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DIVISION S-10—WETLAND SOILS

Redox Range with Minimum Nitrous Oxide and Methane Production in a Rice Soil under Different pH

Wetland Biogeochemistry Inst., Louisiana State Univ., Baton Rouge, LA 70803

^* Corresponding author (bpatrick{at}premier.net).

A Louisiana rice soil was incubated from the most oxidizing to the most reducing conditions that could be maintained. Four different pH levels were used. Nitrous oxide production initiated shortly after the incubation started under oxidized conditions. As the soil suspension became more reducing, a large amount of CH₄ was produced at a critical reducing point and increased exponentially with a further decrease of redox potential (E_H). The results indicated that there was no statistically significant difference between the theoretically predicted decrease of E_H with increase of pH and the observed change of E_H with pH for N₂O production (60 mV, P = 0.932), and for significant CH₄ production (93 mV, P = 0.204), respectively. Consequently, the E_H range with minimum N₂O and CH₄ production shifted to lower values of the E_H scale when pH increased. Global warming potential (GWP) contribution from the studied soil mainly comes from N₂O production at moderately reducing conditions, and from CH₄ production under very reducing conditions. There was a slight production of CH₄ at high E_Hs soon after incubation started, but this CH₄ production was not a significant source of GWP from soils because of its small quantity and transient occurrence. If pH is neutral, the calculated E_H range with minimum GWP is generally in the range of -150 to +180 mV. This redox window accounts for >40% of the entire E_H range in this study, which makes appropriate management of irrigated rice fields possible that will minimize both N₂O and CH₄ production.

Abbreviations: E_H, soil redox potential • IPCC, Intergovernmental Panel on Climate Change • FID, flame ionization detector • GC, gas chromatograph • GWP, global warming potential • OM, organic matter

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