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Iron-dependent anaerobic oxidation of methane (iron-AOM) has recently been reported to occur in paddy soils, but its actual role and regulation remain unclear. Here, we confirmed the occurrence of iron-AOM at different layers of paddy soils (0-10 cm, 10-20 cm, and 30-40 cm) across tillering, elongation, flowering, and ripening periods under three long-term fertilizer management schemes (CK-unamended, CF-chemical fertilizer, and CFS-chemical fertilizer with straw). The iron-AOM activity contributed 41 % to total AOM, which was greater than that of nitrite- (32 %) and nitrate-AOM (27 %). The iron-AOM activity varied significantly with soil layers, growth periods and fertilizer types, with layer being the most important variable. Soil moisture content and organic carbon content were most significant influencing factors on the AOM activity, and a Candidatus Methanoperedens ferrireducens-like lineage potentially catalyzed iron-AOM. Our results suggest that iron-AOM has an important potential for mitigating methane emissions from rice paddies., Competing Interests: Declaration of competing interest The authors declare no conflict of interest., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Blast furnace slag (BFS), a byproduct of iron-producing process, has been applied as silicate fertilizer in rice paddy. Silicate fertilizer contains lime and silicate as main components and iron and manganese as electron acceptors. This amendment improves soil productivity and mitigates methane (CH 4 ) emissions. However, its suppression effect was limited to <20 % at a field level, and its functionality needs improvement to encourage recycling. We hypothesized that the effect of silicate fertilizer on suppressing CH 4 emission might improve by increasing electron acceptor concentration. To investigate the feasibility of electron acceptor added silicate fertilizer on increasing CH 4 flux suppression, four byproducts of the iron-production process (basic oxygen slag-BOF, ferromanganese slag-FerroMn, iron rust, and Kambara reactor slag-KR) were selected and compared through soil incubation test. Iron rust effectively suppressed CH 4 production by 67 %, which is comparable with a 15-30 % reduction of others. To find the optimum mixing ratio of iron rust, it was mixed to BFS with the rate of 0-5 % (wt wt -1 ), and their effect on CH 4 flux was compared. The 3 % mixing ratio highly increased the BFS functionality on suppressing CH 4 production. To confirm the field adaptability of the improved BFS, three types of silicate fertilizer (mixing iron rust with the ratios of 0, 2.5, and 5 %) were applied with the recommendation level (1.5 Mg ha -1 ) before rice transplanting. Seasonal CH 4 flux was significantly decreased by the original silicate fertilizer (BFS 0 ) application to 20 % over control. This effectiveness was enhanced by adding 2.5 % iron rust but thereafter, not more increased. Silicate fertilization (BFS 0 ) significantly increased rice grain productivity by 9 % over control, and the improved silicate fertilizer (BFS 2.5 & 5.0 ) more highly increased by 13 %. In conclusion, the BFS's functionality to increase rice productivity and suppress CH 4 emission could be improved by adding an effective electron acceptor such as Fe 2 O 3 ., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Quantifying methane (CH 4 ) emissions from paddy fields is essential for evaluating the environmental risks of the paddy rice production system, and improving the accuracy of CH 4 modeling is a key issue that needs to be addressed. Based on a database containing 835 field measurements, both single national and region-specific models were established to estimate CH 4 emissions from paddy fields considering different environmental factors and management patterns using 70% of the measurements. The remaining 30% of the measurements were then used for model evaluation. The performance of the region-specific model was better than that of the single national model. The region-specific model could simulate CH 4 emissions in an unbiased manner with R 2 values of 0.15-0.70 and efficiency values of 11-60%. The paddy rice type, water regime, organic amendment, latitude, and soil characteristics (pH and bulk density) were identified as the main drivers in the models. By inputting the high-resolution spatial data of these drivers into the established model, the CH 4 emissions from China's paddy fields were estimated to be 4.75 Tg in 2015, with a 95% confidence interval of 4.19-5.61 Tg. The results indicated that establishing and driving a region-specific model with high-resolution data can improve the estimation accuracy of CH 4 emissions from paddy fields., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Methane (CH 4 ) emission is the consequence of CH 4 production and consumption performed by methanogens and methanotrophs, respectively. Fertilization is an important factor that regulates the behavior of methanogens and methanotrophs; however, the effect of manure and rice straw addition combined with inorganic fertilizers on these communities is not well understood. This study aimed to explore how manure and rice straw amendments together with inorganic fertilizers influenced the methanogenic and methanotrophic communities in a 31-year fertilized rice paddy. Manure amendment significantly increased the abundance of mcrA and pmoA genes by 61.2% and 63.3% compared with the unfertilized control, whereas inorganic NPK fertilization alone or rice straw addition did not affect their abundances. Manure and rice straw amendments greatly decreased the Shannon index and ACE index of the methanogenic communities, whereas inorganic NPK fertilization alone increased the ACE index of the methanotrophic communities compared with the unfertilized control. Methanosarcinaceae and Methylococcaceae dominated at the family level, representing 23.1-35.0% and 48.7-67.2% of the total reads, for the methanogenic and methanotrophic communities, respectively. Application of manure together with inorganic fertilizers suppressed the Methanocellales methanogens and the type I methanotrophs (Methylococcus and Methylobacter). Fertilization greatly altered the community structure of methanogens and methanotrophs, and manure addition had more apparent effects than rice straw. Moreover, total nitrogen, soil organic carbon, available phosphorus, and available potassium correlated significantly to the abundance, composition, and community structure of methanogens and methanotrophs. In conclusion, our study revealed that long-term manure amendment in combination with inorganic fertilizers significantly increased the abundance of methanogens and methanotrophs, but suppressed the type I methanotrophs in rice paddies.

Methane (CH 4 ) is an important greenhouse gas (GHG), and paddy fields are major sources of CH 4 emissions. This pot experiment was conducted to investigate the integrated effects of Azolla inoculation combined with water management and N fertilization on CH 4 emissions in a double-rice cropping system of Southern China. Results indicated that midseason aeration reduced total CH 4 emissions by 46.9%, 38.6%, and 42.4%, followed by N fertilization with 32.5%, 17.0%, and 29.5% and Azolla inoculation with 32.5%, 17.0%, and 29.5%, on average, during the early, late, and annual rice growing seasons, respectively. The CH 4 flux peaks and total CH 4 emissions observed in the late rice growing season were significantly higher than those in the early rice growing season. Additionally, CH 4 fluxes correlated negatively to soil redox potential (Eh) and dissolved oxygen (DO) concentration. Azolla inoculation and N fertilization greatly increased the rice grain yields, whereas midseason aeration had distinct effects on grain yields in both rice seasons. The highest annual rice grain yields of approximately 110 g pot -1 were obtained in the Azolla inoculation and N fertilization treatments. In terms of yield-scaled CH 4 emission, Azolla inoculation combined with midseason aeration and N fertilization generated the lowest yield-scaled CH 4 emissions both in the early and in the late rice growing seasons, as well as during the annual rice cycle. In contrast, the highest yield-scaled CH 4 emission was obtained in the treatment employed continuous flooding, without Azolla and no N application. Our results demonstrated that Azolla inoculation, midseason aeration, and N fertilization practices mitigated total CH 4 emissions by 18.5-42.4% during the annual rice cycle. We recommend that the combination of Azolla inoculation, midseason aeration, and appropriate N fertilization can achieve lower CH 4 emissions and yield-scaled CH 4 emissions in the double-rice growing system.

Knowledge about the impacts of fresh and field-aged biochar amendments on greenhouse gas (CH 4 , N 2 O) emissions is limited. A field experiment was initiated in 2012 to study the effects of fresh and field-aged biochar additions on CH 4 and N 2 O emissions and the associated microbial activity during the entire rice-growing season in typical rice-wheat rotation system in Southeast China. CH 4 and N 2 O fluxes were monitored, and the abundance of methanogen (mcrA), methanotrophy (pmoA), ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), nitrite reductase (nirS, nirK), N 2 O reductase (nosZ), and potential soil enzyme activities related to CH 4 and N 2 O were simultaneously measured throughout different rice developmental stages. There were three treatments: control (urea without biochar), fresh BC (urea with fresh biochar added in 2015), and aged BC (urea with 3-year field-aged biochar added in 2012). Results showed that field-aged biochar significantly decreased seasonal CH 4 emissions by 16.8% in relation to the fresh biochar, though no significant differences were detected between biochars and control treatment. The structural equation model indicated that soil pH, microbial biomass carbon (MBC), pmoA, and mcrA were the main factors directly influenced by fresh and aged biochar amendments; aged biochar showed a negative effect while fresh biochar showed positive effects on CH 4 fluxes. Both fresh and field-aged biochar obviously increased AOA and AOB abundances and reduced the (nirS+nirK)/nosZ ratio during the entire rice-growing season, although no significant effects were observed on seasonal N 2 O emissions. Therefore, biochar amendment produced long-term effects on total CH 4 and N 2 O emissions through observed influences of soil pH and functional gene abundance. The figure shows how fresh and field-aged biochar differentially affected CH4 production and oxidation and N2O production and reduction through related functional gene abundances. Blue arrows indicate suppressing while pink arrows indicate promoting effect.

