Doctor of Philosophy (PhD)


Oceanography and Coastal Sciences

Document Type



Rice fields are a major source of the greenhouse gases methane (CH4) and nitrous oxide (N2O) and contribute to nitrate (NO3-) pollution in waters. Ferric iron (Fe3+) and manganic manganese (Mn4+) are two intermediate alternative electron acceptors (AEAs) capable of regeneration in freshwater soils. In this investigation, the influences of iron-centered intermediate redox processes on NO3- reduction and CH4 formation in rice soils were studied using soil slurries, soil columns, and potted rice. Reduction of Fe3+-centered intermediate AEAs was mainly mediated by obligate anaerobes relying on fermentation products. Ferric iron reducers are bioelectrochemically active, supporting bioelectricity generation through a fuel cell process from the flooded soil coupled to the reduction of O2 or NO3- in the overlying water. As a major electron accepting process in anaerobic carbon decomposition, Fe3+ reduction stimulated N2O production but had little influence on overall NO3- reduction in the homogenized soil slurries under near-neutral pH conditions. In the flooded soil column and pot experiments, intensification of iron-centered intermediate redox processes under amendments of iron and/or manganese oxides changed the fate of NO3- in the overlying water, decreasing heterotrophic denitrification and increasing NO3- percolation and N2O emission. Ferric iron reduction competitively suppressed methanogenic activity in the homogenized soil slurries. The diffusion of the stronger oxidants O2 and NO3- controlled temporal and vertical variations of iron-centered intermediate redox processes, which subsequently controlled temporal and vertical variations of methanogenic activity in the flooded soil columns. In the pot experiment, Fe3+ reduction had small effect on CH4 emission in the early season when CH4 emission was low but effectively reduced CH4 emission after midseason drainage intervals through Fe3+ regeneration. The roles of iron-centered intermediate redox processes need to be considered in the evaluation and predication of NO3- reduction and CH4 formation in rice fields.



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Committee Chair

Robert P. Gambrell