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

Abstract

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 (EH). The results indicated that there was no statistically significant difference between the theoretically predicted decrease of EH with increase of pH and the observed change of EH with pH for N2O production (60 mV, P = 0.932), and for significant CH4 production (93 mV, P = 0.204), respectively. Consequently, the EH range with minimum N2O and CH4 production shifted to lower values of the EH 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 EHs 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 EH range with minimum GWP is generally in the range of −150 to +180 mV. This redox window accounts for >40% of the entire EH range in this study, which makes appropriate management of irrigated rice fields possible that will minimize both N2O and CH4 production.