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., Competing Interests: No competing interests were disclosed.

Background: Rice is understood to enhance methane emissions from paddy fields in IPCC guidelines. However, rice actually has two opposite 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 previous studies have suggested that rice enhances total methane emissions, we aimed to demonstrate in high-emitting paddy fields that the overall methane emission is decreased by rice plants. Methods:  We compared methane emissions with and without rice plants in triple cropping rice paddy fields in the Mekong Delta, Vietnam. The gas samples are collected using chamber method and ware analyzed by gas chromatography. Results:  We found that rice, in fact, suppressed overall methane emissions in high-emitting paddies. The emission reductions increased with the growth of rice to the maximum tillering stage, then decreased after the heading stage, and finally recovered.  Discussion :  Our result indicates that the overall methane emission is larger than that of rice planted area. In addition, although many studies in standard-emitting paddies have found that the contribution of soil organic matter to methanogenesis is small, prior studies in high-emitting paddies suggest that methanogenesis depended mainly on soil organic matter accumulated from past crops. The higher the methane emission level, the lower the contribution of the rice-derived substrate; conversely, the higher the contribution of the rice providing oxygen. Finally, rice plants reduce methane emissions in high-emitting paddies. Conclusion: The present study demonstrates that during the growing season, rice is suppressing methane emissions in high-emitting paddies. This means the significance of using the rice variety which has high suppressing performance in high-emitting paddies., Competing Interests: No competing interests were disclosed.

In recent years, rice paddies have been increasingly converted to vegetable production resulting from economic benefits and changes in demand of diets, potentially altering soil greenhouse gas (GHG) balance. Here, we implemented a parallel field experiment to simultaneously quantify the differences in emissions of CH 4 and N 2 O among rice paddy (RP) and conventional vegetable field (CV) and greenhouse vegetable field (GV), both of which have been recently converted from rice paddy in subtropical China over a full year. The results revealed that CH 4 emission was reduced dramatically by nearly 100% following the initial land management change from rice to vegetable production, with annual emissions of 720.9, 0.9, and 0.2 kg CH 4 -C ha -1 for RP, CV, and GV, respectively. This conversion however substantially increased N 2 O emissions, resulting in the transition from a minor sink of N 2 O in RP (-0.1 kg N ha -1  yr -1 ) to considerable N 2 O sources in CV (31.8 kg N ha -1  yr -1 ) and GV (52.2 kg N ha -1  yr -1 ). Furthermore, annual N 2 O emission from GV significantly exceeded that from CV due to lower soil pH and higher soil temperature facilitating N 2 O production in GV relative to CV. Land management change significantly decreased the annual total emissions of CH 4 and N 2 O from CV and GV by 19-51% as compared to RP, attributing to the reduced CH 4 emissions outweighing the increased N 2 O emissions in CV and GV. These results indicate that expansion of vegetable production at the expense of rice paddies for higher economic benefits also helps mitigate the total emissions of CH 4 and N 2 O.

Methane emissions, along with methanotrophs and methanogens and soil chemical properties, were investigated in a flooded rice ecosystem. Methane emission increased after rice transplantation (from 7.2 to 552 mg day(-1) m(-2) ) and was positively and significantly correlated with transcripts of pmoA and mcrA genes, transcript/gene ratios of mcrA, temperature and total organic carbon. Methane flux was negatively correlated with sulfate concentration. Methanotrophs represented only a small proportion (0.79-1.75%) of the total bacterial 16S rRNA gene reads: Methylocystis (type II methanotroph) decreased rapidly after rice transplantation, while Methylosinus and unclassified Methylocystaceae (type II) were relatively constant throughout rice cultivation. Methylocaldum, Methylobacter, Methylomonas and Methylosarcina (type I) were sparse during the early period, but they increased after 60 days, and their maximum abundances were observed at 90-120 days. Of 33 218 archaeal reads, 68.3-86.6% were classified as methanogens. Methanosaeta, Methanocella, Methanosarcina and Methanobacterium were dominant methanogens, and their maximum abundances were observed at days 60-90. Only four reads were characteristic of anaerobic methanotrophs, suggesting that anaerobic methane metabolism is negligible in this rice paddy system. After completing a multivariate canonical correspondence analysis of our integrated data set, we found normalized mcrA/pmoA transcript ratios to be a promising parameter for predicting net methane fluxes emitted from rice paddy soils., (© 2014 Federation of European Microbiological Societies. Published by John Wiley & Sons Ltd. All rights reserved.)

Purpose: It is unclear whether a higher biochar (BC) application rate enhances rice (Oryza sativa L.) yield and reduces CH4 emissions. This study investigated changes in rice yield and CH4 emissions with varying BC application rates. Methods: Data on rice yield and CH4 emission from paddies amended with or without BC were collected from the literature, and the biochar effects were analyzed using the data set. Results: Across the biochar application rate from 2 to 48 t ha-1, the rice yield increased (by 10.8%) while the area-scaled (by 14.4%) and yield-scaled CH4 emission (by 22.2%) decreased. However, the correlation of BC application rates with rice yield and CH4 mitigation was not significant, implying that a higher BC application rate did not enhance rice yield and CH4 reduction. Interestingly, for a data set showing increased rice yield and decreased CH4 emission by BC, the magnitude of change in the rice yield and CH4 mitigation per unit weight of BC (1 t ha-1) decreased with an increase in the BC application rate. These results suggest that BC effects on rice yield and CH4 mitigation are not additive, probably because of the decreases in the inherent capacity of unit weight of BC to enhance rice yield and reduce CH4 emission, which might be caused by the adverse effects of toxic compounds contained in BC, losses of BC, and a higher degree of nutrient immobilization by BC. Conclusions: Annual BC application at a low rate (e.g., 2 t ha-1) rather than a luxury application may be an effective and economical strategy for long-term rice yield enhancement and CH4 mitigation using BC. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO2) and methane (CH4) are the most important two greenhouse gases in the atmosphere which were closely coupled between terrestrial ecosystems and the atmosphere. However, the relationship between CO2 and CH4 fluxes of rice paddy at different temporal scales is still unclear. Based on 6 years of eddy covariance measurements on a flooded rice paddy field in Eastern China, the relationship between gross primary productivity (GPP) and CH4 fluxes and the effects of biophysical and environmental factors on daily CH4/GPP were investigated. CH4 fluxes and GPP were tightly linked at diurnal and seasonal scales. On average, half‐hourly CH4 fluxes lagged GPP by 1.5 hr while daily GPP lagged CH4 fluxes by 19 days over the growing season. Leaf area index, water table depth (WTD), air temperature, vapor pressure deficit, and canopy conductance significantly affect daily CH4/GPP. A semi‐empirical multiplicative model was able to capture 77% and 84% seasonal variability of daily CH4/GPP for the training data set and testing data set, respectively. WTD is a crucial input affecting the performance of the model. Without WTD included, the agreement between estimated and observed daily CH4/GPP was weakened. Under the condition without WTD data available, separate calibration of the semi‐empirical multiplicative model before and after panicle initiation could improve the estimation of daily CH4/GPP to some extent. The findings in this study enhance our understanding regarding the complex interactions between CH4 fluxes and GPP in the rice paddy and are helpful for better estimation of CH4 fluxes from GPP. Plain Language Summary: This study investigated the relationship between CH4 fluxes and gross primary productivity (GPP) during the growing seasons on a rice paddy field. Results showed CH4 fluxes and GPP were tightly linked at diurnal and seasonal scales. Half‐hourly CH4 fluxes lagged GPP by 1.5 hr while daily GPP lagged CH4 fluxes by 19 days over the growing season. The long time lag of daily GPP relative to daily CH4 fluxes was mainly caused by the water management‐induced fast decline of CH4 fluxes after rice ripened. Due to the significant effects of biophysical and environmental factors on daily CH4/GPP, a semi‐empirical multiplicative model with the above factors as inputs was able to capture 77% and 84% seasonal variability of daily CH4/GPP for the training data set and testing data set, respectively. Without WTD included in the model, the agreement between estimated and observed daily CH4/GPP was weakened. Therefore, it is a very useful finding to promote our understanding of the complex interactions between CH4 fluxes and GPP in the rice paddy for better estimation of CH4 fluxes from GPP. Key Points: The positive relationship between CH4 fluxes and gross primary productivity (GPP) was observed at diurnal and seasonal scales during the rice paddy growing seasonOn average, half‐hourly CH4 fluxes lagged GPP by 1.5 hr while daily GPP lagged CH4 fluxes by 19 days over the growing seasonA semi‐empirical multiplicative model was able to capture the seasonal variability of daily CH4/GPP well [ABSTRACT FROM AUTHOR]

