Date of Award

2000

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

First Advisor

William H. Patrick, Jr

Abstract

Rice fields are an important source of the greenhouse gases methane (CH 4) and nitrous oxide (N2O). In this dissertation study, experiments were conducted at three levels---in soil suspensions, in soil cores, and under actual field conditions, to investigate the impact of soil redox potential on CH4 and N2O emissions to the atmosphere. Methane and N2O emissions occurred at distinctively different redox conditions in the homogenous soil suspensions. No significant amounts of CH4 was produced when the soil redox potentials were above -150 mV. In the denitrification process, both N2O production and reduction occurred in the soil redox potential range of +350 to +400 mV. When N 2O reduction was not inhibited by acetylene, N2O tended to accumulate in the redox potential range of +120 to +250 mV. Therefore, both N2O and CH4 emissions were low in the general redox potential range of +120 to 170 mV where the soil was too oxidized to produce CH4 and too reduced to produce (N2O). Nitrous oxide is a strong chemical oxidant, and the addition of N2O to the reducing soil suspensions could result in a considerable increase of the soil redox potential. In the heterogeneous soil cores and under field conditions, higher CH 4 concentrations were found at greater depths in the soil, while N 2O concentrations tended to form multiple peaks with depth in the soil profile. The seasonal variations of CH4 emissions from rice fields were consistent with the development of strongly reducing conditions in the soils. Non-flooding irrigation management reduced CH4 emissions by about 70 to 80% in the rice growing season. A potential risk exists to increase N2O emissions by the proposed irrigation management, but higher soil organic matter content effectively prevented the increase of N 2O emissions by facilitating N2O reduction to N2. With organic manure application, the rice yields were maintained regardless of different irrigation practices during the rice growing season. Control of both irrigation and organic manure application may be a practical approach for mitigation of greenhouse gas emissions in the irrigated rice fields without adverse effects on rice yield.

ISBN

9780493071787

Pages

128

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