Identifier

etd-03292005-135153

Degree

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

Document Type

Dissertation

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.

Date

2005

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Robert P. Gambrell

DOI

10.31390/gradschool_dissertations.3830

Share

COinS