Crop straw retention in the rice-based rotation cropland has been widely accepted as an effective method to improve soil quality in China. Rice–wheat rotation cropland is one the most prevalent rice-based rotation patterns, where it only exploits a small proportion of the total agricultural land yet feeds the majority of the Chinese population. Previous studies indicated that the incorporation of fore-rotating crop straw can effectively facilitate soil carbon sequestration in rice paddy fields. However, the application of crop straw may increase methane (CH4) emissions from rice paddies due to the anaerobic soil condition. To mitigate CH4 emissions from rice paddies while still preserving their soil carbon sequestration ability, a field experiment was conducted in the 2012–2016 rice growing seasons to determine the optimal low-carbon crop straw retention strategy. Five treatments with different wheat straw retention strategies were employed in this study, including non-fertilization and non-straw (Control), conventional fertilization without straw incorporation (CF), conventional fertilization with wheat straw incorporation (FS), slow-release fertilizer combined with wheat straw (SFS), and conventional fertilization with wheat-straw-derived biochar (FB). The results indicated that FS, SFS, and FB treatments significantly increased soil carbon sequestration in comparison with CF treatment. However, the increment of soil carbon sequestration was offset by raw wheat straw induced excess CH4 emissions under FS and SFS treatments. In contrast, the application of wheat-straw-derived biochar significantly promoted soil carbon sequestration, but showed no significant effect on CH4 emissions. Collectively, to the farmers, who aim to achieve agricultural carbon neutrality, the application of straw-derived biochar is worthy of consideration in rice cultivation processes. [ABSTRACT FROM AUTHOR]

Crop straw retention in the rice-based rotation cropland has been widely accepted as an effective method to improve soil quality in China. Rice–wheat rotation cropland is one the most prevalent rice-based rotation patterns, where it only exploits a small proportion of the total agricultural land yet feeds the majority of the Chinese population. Previous studies indicated that the incorporation of fore-rotating crop straw can effectively facilitate soil carbon sequestration in rice paddy fields. However, the application of crop straw may increase methane (CH4) emissions from rice paddies due to the anaerobic soil condition. To mitigate CH4 emissions from rice paddies while still preserving their soil carbon sequestration ability, a field experiment was conducted in the 2012–2016 rice growing seasons to determine the optimal low-carbon crop straw retention strategy. Five treatments with different wheat straw retention strategies were employed in this study, including non-fertilization and non-straw (Control), conventional fertilization without straw incorporation (CF), conventional fertilization with wheat straw incorporation (FS), slow-release fertilizer combined with wheat straw (SFS), and conventional fertilization with wheat-straw-derived biochar (FB). The results indicated that FS, SFS, and FB treatments significantly increased soil carbon sequestration in comparison with CF treatment. However, the increment of soil carbon sequestration was offset by raw wheat straw induced excess CH4 emissions under FS and SFS treatments. In contrast, the application of wheat-straw-derived biochar significantly promoted soil carbon sequestration, but showed no significant effect on CH4 emissions. Collectively, to the farmers, who aim to achieve agricultural carbon neutrality, the application of straw-derived biochar is worthy of consideration in rice cultivation processes.

Global temperature is projected to increase, which impacts the ecological process in northern mid- and high-latitude ecosystems, but the winter temperature change in ecosystems is among the least understood. Rice paddy represents a significant contributor to global anthropogenic CH _4 emissions and has a strong climate forcing feedback; however, the legacy effects of warming winter on CH _4 emissions in the subsequent growing season remain uncertain. Here, we conducted field and incubation experiments to determine the effects of winter soil temperature changes on CH _4 emissions in the subsequent growing season. First, in the 3 year field experiment, we continuously measured CH _4 emissions from the rice cropping system. The winter soil temperature and its variation showed significant differences over the 3 years. In the warming-winter year, the rice paddy accumulated less NH _4 ^+ –N and more dissolved organic carbon (DOC) in the soil during winter, resulting in high CH _4 emissions. Second, we incubated the paddy soils without flooding at three temperatures (5 °C, 15 °C, and 25 °C) for 4 weeks to simulate warming winter, and subsequently incubated at same temperature (25 °C) under submerged conditions for 4 weeks to simulate growing season. The result was consistent with field experiment, increased soil temperature significantly increased soil DOC content and decreased NH _4 ^+ –N content in ‘winter season’. The CH _4 emissions in the subsequent ‘growing season’ increased by 190% and 468% when previous incubation temperature increased 10 °C and 20 °C. We showed strong and clear links between warming winter and CH _4 emissions in the subsequent growing season for the first time, suggesting that CH _4 related processes respond not only to warming during the growing season but also in the previous winter. Our findings indicate that nonuniform global warming causes a disproportionate increase in climate forcing feedback to emit more CH _4 .

Separate evaluation of methane (CH4) emission dynamics (e.g., oxidation, production, and transportation) at the soil‐plant‐atmosphere and soil‐water‐atmosphere interfaces has been limited in tropical rice paddies, but it is crucial for comprehending the entire CH4 cycles. We investigated CH4 oxidation, production, and transportation through plant and water pathways during the reproductive stage in a tropical Thailand rice paddy field using natural abundance carbon stable isotope ratios (δ13CH4 and δ13CO2). Mass balance equations using δ13CH4 and δ13CO2 in soil gases indicated that CH4 oxidation in the planted soil exceeded those in the interrow soil due to oxygen supply through rice roots. In addition, at 1–11 cm depth acetate fermentation was the dominant process in the planted soil, whereas in the interrow soil the dominant process was H2/CO2 reduction. The water pathway showed a significant negative correlation between CH4 flux and released δ13CH4 over 24 hr, driven by a diel change in episodic ebullition, steady ebullition, and diffusion, all due to diel changes in soil temperature and atmospheric pressure. In contrast, the plant pathway showed a significant positive relationship between CH4 flux and emitted δ13CH4 throughout one day. A comparison of the diel change in emitted δ13CH4 between the water and plant pathways showed that the rice plants transported CH4 in soil bubbles without any large isotopic fractionation. The diel change in the plant‐mediated CH4 transportation was mainly controlled by diel changes in soil bubble expansion and CH4 diffusion through plants, which were probably regulated by diel changes in soil temperature and atmospheric pressure. Plain Language Summary: Methane (CH4) emissions from paddy soil are mainly controlled by three processes: CH4 production, CH4 oxidation (consumption), and CH4 transportation from soil to plant, water, and ultimately the atmosphere. There are two emission pathways, the soil‐plant‐atmosphere and soil‐water‐atmosphere interfaces, but there has not been much detailed evaluation of the different characteristics of the three processes in the two pathways. Here we evaluated CH4 production, oxidation, and transportation at the soil‐plant‐atmosphere and soil‐water‐atmosphere interfaces during the reproductive stage of rice in a tropical Thailand paddy field. We found that in planted soil there was more CH4 production by acetate fermentation and more CH4 oxidation, due to more organic matter and oxygen supply, respectively, through plant roots. Methane was transported through water by three modes: episodic bubble ebullition, steady bubble ebullition, and diffusion. Diel variation in the rates of these three transportation modes was related to diel changes in soil temperature and atmospheric pressure. Methane in soil bubbles was also transported into the atmosphere through rice plants. Diel variation in the CH4 transportation through rice plants was related to diel change in bubble expansion, which was also mainly regulated by diel changes in soil temperature and atmospheric pressure. Key Points: Rice roots significantly enhanced both CH4 oxidation and production processes in flooded paddy soil during the reproductive stageEpisodic bubble ebullition was the dominant water pathway for CH4 transportation in daytime, whereas diffusion was dominant at nightMethane transportation via rice plants was regulated by diel variation in bubble expansion and CH4 diffusion [ABSTRACT FROM AUTHOR]

