Nitrate Reduction and Methane Formation as Influenced by Iron-Centered Intermediate Redox Processes in Rice Soils

• Main Author: Huang, Bin
• Other Authors: William H. Patrick
• Format: Others
• Language: en
• Published: LSU 2005
• Subjects: Oceanography & Coastal Sciences
• Online Access: http://etd.lsu.edu/docs/available/etd-03292005-135153/

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.