decrease. This means that the methane uptake increases in proportial to the water uptake rate until the water uptake rate exceeds Eta_limit. When the water uptake rate becomes higher than ETa_limit the efficiency is reduced.
The approach used in the present model gives a simplified description about the real process, which is complex and still not well-known (Le Mer, 2001). To define the influence of plant activity on emission other authors followed similar ways. Yu (2003), for example, connected the emission rate with the GSI of the plant, introducing a function which is constant during winter time and decreases linearly from the time the plants emerge from the peat until maturity, followed by a linear decrease until most of the plant stems are killed by frost. On the other hand, some author tried to introduce more detailed models. Beckett (2001), for example, described a multicylinder model for Phragmites that accommodates detailed structure into a cylindrically symmetric set of time-dependant partial differential equations, by regarding the root as a set of coaxial regions, each of which has diffusion and respiration properties that are (on average) appropriate to the local structure and metabolism.