How to achieve high soil organic carbon (SOC) sequestration and low greenhouse gases emissions but without compromising yields is the challenge of rice cultivation in China. This study explored the potential to simultaneously increase SOC stocks and reduce methane (CH 4) emissions by optimizing rice cropping rotation and fertilization based on a 2-year field experiment in southeast China. We tested five optimized N management practices including directly reducing chemical N fertilizer input and replacing chemical N fertilizers with organic and controlled-release fertilizers in double-rice cropping and Chinese milk vetch-rice cropping rotation systems. Results showed higher seasonal rice yields, nitrogen use efficiency (NUE) and SOC stocks for Chinese milk vetch-rice rotation systems (CR) as compared to double-rice cropping systems (DR). Seasonal rice yields were the highest in the 2019 single-rice season (2019-SR), followed by the 2018 late (2018-LR) and early rice seasons (2018-ER). SOC stocks of CR for the treatment receiving organic N fertilizer (OF) were the largest among all rice seasons and fertilization treatments. Seasonal CH 4 emissions were 8–12% greater in the OF than in the chemical N fertilizer treatments across all rice seasons, with the greatest in 2019-SR and lowest in 2018-ER. Fertilization stimulated CH 4 emissions through enhancing mcrA gene activity, but regulated by soil properties, while rice rotation models indirectly influenced CH 4 emissions through direct effects on soil properties. However, yield-scaled CH 4 emissions remained unaffected by N fertilization in the two-year rice rotation systems. The NUE was significantly enhanced in OF than in other fertilizer treatments over all seasons. Seasonal net ecosystem C budget (NECB) was 24–41% greater in OF than in chemical N fertilizer treatments across all rice seasons, with the greatest in 2019-SR and lowest in 2018-LR. The NECB was increased by 46% in CR as compared to DR. Our results revealed that the optimized rice cropping rotation by incorporation of Chinese milk vetch with organic N fertilization is beneficial for enhancing soil C sequestration and maintaining rice yields in southeast China. [Display omitted] • Green manure rotation benefits for the enhancement of paddy soil carbon sinks. • Green manure rotation stimulates CH 4 emissions during the rice growing season. • Yield-scaled CH 4 emissions remain unaffected by N fertilization and crop rotation. • Green manure rotation can reconcile low climatic impacts with high yield of rice. [ABSTRACT FROM AUTHOR]

Abstract Methane (CH 4) oxidation and nitrogen (N 2) fixation are simultaneously activated in the roots of rice plants grown in paddy fields with low N input (LN). However, the mechanism of CH 4 oxidation-dependent N 2 fixation remains largely unknown. In the present study, a15N 2 -feeding experiment was adopted to evaluate methanotrophic N 2 fixation in LN rice roots and in a methanotroph isolated from the rice roots. The presence of CH 4 significantly enhanced 15N incorporation from 15N 2 gas in LN rice roots, indicating methanotrophic N 2 fixation. Methylosinus sp. strain 3S-1 was isolated from LN rice roots, and it grew well in N-free liquid medium under different levels of oxygen stress (2%, 10%, and 20% O 2). N 2 fixation of strain 3S-1 was directly measured using 15N in biomass; when 3S-1 cells were exposed to a gas mixture consisting of 15N 2 (35%), CH 4 (5%), and O 2 (2% or 10%) in argon (Ar) balance, the 15N concentration in the cells rapidly increased at both O 2 concentrations. The addition of difluoromethane, a potent inhibitor of methane monooxygenase, immediately stopped CH 4 oxidation and reduced the 15N enrichment rate, indicating that N 2 fixation depended on CH 4 oxidation in pure culture. These results suggest that N 2 fixation was stimulated by CH 4 oxidation in LN rice roots and that type II methanotrophs in LN rice roots, including Methylosinus , are responsible for CH 4 oxidation-dependent N 2 fixation. Highlights • CH 4 oxidation–dependent N 2 fixation occurred in low-N rice roots. • Type II methanotrophs Methylosinus sp. 3S-1 was isolated from Low-N roots. • CH 4 oxidation–dependent N 2 fixation by 3S-1 also occurred in high O 2 conditions. [ABSTRACT FROM AUTHOR]

Highlights • Mid-season drainage decreased CH 4 emission while increased N 2 O emission. • Mid-season drainage decreased the global warming potential (GWP). • Increasing drainage times did not affect the response of GWP to mid-season drainage. • Organic matter amendment did not affect the response of GWP to mid-season drainage. • GWP decreased under mid-season drainage when N fertilizer rate increases. Abstract Paddy rice cultivation is an important source of global anthropogenic methane emissions. Drainage the flooded soils can reduce methane substantially, but N 2 O emission occur concurrently, which would offset the reduction of methane emission. It remains unclear how mid-season drainage affects the global warming potential (GWP) of CH 4 and N 2 O emissions. In this study, a meta-analysis was conducted to investigate the effect of mid-season drainage on GWP and the factors that control the response of GWP to mid-season drainage. Results showed that mid-season drainage decreased CH 4 emission by 52% while increased N 2 O emission by 242%. The GWP under mid-season drainage decreased by 47% compared to continuously flooding. The yield-scaled GWP under mid-season drainage decreased by 48%. Mid-season drainage had no effect on rice grain yield. Although soil drainage times and organic matter amendment are important factors affecting CH 4 and N 2 O emissions in rice paddy field, the study showed that neither of them had effect on the response of GWP to mid-season drainage. The reduction rate of the GWP under mid-season drainage increased when N fertilization application rate increases from 50 kg ha−1 to >200 kg ha−1. This study demonstrated that CH 4 is still a dominant greenhouse gas in rice paddies under water management with mid-season drainage. Nitrogen fertilization is an important factor that regulates the response of GWP to mid-season drainage. High nitrogen fertilization rate would decrease the overall emission of CH 4 and N 2 O under mid-season drainage. However, increasing drainage times or applying organic fertilizer under mid-season does not change the overall emission rate of CH 4 and N 2 O. [ABSTRACT FROM AUTHOR]

We investigated the response of aerobic methane oxidation and the associated methanotrophs to salt-stress in a NaCl gradient ranging from 0 M (un-amended reference) to 0.6 M NaCl (seawater salinity) using a rice paddy soil as a model system. Salt-stress significantly inhibited methanotrophic activity at > 0.3 M NaCl; at 0.6 M NaCl amendment, methanotrophic activity fully ceased. MiSeq sequencing of the pmoA gene and group-specific qPCR analyses revealed that type Ia methanotroph ( Methylobacter ) appeared to be favored under salinity up to 0.3 M NaCl, increasing in numerical abundance, while the type Ib was adversely affected. This suggests niche differentiation within members of the gammaproteobacterial methanotrophs. Overall, rice paddy soil methanotrophs showed remarkable resistance to salt-stress. [ABSTRACT FROM AUTHOR]

Rice paddies are considerable sources of methane because of the highly reduced soil oxidation–reduction conditions during rice cultivation. In this ecosystem, active methane-oxidizing bacteria are important, because they consume methane while reducing the overall emissions from rice paddy soils. However, the biogeographic distribution of active methanotrophs in paddy soils across Taiwan remains poorly understood. We used DNA-based stable isotope probing (DNA-SIP) to show that phylogenetically distinct type I and type II methanotrophs dominated methane oxidation in geographically different paddy soils across Taiwan. High-throughput sequencing of soil 16S rRNA and pmoA genes under field conditions revealed that a type II methanotroph, Methylocystis , was predominant in rice paddy soils. In addition, an uncultured novel type I methanotroph cluster (Rice Paddy Clusters) was detected with the closest relatedness to Methylocaldum 16S rRNA genes in all rice field soils tested. SIP microcosm incubation, however, suggested that the type I methanotrophs Methylosarcina and Methylobacter were significantly stimulated during the consumption of high-concentration methane in five paddy soils with pH > 6.07, whereas the Methylosinus -like microorganisms of type II methanotrophs dominated aerobic methane-oxidizing communities in acidic soil with pH = 5.17. Furthermore, high-throughput sequencing of 13 C- pmoA genes indicated the presence of novel methanotrophs that are phylogenetically distantly related to the type I methanotrophs Methylosarcina in four out of five non-acidic paddy soils studied, and the high proportions in the 13 C-DNA suggested that these uncultured methanotrophs play an important role in methane oxidation. These results provide strong evidence for the environmental selection of phylogenetically distinct methanotrophs under field conditions. Moreover, community shifts in active methanotrophs likely occurred in response to environmental variations with fluctuating methane concentrations. [ABSTRACT FROM AUTHOR]

