稻鸭共作对不同栽培环境稻季CH4和N2O排放的影响

稻田是大气温室气体甲烷(CH₄)和氧化亚氮(N₂O)的重要排放源, 稻田温室气体减排一直是生态农业研究的热点。目前, 采用水稻品种选择利用、水分控制管理、肥料运筹管理、耕作制度调整以及种养结合模式等方法来减少稻田温室气体排放有较好实践效应, 但不同稻田栽培环境(露地、网室)基础上的稻鸭共作对麦秸全量还田的稻田温室气体排放特征及相关土壤理化特性关联性的影响尚为少见。本研究采用裂区设计, 在两种栽培环境条件下, 以无鸭子放养的常规稻作和麦秸不还田为对照, 在等养分条件下分析麦秸全量还田与稻鸭共作模式对稻田土壤氧化还原电位、CH₄排放量、产CH₄潜力及CH₄氧化能力、N₂O排放量及N₂O排放高峰期土壤反硝化酶活性、全球增温潜势、水稻产量的影响, 为稻田可持续生产和温室气体减排提供参考。结果表明, 麦秆还田增加了稻田产CH₄潜力、提高了CH₄排放量, 降低了稻田土壤反硝化酶活性、土壤氧化还原电位和N₂O排放量, 整体上导致全球增温潜势上升96.89%~123.02%; 稻鸭共作模式, 由于鸭子的不间断活动提高了稻田土壤氧化还原电位, 降低了稻田产CH₄潜力, 增强了稻田CH₄氧化能力, 从而降低稻田CH₄排放量, N₂O排放量虽有提高, 整体上稻鸭共作模式的全球增温潜势较无鸭常规稻田下降8.72%~14.18%; 网室栽培模式显著提高了稻田土壤氧化还原电位, 降低稻田产CH₄潜力、CH₄氧化能力和土壤反硝化酶活性, 减少了稻田CH₄和N₂O排放量, 全球增温潜势降低6.35%~13.14%。本试验条件下, 稻田土壤的CH₄氧化能力是产CH₄潜力的2.21~3.81倍; 相同环境条件下, 稻鸭共作和麦秸还田均能增加水稻实际产量, 网室栽培的所有处理较相应的露地栽培减少了水稻实际产量1.19%~5.48%。本试验表明, 稻鸭共作和网室栽培可减缓全球增温潜势, 稻鸭共作和麦秸还田能够增加水稻实际产量。     

Incorporation of rice straw mitigates CH4 and N2O emissions in water saving paddy fields of Central Vietnam

This study evaluated the effects of rice straw and water regimes on CH₄ and N₂O emissions from paddy fields for two rice growing seasons (summer 2014 and spring 2015). Water regimes included alternating wet–dry irrigation (AWD) maintained at three levels (–5 cm, – 10 cm and –15 cm) in comparison to continuous flooding irrigation (CF). Rice straw (5 t ha^–1) was incorporated into the top soil (0 – 15 cm), distributed and burned in situ. Results showed that using burned in situ rice straw was found to reduce seasonal cumulative CH₄ emission (24–34% in summer; 18–28% in spring), N₂O emission (21–32% in summer; 22–29% in spring) and lower rice yield (8–9%) than rice straw incorporation into top soil. AWD methods reduced the amount of CH₄ production (22.6–41.5%) and increased N₂O emission (25–26%) without any decrease in rice yield. Rice straw incorporation into the top soil with AWD had higher water productivity (23–37%) than rice straw when burned in situ with CF. The results conclude that AWD and rice straw management can be employed as mitigation strategy for CH₄ and N₂O emissions from paddy fields in Central Vietnam.     

