A mechanistic model on methane oxidation in a rice rhizosphere

A mechanistic model is presented onthe processes leading to methane oxidation inrice rhizosphere. The model is driven byoxygen release from a rice root into anaerobicrice soil. Oxygen is consumed by heterotrophicand methanotrophic respiration, described bydouble Monod kinetics, and by iron oxidation,described by a second order reaction.Substrates for these reactions – ferrous iron,acetate and methane – are produced by anexponential time dependent organic mattermineralisation in combination with modifiedMichaelis Menten kinetics for competition foracetate and hydrogen. Compounds diffusebetween rhizosphere, root and atmosphere. Adiffusion resistance between the rice root andshoot is included. Active transport across theroot surface occurs for root exudation andplant nutrient uptake. Iron adsorption isdescribed dependent on pH. The model predictswell root oxygen release, compound gradientsand compound concentrations in a ricerhizosphere. Methane oxidation estimates arecomparable to experimental estimates. Asensitivity analysis showed however thatmethane oxidation is highly dependent on modelinitialisation and parameterisation, which ishighly dependent on the history of therhizosphere and root growth dynamics.Equilibrium is not obtained within the periodthat a single root influences a soil micrositeand results in a large change in methanestorage. Equilibrium is moreover dependentupon the diffusion resistance across the rootsurface. These factors make methane oxidationdynamics highly variable in space and time anddependent on root dynamics. The increasedunderstanding of methane oxidation does notdirectly lead to increased predictiveabilities, given this high variability and theuncertainties involved in rhizospheredynamics.