Irrigation regime and fertilizer nitrogen (N) are considered as the most effective agricultural management systems to mitigate greenhouse gas (GHG) emissions from crop fields, but few studies have involved saline–alkaline paddy soil. Gas emitted from saline–alkaline paddy fields (1-year-old and 57-year-old) was collected during rice growing seasons by the closed chamber method. Compared to continuous flooding irrigation, lower average CH4 flux (by 22.81% and 23.62%), but higher CO2 flux (by 24.84% and 32.39%) was observed from intermittent irrigation fields. No significant differences of N2O flux were detected. Application rates of N fertilizer were as follows: (1) No N (N0); (2) 60 kg ha−1 (N60); (3) 150 kg ha−1 (N150); and (4) 250 kg ha−1 (N250). The cumulative emissions of GHG and N fertilizer additions have positive correlation, and the largest emission was detected at the rate of 250 kg N ha−1 (N250). Global warming potential (GWP, CH4 + N2O + CO2) of the 57-year-old field under the N250 treatment was up to 4549 ± 296 g CO2-eq m−2, approximately 1.5-fold that of N0 (no N application). In summary, the results suggest that intermittent irrigation would be a better regime to weaken the combined GWP of CH4 and N2O, but N fertilizer contributed positively to the GWP. [ABSTRACT FROM AUTHOR]

There has been an increased interest in and wide application of biochar and slow-release fertilizer (SRF) to agricultural soils in recent years because they can reduce greenhouse gas emissions, but increase rice productivity. However, the studies considering combined effects of biochar and SRF are rare. This study examined the combined effects of biochar and SRF on the biogeochemistry, rice productivity, methane emission, and microbial abundances in rice paddy. The study sites included six different treatment combinations: urea (NU), SRF (NS), straw + urea (SU), straw + SRF (SS), biochar + urea (BU), and biochar + SRF (BS). Both the biochar and SRF reduced the methane emission, and the BS paddy soil had the lowest methane emission, while it had the highest rice yield. The biochar inhibited methanogenesis by increasing the soil aeration and oxygen availability. The SRF decreased the plant biomass, thus they may decrease plant-mediated methane transport and carbon substrate from plant debris and root exudates. Increasing in the abundance of methane-oxidizing bacteria was assumed to have critical impact on the reduction in methane emission by biochar. In conclusion, combined application of biochar and SRF highly recommended in rice cultivation, because they can minimize the methane emission but maximize rice yield. [ABSTRACT FROM AUTHOR]

The diverse habitat of the mangrove ecosystems all over the globe are under continuous threat of conversion for immediate and/or short-term economic benefits. Nonetheless, the emission of climatically relevant greenhouse gases increases with the disturbance of the mangrove sediment −this might undermine the credible reservoir of carbon within the sediment. This article attempts to estimate the environmental (carbon emission) and economic consequences of converting mangrove to cropland (especially rice paddy) based on field-scale study at three different sites (Khola, Gupti and Damra) within the Bhitarkanika mangrove for two consecutive years. The study suggests that the cumulative methane (CH 4 ) emission was significantly higher from the rice paddy (211.3 kg ha −1 ) compared to the mangrove sediment (50.8 kg ha −1 ), while the average nitrous oxide (N 2 O) emission was significantly higher from the later (2.1 kg ha −1 ). Multivariate statistical analysis suggests that the land use was the prime controlling factor for variation in CH 4 and N 2 O emission. Total carbon equivalent emission (CEE TOT ) from the rice paddy was significantly higher than mangrove during the study period. The study suggests that the economic value of the mangrove ecosystem was several folds higher than that of the rice paddy. The CEE TOT of the Bhitarkanika mangrove has increased approximately 212 Gg over last few decades due to the conversion of the mangrove area to the rice paddy. Such studies are imperative in developing effective regional climate change adaptation strategies. The study advocates urgent need to educate and aware people about the benefits of the mangrove compared to the cropland. [ABSTRACT FROM AUTHOR]

Rice paddies and rivers draining agricultural watersheds have been documented as two major sources of atmospheric methane (CH 4), while parallel flux measurements have been rarely taken on CH 4 from rice paddies against the drainage rivers in agricultural watersheds. Moreover, the drainage events during the rice growing season deliver sufficient organic and inorganic nutrients to rivers, which makes there an increasing concerned source of atmospheric CH 4. Here, we compared CH 4 emissions from rice paddies and the associated rivers draining agricultural watersheds by a parallel field experiment in southeast China. Over the whole rice–growing season (June–November), CH 4 fluxes from drainage rivers averaged 25 mg m–2 h–1, which were 82 % higher than those from rice paddies (13.7 mg m–2 h–1). Besides the dependence of riverine CH 4 fluxes on water dissolved oxygen (DO), both ecosystem CH 4 fluxes were significantly correlated with water/soil dissolved organic carbon (DOC), oxidation-reduction potential (Eh) and the microbial functional genes that drive CH 4 production or uptake [methanogens (mcrA and methanogenic archaeal 16S rRNA) and methanotrophs (pmoA)]. Our results highlighted that the CH 4 mitigation potential benefited from the drainage practice in rice paddies might have been totally offset by its induced increases in CH 4 emissions from rivers draining agricultural watersheds at the regional scale. ● Rivers draining agricultural watersheds are an important source of atmospheric CH 4. ● CH 4 fluxes from drainage rivers have been rarely related to the abundance of microbes. ● CH 4 fluxes from drainage rivers were 82 % greater than those from rice paddies. ● The increase in CH 4 emissions in rivers may offset the mitigation benefit of CH 4 in rice paddies. [ABSTRACT FROM AUTHOR]

The mitigation of methane (CH 4 ) and nitrous oxide (N 2 O) emissions from rice paddy fields is pivotal in minimizing the impact of rice production on global warming. The large majority of the world rice is cropped in continuously flooded paddies, where soil anaerobic conditions lead to the production and emission of significant amounts of CH 4 . In this work we evaluated the effectiveness of water management techniques alternative to the conventional flooding on the mitigation of CH 4 emissions from paddy soils, and verified whether any concurrent increase in N 2 O emissions can totally or partially offset their environmental benefit. Two alternative water management systems were compared to the conventional continuous flooding system (WFL): dry seeding with delayed flooding (DFL) and intermittent irrigation (DIR). Methane and N 2 O emissions were monitored at field-scale over two years including both rice cropping and fallow seasons, using non-steady-state closed chamber approach. The DFL system resulted in a 59% decrease (average of the two measured years) in total CH 4 emissions with respect to WFL, while DIR annulled CH 4 emissions. The effect of CH 4 mitigation of DFL with respect to WFL was mainly concentrated within the vegetative stage, while any significant flux from DIR was recorded throughout the growing and non-growing season. However, DIR resulted in the highest emission peaks and cumulative fluxes of N 2 O, almost totally occurred during the vegetative stage. In contrast, DFL and WFL showed N 2 O emissions that were 77 and 93% lower with respect to DIR, respectively. Total annual fluxes suggest that the adoption of alternative water management practices that involve dry seeding and subsequent delayed flooding or intermittent irrigation can contribute to significantly reduce the global warming potential of rice cropping systems by 56 and 83%, respectively with respect to continuous flooding. [ABSTRACT FROM AUTHOR]

Mid-season drainage (MSD) decreases methane emission from an irrigated rice paddy. For this reason, the 2006 IPCC Guidelines and several methane estimation models consider MSD as a static input factor. However, if rainfall occurs during the MSD period, the reduction effect on methane emission may become small. In this paper, we report the methane emission from an intermittently irrigated rice paddy with MSD for three growing seasons. Methane flux was measured using eddy covariance technique with an open-path methane analyzer. Our objective is to ascertain the seasonal and interannual variations in methane emission caused by rainfalls during the MSD period. The growing season (from June to Oct.) methane emission during the three year study period ranged from 198 to 450 kg CH 4 ha −1 , showing significant interannual variability with changes in rainfall during the MSD period. In order to implement the effect of rainfall during the MSD on methane emission (not considered in the IPCC method), we developed a simple method for a scaling factor. The estimated and the observed CH 4 emissions agreed within 40%, and additional consideration of soil pH further improved the agreement within 10%. These results suggest that the rainfall effect should be considered when estimating national or global methane emission from irrigated rice paddies. [ABSTRACT FROM AUTHOR]

