Abstract
Background: Rice is typically understood to enhance methane emissions from paddy fields. However, rice actually has two separate functions related to methane: i) emission enhancement, such as by providing emission pathways (aerenchyma) and methanogenetic substrates; and ii) emission suppression by providing oxygen pathways, which suppress methanogenesis or enhance methane oxidation. The overall role of rice is thus determined by the balance between its enhancing and suppressing functions. Although existing studies have suggested that rice enhances total methane emissions, we aimed to demonstrate that the balance between rice’s emitting and suppressing functions changes according to overall methane emission levels, which have quite a large range (16–500 kg methane ha−1 crop−1). Methods: Using PVC chambers, we compared methane emissions emitted by rice paddy fields with and without rice plants in rice fields in the Mekong Delta, Vietnam. Samples were analyzed by gas chromatograph. Results: We found high overall methane emission levels and our results indicated that rice in fact suppressed methane emissions under these conditions. Emission reductions increased with the growth of rice, up to 60% of emission rate at the maximum tillering stage, then decreased to 20% after the heading stage, and finally recovering back to 60%. Discussion: It is known that methane is emitted by ebullition when the emission level is high, and methane emission reductions in rice-planted fields are thought to be due to oxidation and methanogenesis suppression. However, although many studies have found that the contribution of soil organic matter to methanogenesis is small, our results suggested that methanogenesis depended mainly on soil organic matter accumulated from past crops. The higher the methane emission level, the lower the contribution of rice-providing substrate. Conclusion: As a result, during the growing season, rice enhanced methane emissions in low-emission paddy fields but suppressed methane emissions in high-emission paddy fields.
Highlights
The role of rice in methane (CH4) emissions changes according to emission levels
The presence of rice plants has been determined to be the cause of CH4 emissions in paddy fields
Our results showed that rice decreased CH4 emissions by half relative to paddy fields without rice
Summary
The role of rice in methane (CH4) emissions changes according to emission levels. Rice performs three key functions related to CH4 emissions: i) providing a CH4 pathway through a well-developed system of intercellular air spaces (aerenchyma), ii) providing a substrate for methanogenesis, and iii) oxidising CH4 in rhizosphere by supporting O2 counter-transport through aerenchyma system[1,2,3,4,5,6]. Other recent studies have provided additional evidence that the primary source of CH4 is current-season photosynthates—root exudates or decaying tissues[8,9,10,11] This results in CH4 emissions that peak during the late stage of rice growth. The overall role of rice is determined by the balance between its enhancing and suppressing functions. Existing studies have suggested that rice enhances total methane emissions, we aimed to demonstrate that the balance between rice’s emitting and suppressing functions changes according to overall methane emission levels, which have quite a large range (16–500 kg methane ha−1 crop−1). Discussion: It is known that methane is emitted by ebullition when the emission level is high, and methane emission reductions in rice-planted fields are thought to be due to oxidation and methanogenesis suppression. Conclusion: As a result, during the growing season, rice enhanced methane emissions in low-emission paddy fields but suppressed methane emissions in high-emission paddy fields
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