Dynamics of the methanogenic archaeal community in anoxic rice soil upon addition of straw

Addition of rice straw, which is a common practice in rice agriculture, generally results in enhanced production and emission of the greenhouse gas methane (CH₄). However, it is unclear whether straw addition affects only the activity or also the composition of the methanogenic microbial community. It is also unclear to what extent methanogenic archaea would be able to proliferate in the soil. Anoxic slurries of Italian rice‐field soil produced CH₄ after a lag, during which ferric iron and sulfate were reduced. Addition of rice straw slightly decreased this lag and greatly enhanced the subsequent production of CH₄. At the same time, addition of rice straw enhanced the intermediate production of H₂ and acetate that served as the methanogenic substrates. Compared with the unamended control, the addition of rice straw resulted in an increased concentration of phospholipid fatty acids in the soil. Quantitative ‘real‐time’ PCR targeting the 16S rRNA gene also showed increased copy numbers of both Bacteria and Archaea in the straw‐amended soil at the end of the experiment. The composition of the archaeal community was followed over time by terminal restriction length polymorphism (T‐RFLP) analysis of the archaeal 16S rRNA genes extracted from straw‐amended soil and the control. Rice Cluster‐I (RC‐I) methanogens and Methanosarcinaceae were the most abundant methanogenic populations, followed by Methanobacteriales, Methanomicrobiales and Methanosaetaceae. Addition of rice straw resulted in a relative increase of Methanosarcinaceae and Methanobacteriales and a relative decrease of RC‐I methanogens and Methanomicrobiales. Our results revealed a dynamic methanogenic community in anoxic rice‐field soil and showed that addition of organic matter selectively enhanced the growth of particular methanogenic populations, which were apparently better adapted to the presence of straw than the others. The extent of archaeal growth was consistent with that expected theoretically from the ambient Gibbs free energies of hydrogenotrophic and acetoclastic methanogenesis.     

Responses of methane emissions and rice yield to applications of biochar and straw in a paddy field

Purpose

Directly returning straw back to the paddy field would significantly accelerate methane (CH₄) emission, although it may conserve and sustain soil productivity. The application of biochar (biomass-derived charcoal) in soil has been proposed as a sustainable technology to reduce methane (CH₄) emission and increase crop yield. We compared the effects of either biochar or rice straw addition with a paddy field on CH₄ emission and rice yield.

Materials and methods

A 2-year field experiment was conducted to investigate a single application of rice straw biochar (SC) and bamboo biochar (BC) (at 22.5 t ha^?1) in paddy soil on CH₄ emission and rice yield as compared with the successive application (6 t ha^?1) of rice straw (RS). Soil chemical properties and methanogenic and CH₄ oxidation activities in response to the amendment of biochar and rice straw were monitored to explain possible mechanism.

Results and discussion

SC was more efficient in reducing CH₄ emission from paddy field than BC. Incorporating SC into paddy field could decrease CH₄ emission during the rice growing cycle by 47.30 %–86.43 % compared with direct return of RS. This was well supported by the significant decrease of methanogenic activity in paddy field with SC. In comparison to a non-significant increase with BC or RS application, rice yield was significantly raised with SC amendment by 13.5 % in 2010 and 6.1 % in 2011. An enhancement of available K and P and an improvement in soil properties with SC amendment might be the main contributors to the increased crop yield.

Conclusions

These results indicated that conversion of RS into biochar instead of directly returning it to the paddy field would be a promising method to reduce CH₄ emission and increase rice yield.     