Studies on earthworms in rice-based ecosystems tend to focus on some pest species, while the potential of these important soil engineers for beneficially affecting carbon storage and cycling is widely ignored. We carried out a microcosm experiment to quantify the impact of the tropical earthworm Pheretima sp. on the C turnover in paddy soils under different conditions of water saturation and N fertilization. The soil was sampled at the lowland farm of the International Rice Research Institute (Philippines). In the absence of earthworms, soil respiration showed a distinct hump-shaped maximum at intermediate levels of water saturation (4-fold higher than in hand-dry soil) and increased 1.5-fold with increasing amounts of N fertilization. Amounts of CH 4 emitted, in contrast, were small at low to moderate soil humidity and became very high under conditions of water saturation (80-fold higher than hand-dry soil). No response to nitrogen addition was observed. Earthworms suppressed both the respiration maximum at intermediate saturation levels (by a factor of 1.4) and the stimulating impact of N fertilization (1.7-fold at maximum fertilizer level). On the other hand, earthworms strongly increased CH 4 release under conditions of high water saturation (3-fold). No consistent response of the soil microflora (bacterial abundance, soil enzymes) to earthworm activity could be established. Our findings suggest that the stabilization of soil organic C via earthworm bioturbation is confined to the range of soil humidity that allows high activity of Pheretima sp. Under conditions of intensive agriculture, the stabilizing effect of the worms may even be augmented by the fact that they offset the positive effect of N fertilization on microbial respiration. Earthworms may thus play a vital role in reducing the CO 2 flush from paddy soils after the conversion to non-flooded crops such as aerobic rice or maize. Acceleration of methane emission in very humid soils nevertheless points to a certain risk that is associated with increasing earthworm abundance in production systems that are still exposed to temporary flooding during the wet season. [ABSTRACT FROM AUTHOR]

Abstract: We have studied the effects of application of different nitrification inhibitors on methane (CH4) and nitrous oxide (N2O) emissions from rice paddy and associated soil chemical and biological dynamics during wet and dry seasons of rice crop in a tropical climate of eastern India. The experiment consisted of four treatments viz. (i) Prilled urea amended control (ii) urea + Dicyandiamide (DCD), (iii) urea + Nimin and (iv) urea + Karanjin. CH4 emission was significantly higher from the DCD (372.36 kg ha−1) and Karanjin (153.07 kg ha−1) applied plots during the wet and dry season, respectively. N2O emission was significantly inhibited in the Nimin applied plots during both seasons (69% and 85% over control during wet season and dry season respectively). CH4 and N2O emissions per Mg of rice grain yield were lowest from the Nimin applied plots during both seasons. Global warming potential (GWP) of the plot treated with DCD (13.93) was significantly higher during the experimental period. CH4 production potential was significantly higher from the nitrification inhibitor applied plots compared to control. While, CH4 oxidation potential followed the order; urea + Nimin > urea + Karanjin > urea + DCD > control. Application of Nimin significantly increased the methanotrophic bacterial population in the soil during the maximum tillering to flowering stage and may be attributed to low CH4 emission from the plots. Denitrification enzyme activity (DEA) of the soil was significantly low from the Nimin and Karanjin applied plots. Results suggest that apart from being potent nitrification inhibitors, Nimin and Karanjin also have the potential to reduce the denitrification activity in the soil. This in turn, would reduce N2O emission from flooded paddy where both nitrification and denitrification processes causes N2O emission. [Copyright &y& Elsevier]

Methane ( $${\mathrm {CH}}_{4}$$ ) fluxes observed with the eddy-covariance technique using an open-path $${\mathrm {CH}}_{4}$$ analyzer and a closed-path $${\mathrm {CH}}_{4}$$ analyzer in a rice paddy field were evaluated with an emphasis on the flux correction methodology. A comparison of the fluxes obtained by the analyzers revealed that both the open-path and closed-path techniques were reliable, provided that appropriate corrections were applied. For the open-path approach, the influence of fluctuations in air density and the line shape variation in laser absorption spectroscopy (hereafter, spectroscopic effect) was significant, and the relative importance of these corrections would increase when observing small $${\mathrm {CH}}_{4}$$ fluxes. A new procedure proposed by Li-Cor Inc. enabled us to accurately adjust for these effects. The high-frequency loss of the open-path $${\mathrm {CH}}_{4}$$ analyzer was relatively large (11 % of the uncorrected covariance) at an observation height of 2.5 m above the canopy owing to its longer physical path length, and this correction should be carefully applied before correcting for the influence of fluctuations in air density and the spectroscopic effect. Uncorrected $${\mathrm {CH}}_{4}$$ fluxes observed with the closed-path analyzer were substantially underestimated (37 %) due to high-frequency loss because an undersized pump was used in the observation. Both the bandpass and transfer function approaches successfully corrected this flux loss. Careful determination of the bandpass frequency range or the transfer function and the cospectral model is required for the accurate calculation of $${\mathrm {CH}}_{4}$$ fluxes with the closed-path technique. [ABSTRACT FROM AUTHOR]

Aims: Two pot experiments in a “walk-in” growth chamber with controlled day and night temperatures were conducted to investigate the influence of elevated temperatures along with rice straw incorporation on methane (CH 4) and nitrous oxide (N 2O) emissions as well as rice yield. Methods: Three temperature regimes–29/25, 32/25, and 35/30 °C (Exp. I) and 29/22, 32/25, and 35/28 °C (Exp. II), representing daily maxima/minima were used in the study. Two amounts of rice straw (0 and 6 t ha −1) were applied with four replications in each temperature regime. CH 4 and N 2O emissions as well as soil redox potential (Eh) were monitored weekly throughout the rice-growing period. Results: Elevated temperatures increased CH 4 emission rates, with a more pronounced effect from flowering to maturity. The increase in emissions was further enhanced by incorporation of rice straw. A decrease in soil Eh to <−100 mV and CH 4 emissions was observed early in rice straw–incorporated pots while the soil without straw did not reach negative Eh levels (Exp. I) or showed a delayed decrease (Exp. II). Moreover, soil with high organic C (Exp. II) had higher CH 4 emissions. In contrast to CH 4 emissions, N 2O emissions were negligible during the rice-growing season. The global warming potential (GWP) was highest at high temperature with rice straw incorporation compared with low temperature without rice straw. On the other hand, the high temperature significantly increased spikelet sterility and reduced grain yield ( p < 0.05). Conclusions: Elevated temperature increased GWP while decreased rice yield. This suggests that global warming may result in a double negative effect: higher emissions and lower yields. [ABSTRACT FROM AUTHOR]

Methane concentrations have been monitored at the Anmyeon-do Observatory, Korea, since 1999. In recent years, the methane concentration has increased, but the sources of this increase have yet to be identified. This study was designed to identify the major source contributing to the increase by using World Data Centre for Greenhouse Gases (WDCGG) data and the Greenhouse Gases Emission Presumption (GEP) method. The data were collected at Anmyeon-do between 2003 and 2009 (except 2008), and the analyses showed that the increase in methane concentration originated mainly in rice paddies around the observation point. The annual average methane concentration at Anmyeon-do was 1894 ppb, of which 100-103 ppb (5.3-5.4%) was shown to originate mainly from rice paddies. The seasonal fluctuation in methane concentration from May to October estimated by the GEP method was compared with experimental data of previous research conducted on rice paddies. The close match obtained through this comparison shows that the GEP method is effective. The difference in methane concentration was also analyzed in terms of land use and land cover. It was shown that although rice paddies account for only 14.7% of the area surveyed, they accounted for between 69 and 90% of the total increase in methane concentration. These results confirm that rice paddies are the main source of the increase in methane concentration observed at Anmyeon-do. [ABSTRACT FROM AUTHOR]

Methane (CH4) emission from flooded rice fields was measured hourly over 24 h for rice (Oryza sativaL.) seasons in 2008 and 2009. The objectives of this study were to identify typical diel variation in CH4emission and to estimate the best time of day for optimum extrapolation of daily CH4emission. Our results showed distinct diel variation in CH4emission, which exhibited a maximum at 14:00–15:00 and a minimum at midnight. About 5.2–5.6% of total CH4emitted per day (110–160 mg CH4m−2d−1) was released at 14:00–15:00. The diel pattern of CH4emission resembled that of air temperature (Ta). The Tacoupled with solar radiation could cause a difference in partial pressure of CH4(DPPC) through the gas conduit of the plant. The best extrapolation of daily CH4emission was achieved with data observed at 10:00–11:00. We concluded that DPPC-induced CH4emission is an important mechanism causing diel variation.Present address of Seok-In Yun, Department of Bio-Environmental Chemistry, Wonkwang University, 460 Iksandae-ro, Iksan, Jeonbuk 570-749, Korea. [ABSTRACT FROM AUTHOR]

A field experiment was conducted in Bangladesh Agricultural University Farm to investigate the mitigating effects of soil amendments such as calcium carbide, calcium silicate, phosphogypsum, and biochar with urea fertilizer on global warming potentials (GWPs) of methane (CH) and nitrous oxide (NO) gases during rice cultivation under continuous and intermittent irrigations. Among the amendments phosphogypsum and silicate fertilizer, being potential source of electron acceptors, decreased maximum level of seasonal CH flux by 25-27 % and 32-38 % in continuous and intermittent irrigations, respectively. Biochar and calcium carbide amendments, acting as nitrification inhibitors, decreased NO emissions by 36-40 % and 26-30 % under continuous and intermittent irrigations, respectively. The total GWP of CH and NO gases were decreased by 7-27 % and 6-34 % with calcium carbide, phosphogypsum, and silicate fertilizer amendments under continuous and intermittent irrigations, respectively. However, biochar amendments increased overall GWP of CH and NO gases. [ABSTRACT FROM AUTHOR]

