Type of Document	Dissertation
Author	Huang, Bin
Author's Email Address	bhuang3@lsu.edu
URN	etd-03292005-135153
Title	Nitrate Reduction and Methane Formation as Influenced by Iron-Centered Intermediate Redox Processes in Rice Soils
Degree	Doctor of Philosophy (Ph.D.)
Department	Oceanography & Coastal Sciences
Advisory Committee	Advisor Name Title Robert P. Gambrell Committee Co-Chair William H. Patrick Committee Co-Chair Harry H. Roberts Committee Member Jaye E. Cable Committee Member Ralph J. Portier Committee Member Lewis A. Gaston Dean's Representative
Keywords	wetland soil redox potential ferric iron reduction denitrification mehanogenesis greenhouse gases nitrous oxide
Date of Defense	2005-01-28
Availability	unrestricted
Abstract 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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