Mechanisms of biochar decreasing methane emission from Chinese paddy soils

Paddy fields are one of the largest anthropogenic sources of global CH₄ emission. A decrease in paddy CH₄ emission can contribute significantly towards the control of global warming. Recent studies have demonstrated that the application of biochar in paddy soils has such a capability, but its underlying mechanism has yet to be elucidated. In this investigation, we studied CH₄ emission, methanogenic archaeal, as well as methanotrophic proteobacterial communities, from microcosms derived from two paddy soils, Inceptisol and Ultisol. Both soils were amended with biochar at different pyrolysis temperatures (300 °C, 400 °C and 500 °C) at field condition. The soil CH₄ flux was monitored across whole rice season in 2010; the functional guilds communities were analyzed by PCR–DGGE and real-time quantitative PCR (qPCR). It is found that paddy CH₄ emissions significantly decreased under biochar amendments, which, interestingly, didn't result from the inhibition of methanogenic archaeal growth. qPCR further revealed that biochar amendments (1) increased methanotrophic proteobacterial abundances significantly, and (2) decreased the ratios of methanogenic to methanotrophic abundances greatly. These results shed insight on the underlying mechanism of how biochar decreases paddy CH₄ emission. This knowledge can be applied to develop a more effective greenhouse gas mitigation process for paddy fields.     

Methanogenesis during rice growth as related to the water regime between crop seasons

In a long-term pot experiment with paddy rice, the effect of a wet or dry fallow on methanogenesis was studied during the ninth season. At times of CH₄ measurement, the oxidation of CH₄ was suppressed by C₂H₂. Methanogenesis started earlier in continuously flooded soil and was higher during the entire rice-growing period than in soil kept dry during the fallow and rewetted again before transplanting the rice seedlings. Increased CH₄ emission from the wet fallow treatment was, in contrast to the dry fallow treatment, associated with constantly low redox potentials. The experiment shows that the water regime during the fallow period is important to methanogenesis during the growth of rice plants.     

Methane emissions from paddy fields in Bali Island,Indonesia

Abstract

Methane emission rates from plots with and without fertilizer and rice straw application, and growth of two rice varieties (an improved variety, IR74 or IR64, and a local variety, Krueng Aceh) in two Indonesian paddy fields (Inceptisol and Alfisol soils of volcanic ash origin) were measured every week throughout the growth period in the first and the second cropping seasons, 1994. The CH₄ emission rates from the fields were similar between the two varieties. The effect of chemical fertilizer on the increase of the emissions was observed only in the Tabanan paddy field for the plots treated with rice straw. Application of rice straw increased the CH₄ emission rates. The mean rates of CH₄ emission were 1.37-2.13 mg CH₄?C m^?2 h^?1 for the plots without rice straw and 2.14–3.62 mg CH₄?C m^?2 h^?1 for the plots with rice straw application in the Alfisol plots, and 2.32–3.32 mg CH₄ -C m-2 h-1 for the plots without rice straw and 4.18–6.35 mg CH₄?C m^?2 h^?1 for the plots with rice straw application in the Inceptisol plots, respectively. Total amounts of CH₄ emitted during the growth period were 3.9–6.8 and 2.6–3.3 g CH₄?C m^?2 for the Alfisol plots and 6.9–10.7 and 4.2–5.8 g CH₄?C m^?2 for the Inceptisol plots with and without rice straw application, respectively. These findings suggested that CH₄ emission from tropical paddy fields with soils of volcanic ash origin is low.     

淹水土壤有机酸积累与秸秆碳氮比及氮供应的关系

有机酸积累和毒害是稻田秸杆还田中受到广泛关注的问题。本文以水稻与小麦秸杆为材料，采用淹水培养研究了甲酸、乙酸、丙酸及丁酸在士壤中的积累及其与秸秆碳氮比、氮肥添加量的关系。结果表明，在不施用氮肥的情况下。随秸秆用量的增加，秸秆处理的有机酸积累均显著增多。与稻秸处理相比，麦秸处理的有机酸（尤其足丙酸）积累量显著较高，土壤溶液中NH4^＋浓度显著较低。加入尿素明显减少有机酸积累，促进CH4排放，但对CO2的排放无显著影响；氮素的影响在麦秸处理中表现的尤为明显。上述结果说明麦秸的高碳氮比增加了无机氮的生物固定，抑制有机酸向CH4转化，从而导致麦秸处理有机酸积累量高于稻秸处理。施用氮肥是减少麦秸还田后有机酸积累的有效措施之一，但此措施将可能促进CH4的排放。     

