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

Description

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.

Links & Downloads

1. http://etd.lsu.edu/docs/available/etd-03292005-135153/

Tags

Oceanography & Coastal Sciences

Additional Fields

Identifer	oai:union.ndltd.org:LSU/oai:etd.lsu.edu:etd-03292005-135153
Date	04 April 2005
Creators	Huang, Bin
Contributors	William H. Patrick, Robert P. Gambrell, Harry H. Roberts, Ralph J. Portier, Jaye E. Cable, Lewis A. Gaston
Publisher	LSU
Source Sets	Louisiana State University
Language	English
Detected Language	English
Type	text
Format	application/pdf
Source	http://etd.lsu.edu/docs/available/etd-03292005-135153/
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