Purpose: We attempted to determine the contribution of entrapped gas bubbles to the soil methane (CH) pool and their role in CH emissions in rice paddies open to the atmosphere. Methods: We buried pots with soil and rice in four treatments comprising two atmospheric CO concentrations (ambient and ambient +200 μmol mol) and two soil temperatures (ambient and ambient +2 °C). Pots were retrieved for destructive measurements of rice growth and the gaseous CH pool in the soil at three stages of crop development: panicle formation, heading, and grain filling. Methane flux was measured before pot retrieval. Results: Bubbles that contained CH accounted for a substantial fraction of the total CH pool in the soil: 26-45 % at panicle formation and 60-68 % at the heading and grain filling stages. At panicle formation, a higher CH mixing ratio in the bubbles was accompanied by a greater volume of bubbles, but at heading and grain filling, the volume of bubbles plateaued and contained ~35 % CH. The bubble-borne CH pool was closely related to the putative rice-mediated CH emissions measured at each stage across the CO concentration and temperature treatments. However, much unexplained variation remained between the different growth stages, presumably because the CH transport capacity of rice plants also affected the emission rate. Conclusions: The gas phase needs to be considered for accurate quantification of the soil CH pool. Not only ebullition but also plant-mediated emission depends on the gaseous-CH pool and the transport capacity of the rice plants. [ABSTRACT FROM AUTHOR]

Abstract: Cover crop cultivation during the cold fallow season has been strongly recommended in mono-rice cultivation system to improve soil quality. Its addition as green manure could stimulate methane (CH4) emission in the submerged paddy soil during rice cultivation, but its effect has not been evaluated well. Leguminous Chinese milk vetch (hereafter milk vetch) and non-leguminous rye were seeded after rice harvesting in 2008 and 2009, and total above-ground biomasses were incorporated with the recommended mineral fertilizers (NPK+milk vetch, and NPK+rye, respectively) before rice transplanting in 2009 and 2010. No fertilization (control) and mineral fertilization alone (NPK) plots were installed for comparison. Methane flux which was investigated by the closed chamber method during rice cultivation was significantly increased by chemical fertilization to ca. 80–250% over the control (138–141kgCH4 ha−1), but increased more markedly by cover crop biomass addition to ca. 28–121% over the NPK treatment. Milk vetch was more effective on stimulating less CH4 emission (28–61% increase over NPK) and improving rice productivity (18–31% increase over NPK) than rye (86–121% increase of CH4 emission and −3–6% of yield increase over NPK). As a result, the total CH4 flux per grain yield was significantly increased by rye addition, but were similar between the NPK+milk vetch and NPK treatments. Conclusively, low C/N ratio cover crop like milk vetch might be more recommendable green manure to minimize CH4 emission impact and increase rice productivity than high C/N ratio cover crop like rye in paddy soil. [Copyright &y& Elsevier]

Abstract: Methane (CH4) production and emission from three rice-growing soils (Luisiana, Maahas and Pila) of Luzon Island in the Philippines were estimated by incubation and pot culture studies, respectively, to understand their interrelationship. Topsoil (0–20cm depth) and subsoil (30–50cm depth) were used for the study along with their blend (1:1) for generating a gradient in soil properties, which allowed evaluating the importance of organic carbon content in regulating CH4 production in both layers of soil. For all the three soil samples, total CH4 production showed a decreasing trend in the order Topsoil>1Topsoil:1Subsoil>Subsoil. Inoculation by non-sterilized soil suspension in sterilized soils triggered higher CH4 production. It was observed that with the addition of external organic substrates like rice straw, even subsoil produced appreciable amount of CH4. Methane emission was also studied from the same soils cultivated with a rice seedling (IR-72) grown in pots. Temporal pattern of CH4 emission deviated from temporal pattern of its production, i.e. emission patterns either showed a certain time lag (Luisiana and Pila soil) or no significant correlation to the CH4 production (Maahas soil). Methane production and emission rates recorded in this study yielded significant relationship. [Copyright &y& Elsevier]

Abstract: Greenhouse gas fluxes from rice paddies under nitrogen fertilization merit serious attention because nitrogen fertilizer is increasingly used for the intensification of rice cultivation. A 3-year field study was conducted to measure methane (CH4) and nitrous oxide (N2O) fluxes simultaneously in a sandy loam paddy field under three nitrogen application rates (0, 150 and 250kgNha−1) in the Yangtze River Delta from 2005 through 2007. The rice paddies were under a typical Chinese water regime, characterized by intermittent irrigation with midseason drainage. The results revealed a trade-off between CH4 and N2O emissions as influenced by the application of urea-based fertilizers, i.e., the nitrogen fertilization reduced CH4 emissions from rice paddies but increased N2O emissions. The seasonal CH4 emissions averaged 155.9kgCha−1 in the absence of nitrogen amendment. Compared to no nitrogen addition, the seasonal CH4 emissions were decreased by 27% and 53% in the fertilized plots at rates of 150 and 250kgNha−1, respectively. It was most likely that the sandy loam texture combined with the addition of urea-based fertilizers stimulated growth and activity of methane oxidizers. In contrast, nitrogen addition increased N2O emissions 2.5 times for an application rate of 150kgNha−1 and 6.0 times at 250kgNha−1, compared to no nitrogen addition (0.38kgNha−1). The direct emission factor of fertilizer N for N2O was estimated to be 0.39–1.22% for rice fields, with a mean value of 0.77%. The overall emissions of CH4 and N2O, expressed as carbon dioxide equivalents were affected by the nitrogen addition rate, with the minimum emissions occurring at 250kgNha−1. This result indicated that the commonly applied nitrogen rate in this region might be an effective option for mitigating the combined impacts of rice production. Our results also demonstrate the presence of large interannual variations in the CH4 and N2O fluxes. It is probable that these variations resulted from differences in the amount and distribution of precipitation. [Copyright &y& Elsevier]

A complete accounting of net greenhouse gas balance (NGHGB) and greenhouse gas intensity (GHGI) affected by Fe(III) fertilizer application was examined in typical annual paddy rice-winter wheat rotation cropping systems in southeast China. Annual fluxes of soil carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) were measured using static chamber method, and the net ecosystem exchange of CO2 (NEE) was determined by the difference between soil CO2 emissions (R H) and net primary production (NPP). Fe(III) fertilizer application significantly decreased R H without adverse effects on NPP of rice and winter wheat. Fe(III) fertilizer application decreased seasonal CH4 by 27–44%, but increased annual N2O by 65–100%. Overall, Fe(III) fertilizer application decreased the annual NGHGB and GHGI by 35–47% and 30–36%, respectively. High grain yield and low greenhouse gas intensity can be reconciled by Fe(III) fertilizer applied at the local recommendation rate in rice-based cropping systems. [Copyright &y& Elsevier]

Abstract: Response of methanogenesis and methanotrophy to elevated carbon dioxide (CO2) could be affected by changes in soil moisture content and temperature. In soil microcosms contained in glass bottles and incubated under laboratory conditions, we assessed the impact of elevated CO2 and temperature interactions on methanogenesis and methanotrophy in alluvial and laterite paddy soils of tropical origin. Soil samples were incubated at ambient (370 μmol mol−1) and elevated (600 μmol mol−1) CO2 concentrations at 25, 35 and 45 °C under non-flooded and flooded conditions for 60 d. Under flooded condition, elevated CO2 significantly increased methane (CH4) production while under non-flooded condition, only marginal increase in CH4 production was observed in both the soils studied and the increase was significantly enhanced by further rise in temperature. Increased methanogenesis as a result of elevated CO2 and temperature interaction was mostly attributed to decreased soil redox potential, increased readily mineralizable carbon, and also noticeable stimulation of methanogenic bacterial population. In contrast to CH4 production, CH4 oxidation was consistently low under elevated CO2 concentration and the decrease was significant with rise in temperature. The low affinity and high affinity CH4 oxidation were faster under non-flooded condition as compared to flooded condition. Admittedly, decreased low and high affinity CH4 oxidation as a result of elevated CO2 and temperature interaction was related to unfavorable lower redox status of soil and the inhibition of CH4-oxidizing bacterial population. [Copyright &y& Elsevier]