Effect of the long-term application of organic matter on soil carbon accumulation and GHG emissions from a rice paddy field in a cool-temperate region,Japan-II. Effect of different compost applications-

ABSTRACT

The influence of long-term application of different types of compost on rice grain yield, CH₄ and N₂O emissions, and soil carbon storage (0 ? 30 cm) in rice paddy fields was clarified. Two sets of paddy fields applied with rice straw compost or livestock manure compost mainly derived from cattle were used in this study. Each set comprised long-term application (LT) and corresponding control (CT) plots. The application rates for rice straw compost (42 years) and livestock manure compost (41 years in total with different application rates) were 20 Mg fresh weight ha^–1. Soil carbon storage increased by 33% and 37% with long-term application of rice straw compost and livestock manure compost, respectively. The soil carbon sequestration rate by the organic matter application was 23% higher with the livestock manure compost than with the rice straw compost. The rice grain yield in the LT plot was significantly higher than that in the corresponding CT plot with both types of compost. Although the difference was not significant in the rice straw compost, cumulative CH₄ emissions increased with long-term application of both composts. Increase rate of CH₄ emission with long-term application was higher in the livestock manure compost (99%) than that in the rice straw compost (26%). In both composts, the long-term application did not increase N₂O emission significantly. As with the rice straw compost, the increase in CH₄ emission with the long-term application of livestock manure compost exceeded the soil carbon sequestration rate, and the change in the net greenhouse gas (GHG) balance calculated by the difference between them was positive, indicating a net increase in the GHG emissions. The increase in CH₄ and net GHG emissions owing to the long-term application of the livestock manure compost could be higher than that of the rice straw compost owing to the amount of applied carbon, the quality of compost and the soil carbon accumulation. The possibility that carbon sequestration in the subsoil differs depending on the type of composts suggests the importance of including subsoil in the evaluation of soil carbon sequestration by long-term application of organic matter.     

Effects of rice‐straw and phosphorus application on production and emission of methane from tropical rice soil

Rice‐straw amendment increased methane production by 3‐fold over that of unamended control. Application of P as single superphosphate at 100 μg (g soil)^–1 inhibited methane (CH₄) production distinctly in flooded alluvial rice soil, in the absence more than in the presence of rice straw. CH₄ emission from rice plants (cv. IR72) from alluvial soil treated with single superphosphate as basal application, in the presence and absence of rice straw, and held under non‐flooded and flooded conditions showed distinct variations. CH₄ emission from non‐flooded soil amended with rice straw was high and almost similar to that of flooded soil without rice‐straw amendment. The cumulative CH₄ efflux was highest (1041 mg pot^–1) in rice‐straw‐amended flooded soil. Appreciable methanogenic reactions in rice‐straw‐amended soils were evident under both flooded and non‐flooded conditions. Rice‐straw application substantially altered the balance between total aerobic and anaerobic microorganisms even in non‐flooded soil. The mitigating effects of single‐superphosphate application or low‐moisture regime on CH₄ production and emission were almost nullified due to enhanced activities of methanogenic archaea in the presence of rice straw.     

Diversity of methanogenic archaeal communities in Japanese paddy field ecosystem, estimated by denaturing gradient gel electrophoresis

