Simultaneous minimization of nitrous oxide and methane emission from rice paddy soils is improbable due to redox potential changes with depth in a greenhouse experiment without plants

Rice (Oryza sativa L.) paddy soils have the potential to emit both of the greenhouse gases methane (CH4) and nitrous oxide (N2O), depending on redox potential. Results from incubation studies with homogenized soil led to a proposed “healthy redox potential” range that minimizes both CH4 and N2O emission. We examined whether controlled irrigation with the water-saving practice of “safe alternate wetting and drying” (AWD) could maintain a healthy redox potential throughout the rooting layer of rice paddy soil in a greenhouse experiment without plants. We hypothesized that a surface redox potential for N2O and a rooting-depth redox potential for CH4 would both be important in determining gas emissions, making it difficult to minimize rice paddy greenhouse gas emissions through water management. We monitored gas emission for 7 weeks from soil treated by continuous flooding, AWD, or “healthy redox” water management, with and without rice residue incorporation, with no fertilizer application. Nitrous oxide, after the initial flush, was emitted only when the redox potential at 2-cm soil depth was above + 140 mV. Methane was emitted even when the 7.5-cm redox potential was above –150 mV. Measurement of dissolved CH4 in soil solution confirmed that there was 17 to100 times more methane at 20-cm depth than 7.5 cm, with no significant differences between water treatments at 20 cm (P < 0.05). The AWD treatment emitted significantly more N2O than the continuously flooded treatment (P < 0.05), while the reverse was true for CH4 (P < 0.05). Residue incorporation significantly increased CH4, but not N2O, emission in both water treatments (P < 0.05), and thus had more impact than water management. We conclude that simultaneous minimization of both CH4 and N2O emission cannot be maintained in rice soils due to the redox potential changes with depth, but that appropriate water and residue management can reduce greenhouse gas emissions. Without residue incorporation, continuous flooding has the lowest combined risk of N2O and CH4 emission, but with residue incorporation, AWD has similar or lower risk.