Abstract: Elevated atmospheric CO2 concentration ([CO2]) has the potential to increase CH4 emissions from rice fields. However, there is still inconclusive evidence due to limited data on whether elevated temperature (T a) and [CO2] combined can modify CH4 emission. To study this issue further, we conducted a temperature gradient field chamber (TGC) experiment in Gwangju, Korea (126°53′E, 35°10′N, alt. 33m). Rice (Oryza sativa L.) was grown at two [CO2] (396 vs 673ppmV) and two T a [24.8 (≈ambient) vs 26.5°C] regimes in six independent field TGCs (three each for ambient and elevated [CO2]). CH4 fluxes were measured hourly using an automated gas sampling and analyzing system during the entire season. Elevated [CO2] significantly increased total CH4 emission by 17.4% (14.37g/12.24g CH4 m−2) at maturity, whereas elevated T a only had a minor or insignificant effect (ca. +8.0%; 13.22g/12.24g CH4 m−2). However, the elevated T a effect was significant when combined with elevated [CO2], resulting in an additive effect on CH4 emission (+29.3%; 15.83g/12.24g CH4 m−2). This suggests that ongoing rising atmospheric [CO2] and T a may have a positive feedback on projected global warming. Nevertheless, the positive effects of elevated [CO2] on CH4 emission were greatly reduced with plant development, displaying an increase of 37.5% (or 53.2% in combination with elevated T a), 23.4% (40.1%) and 17.4% (29.3%) at panicle initiation, full heading and grain maturity, respectively. We conclude that such seasonal dynamics of CH4 emission were attributed to the dwindling response of plant growth, including tiller number, above- and below-ground biomass, to elevated [CO2]. These are assumed to result in the reducing potential of C substrate availability for methanogens, as well as CH4 transport capacity. [Copyright &y& Elsevier]

The impact of agricultural management on global warming potential (GWP) and greenhouse gas intensity (GHGI) is not well documented. A long-term fertilizer experiment in Chinese double rice-cropping systems initiated in 1990 was used in this study to gain an insight into a complete greenhouse gas accounting of GWP and GHGI. The six fertilizer treatments included inorganic fertilizer [nitrogen and phosphorus fertilizer (NP), nitrogen and potassium fertilizer (NK), and balanced inorganic fertilizer (NPK)], combined inorganic/organic fertilizers at full and reduced rate (FOM and ROM), and no fertilizer application as a control. Methane (CH) and nitrous oxide (NO) fluxes were measured using static chamber method from November 2006 through October 2009, and the net ecosystem carbon balance was estimated by the changes in topsoil (0-20 cm) organic carbon (SOC) density over the 10-year period 1999-2009. Long-term fertilizer application significantly increased grain yields, except for no difference between the NK and control plots. Annual topsoil SOC sequestration rate was estimated to be 0.96 t C ha yr for the control and 1.01-1.43 t C ha yr for the fertilizer plots. Long-term inorganic fertilizer application tended to increase CH emissions during the flooded rice season and significantly increased NO emissions from drained soils during the nonrice season. Annual mean CH emissions ranged from 621 kg CH ha for the control to 1175 kg CH ha for the FOM plots, 63-83% of which derived from the late-rice season. Annual NO emission averaged 1.15-4.11 kg NO-N ha in the double rice-cropping systems. Compared with the control, inorganic fertilizer application slightly increased the net annual GWPs, while they were remarkably increased by combined inorganic/organic fertilizer application. The GHGI was lowest for the NP and NPK plots and highest for the FOM and ROM plots. The results of this study suggest that agricultural economic viability and GHGs mitigation can be simultaneously achieved by balanced fertilizer application. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this study was to evaluate how former upland cultivation history affects CH4 emission from rice paddies. We measured CH4 flux, methanogen population and in situ Fe(III) reduction in the rice paddies following three different lengths of time since upland crop (Soybean) cultivation. Results showed that CH4 emissions from long-term rice paddy (19 year''s continuous cultivation) were significantly higher than recently converted ones. Temporal dynamics of methanogens on rice roots also varied among the plots, and showed a good correlation with CH4 emission rates. Cumulative Fe(III) reduction acted as the dominant electron acceptor in all plots, accounting for 68–94% of the total electron consumption. Fe(II) concentration was highest in the 19-year plot and lowest in the 1-year plots, indicating lower electron availability in recently converted paddies necessary for Fe reduction and CH4 production. Anoxic laboratory soil incubation also suggested poor availability of electron donors in the recent paddies. Collectively, our results demonstrate that the conversion of upland to paddy rice cultivation significantly affected CH4 emission through changing availability of electron donors, redox status of soil Fe and activity of methanogens, which ultimately caused low CH4 emissions. [Copyright &y& Elsevier]

Abstract: A field trial was performed to investigate the effect of biochar at rates of 0, 10 and 40tha−1 on rice yield and CH4 and N2O emissions with or without N fertilization in a rice paddy from Tai Lake plain, China. The paddy was cultivated with rice (Oryza sativa L., cv. Wuyunjing 7) under a conventional water regime. Soil emissions of CH4 and N2O were monitored with a closed chamber method throughout the whole rice growing season (WRGS) at 10 day intervals. Biochar amendments of 10tha−1 and 40tha−1 increased rice yields by 12% and 14% in unfertilized soils, and by 8.8% and 12.1% in soils with N fertilization, respectively. Total soil CH4-C emissions were increased by 34% and 41% in soils amended with biochar at 40tha−1 compared to the treatments without biochar and with or without N fertilization, respectively. However, total N2O emissions were sharply decreased by 40–51% and by 21–28%, respectively in biochar amended soils with or without N fertilization. The emission factor (EF) was reduced from 0.0042kgN2O-Nkg−1 N fertilized with no biochar to 0.0013kgN2O-Nkg−1 N fertilized with biochar at 40tha−1. The results show that biochar significantly increased rice yields and decreased N2O emission, but increased total CH4 emissions. Summary calculations based on this experiment data set provide a basis for estimating the potential reductions in GHG emissions that may be achieved by incorporating biochar into rice paddy soils in south-eastern China. [ABSTRACT FROM AUTHOR]

Based on the long-term datasets of soil organic matter content and the observation data of rice paddies' methane (CH4) emission collected from the agricultural experiment stations across the country, the rice paddies in China were divided into single cropping and double cropping regions. The soil carbon sequestration potential of straw return in three types of rice paddies in the two regions, i.e., single cropping rice paddies, upland/paddy alternated rice paddies, and double cropping rice paddies, was evaluated, based on the datasets of soil organic matter content; and the total CH4 emission from rice paddies without straw return was estimated, with reference to the experimental data of paddies CH4 emission and by the method of mean emission coefficient. The total CH4 emission from our paddies after straw return and the global warming potential of the increased CH4 emission were also estimated by using the related methods and parameters given by IPCC. It was estimated that the full popularization of straw return to China's rice paddies would sequester 10.48 Tg ·a-1 of C, and the contribution to the global warming mitigation was 38.43 Tg CO2-eqv·a-1. In the meanwhile, the CH4 emission from our rice paddies would be increased from 5.796 Tg·a-1 to 9.114 Tg·a-1, and the increased 3.318 Tg·a-1 of CH4 emission would lead to a global warming potential of 82.95 Tg CO2-eqv·a-1, which was 2.158 times of the mitigation from carbon sequestration in rice paddies. Therefore, the increased CH4 emission due to straw return should be regarded as an important greenhouse gas leakage, since it could greatly offset the mitigation benefits of soil carbon sequestration in China's rice paddies. [ABSTRACT FROM AUTHOR]

A 13C pulse-labeling experiment was conducted under ambient CO2 and free-air CO2 enrichment (FACE) conditions to understand how elevated CO2 affects the CH4 in soil bubbles and CH4 emissions from rice paddies. Two hills of rice plants enclosed by frames in ambient CO2 and FACE plots were labeled with 13CO2 at the panicle formation stage, and plants and soil bubbles were collected 2 and 51 days after the pulse labeling. CH4 and 13CH4 emissions were also measured by the closed-chamber method between the 2 bubble samplings. Conversion of photosynthate carbon to CH4 entrapped in soil bubbles and CH4 emission to the atmosphere occurred quickly, suggesting that recent photosynthates can be an important source of carbon for methanogenesis. About 1% of the 13C-labeled C taken up through photosynthesis was emitted as 13CH4 during the 7 weeks of measurement, most during the first 3 weeks after labeling. Total CH4 and 13CH4 emissions did not differ between the ambient CO2 and FACE treatments, because the entrapped CH4 in the soil and the root biomass were not affected by elevated CO2 levels in this study. [ABSTRACT FROM AUTHOR]