Diversity of methanogenic archaeal communities in Japanese paddy field ecosystem was evaluated by the denaturing gradient gel electrophoresis (DGGE) after PCR amplification of the 16S rRNA genes (16S rDNAs), sequencing analysis and data evaluation by principal component analysis. Data were obtained from samples collected from the plowed soil layer, rice roots, rice straws incorporated in soil, plant residues (mixture of weeds, rice litters, rice roots, and rice stubbles) in soil, and composing rice straw. The number of bands of DGGE profiles ranged from 12 to 26 with the highest numbers in rice roots and rice straws incorporated in soil. However, the diversity indices based on both the numbers and intensity of bands indicated that the community of the plowed soil layer was the most diverse, even, and stable. Sequencing of the main DGGE bands showed the presence of Methanomicrobiales, Methanosarcinales, Methanobacteriaceae, and Methanocellales. The plowed soil layer included all phylogenetic groups of the methanogenic archaea of the other studied habitats, with prevalence of the members of Methanomicrobiales and Methanocellales. The phylogenetic diversity was compared with that of paddy soils collected in Italy, China, and the Philippines and that of 12 anaerobic environments (fen, waste, coast, permafrost, natural gas field, bovine rumen, riparian soil, termite, ciliate endosymboints, lake sediment, landfill, and seep rumen). The phylogenetic diversity was more similar among paddy soils than with the other anaerobic environments. Probably, the methanogenic archaeal communities of the paddy field soils were characterized by indigenous members and some of the members of the community of the plowed soil layer colonized rice roots, rice straws, and plant residues.     

Decreasing of methanogenic activity in paddy fields via lowering ponding water temperature: A modeling investigation

Methane (CH₄) is a potent greenhouse gas and the huge CH₄ fluxes emitted from paddy fields can prejudice the eco-compatibility of rice cultivation. CH₄ production in submerged rice crops is known to be highly influenced by water temperature. Hence, lowering ponding water temperature (LPWT) could be an option to mitigate CH₄ emissions from paddy environments when it is possible either to irrigate with slightly colder water or to increase ponding water depth. However, paddy soil is a complex environment in which many processes are simultaneously influenced by temperature, leading to a difficult prediction of LPWT effects. For this reason, LPWT efficiency is here theoretically investigated with a one-dimensional process-based model that simulates the vertical and temporal dynamics of water temperature in soil and the fate of chemical compounds that influence CH₄ emissions. The model is validated with literature measured data of CH₄ emissions from a paddy field under time-variable temperature regime. Based on modeling results, LPWT appears promising since the simulated reduction of CH₄ emissions reaches about −12% and −49% for an LPWT equal to −5 °C during the ripening stage only (last 30 days of growing season, when rice is less sensitive to temperature variations) and −2 °C over the whole growing season, respectively. LPWT affects CH₄ emissions either directly (decreasing methanogenic activity), indirectly (decreasing activity of bacteria using alternative electron acceptors), or both. The encouraging results provide the theoretical ground for further laboratory and field studies aimed to investigate the LPWT feasibility in paddy environments.     

Effect of the long-term application of organic matter on soil carbon accumulation and GHG emissions from a rice paddy field in a cool-temperate region,Japan. -I. Comparison of rice straw and rice straw compost -

ABSTRACT

The influence of the long-term combination of rice straw removal and rice straw compost application on methane (CH₄) and nitrous oxide (N₂O) emissions and soil carbon accumulation in rice paddy fields was clarified. In each of the initial and continuous application fields (3 and 39?51 years, respectively), three plots with different applications of organic matter were established, namely, rice straw application (RS), rice straw compost application (SC) and no application (NA) plots, and soil carbon storage (0?15 cm), rice grain yield and CH₄ and N₂O fluxes were measured for three years. The soil carbon sequestration rate by the organic matter application was higher in the SC plot than in the RS plot for both the initial and continuous application fields, and it was lower in the continuous application field than in the initial application field. The rice grain yield in the SC plot was significantly higher than those in the other plots in both the initial and continuous application fields. Cumulative CH₄ emissions followed the order of the NA plot < the SC plot < the RS plot for both the initial and continuous application fields. The effect of the organic matter application on the N₂O emissions was not clear. In both the initial and continuous application fields, the increase in CH₄ emission by the rice straw application exceeded the soil carbon sequestration rate, and the change in the net greenhouse gas (GHG) balance calculated by the difference between them was a positive, indicating a net increase in the GHG emissions. However, the change in the GHG balance by the rice straw compost application showed negative (mitigating GHG emissions) for the initial application field, whereas it showed positive for the continuous application field. Although the mitigation effect on the GHG emissions by the combination of the rice straw removal and rice straw compost application was reduced by 21% after 39 years long-term application, it is suggested that the combination treatment is a sustainable management that can mitigate GHG emissions and improve crop productivity.     

