『Abstract
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 to 100 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.
Keywords: Greenhouse gas; Methane; Nitrous oxide; Healthy redox
potential; Alternate wetting and drying; Rice cultivation』
1. Introduction
2. Materials and methods
2.1. Soil and straw
2.2. Experimental design
2.3. Redox potential and soil water potential measurements
2.4. Gas measurements, emitted and dissolved
2.5. Statistical analyses
3. Results
3.1. Redox potential and soil water potential
3.2. Nitrous oxide emission
3.3. Methane emission and production
4. Discussion
4.1. Application of the healthy redox concept
4.2. Water and crop residue management strategies
Acknowledgements
References