Methane fluxes were related to the first principles of transport and kinetics (microbial and chemical reactions). The difference between the kinetic scale and the smallest flux scale, the plot scale, is large. A stepwise scaling up procedure was therefore used in a series of three papers. This paper treats kinetics and diffusion around a single gas-transporting root. Kinetic processes include methane production, methane oxidation, electron acceptor reduction, electron acceptor reoxidation, and aerobic respiration. This is the minimum number of processes needed to relate net methane production to the main driving variables: carbon availability and oxygen inflow. Kinetics were integrated with diffusion, leading to a set of partial differential equations. This set was solved directly and also after simplification to a set of spatially averaged ordinary differential equations. Results of the simplified model closely resembled results of the unsimplified (full) model, which implies that the simplified model covers the main interactions of the full model and is suitable for further scaling up. Model results showed that reduction of methane emission after 100% specific inhibition of methane oxidation may not result in a reliable estimate of methane oxidation, firstly because of changes in the oxygen dynamics which directly or indirectly affects methane production and secondly because of transient effects. ¿ 2001 American Geophysical Union