Methane emissions from rice paddies : experiments and modelling

This thesis describes model development and experimentation on the comprehension and prediction of methane (CH 4 ) emissions from rice paddies. The large spatial and temporal variability in CH 4 emissions and the dynamic non-linear relationships between processes underlying CH 4 emissions impairs the applicability of empirical relations. Mechanistic concepts are therefore starting point of analysis throughout the thesis.The process of CH 4 production was investigated by soil slurry incubation experiments at different temperatures and with additions of different electron donors and acceptors. Temperature influenced conversion rates and the competitiveness of microorganisms. The experiments were used to calibrate and validate a mechanistic model on CH 4 production that describes competition for acetate and H 2 /CO 2 , inhibition effects and chemolithotrophic reactions. The redox sequence leading eventually to CH 4 production was well predicted by the model, calibrating only the maximum conversion rates.Gas transport through paddy soil and rice plants was quantified by experiments in which the transport of SF 6 was monitored continuously by photoacoustics. A mechanistic model on gas transport in a flooded rice system based on diffusion equations was validated by these experiments and could explain why most gases are released via plant mediated transport. Variability in root distribution led to highly variable gas transport.Experiments showed that CH 4 oxidation in the rice rhizosphere was oxygen (O 2 ) limited. Rice rhizospheric O 2 consumption was dominated by chemical iron oxidation, and heterotrophic and methanotrophic respiration. The most abundant methanotrophs and heterotrophs were isolated and kinetically characterised. Based upon these experiments it was hypothesised that CH 4 oxidation mainly occurred at microaerophilic, low acetate conditions not very close to the root surface. A mechanistic rhizosphere model that combined production and consumption of O 2