Rice straw incorporation in winter with fertilizer-N application improves soil fertility and reduces global warming potential from a double rice paddy field

Proper rice straw management in paddy fields is necessary in order to sustain soil productivity and reduce greenhouse gas emissions. A field experiment was carried out from 2008 to 2011 in subtropical China: (1) to monitor rice yield, soil available nutrients, CH₄, and N₂O emissions and (2) to evaluate the effects of timing of rice straw incorporation and joint N application rate in a double rice cropping system. The total amount of rice straw from one cropping season was incorporated in winter (W_S) or in spring (S_S) and mineral N was jointly applied with rice straw incorporation at rates of 0, 30, and 60 % of the basal fertilization rate (N_0B, N_30B, and N_60B) for the first rice crop. Soil water was naturally drained during the period of winter fallow (P_WF) and controlled under intermittent irrigation during the period of first rice growth (P_FR). Compared with S_S, W_S significantly (P?<?0.05) increased the first rice yield only in the flooding year (2010), and increased the soil available K concentration after P_WF and P_FR in 2008–2009 and the hydrolysable N concentration after P_WF in 2010–2011. Meanwhile, W_S significantly decreased the total CH₄ emission by about 12 % in 2009–2010 and 2010–2011, but increased the total N₂O emission by 15–43 % particularly during P_WF in all 3 years, resulting in a lower GWP in W_S in the flooding year and no differences in the nonflooding years. Compared with N_0B, joint N application (N_60B and N_30B) increased the soil hydrolysable N after P_WF in all 3 years. Meanwhile, it decreased the total CH₄ emissions by 21 % and increased the N₂O emissions during P_WF by 75–150 % in the nonflooding years, but the net GWP was lower in N_60B than in N_0B. The results suggested that the rice straw incorporation with joint N application in winter is more sustainable compared with the local practices such as rice straw incorporation in spring or open-field burning.     

Activity and composition of the methanogenic archaeal community in soil vegetated with wild versus cultivated rice

Wild rice (Oryza rufipogon) is a problematic weed in fields of cultivated rice (Oryza sativa). We hypothesized that the composition and/or the activity of the methanogenic microbial communities might be different in soil grown with cultivated versus wild rice. We used samples from Hainan, China, where wild rice grew on a field adjacent to cultivated rice. The composition of the methanogenic archaeal community was analyzed in samples of rice soil by targeting the 16S rRNA gene. Analysis of the terminal restriction fragment length polymorphism (T-RFLP) showed similar patterns in soil from wild versus cultivated rice. Sequences of archaeal 16S rRNA genes also showed similar composition in soil from wild versus cultivated rice, revealing the presence of Methanosarcinaceae, Methanosaetaceae, Methanobacteriales, Methanocellales (Rice Cluster I), Rice Cluster II, Crenarchaeota Group I.3 and Crenarchaeota Group I.1b. Incubation of soil samples under anoxic conditions generally resulted in vigorous CH₄ production after a lag phase of 7-8 days. Production of CH₄ was partially inhibited by methyl fluoride, a specific inhibitor of acetoclastic methanogenesis, resulting in nearly stoichiometric accumulation of acetate. CO₂ was produced without lag phase. The δ¹³C of the produced CO₂ was slightly lower in soil grown with cultivated rice versus wild rice, reflecting the δ¹³C of organic matter, which was about −29‰ for cultivated rice soil and about −24‰ for wild rice soil. The δ¹³C of the produced CH₄ and the acetate that accumulated in the presence of CH₃F was much more negative in cultivated versus wild rice soil, mainly since the isotopic fractionation factors for hydrogenotrophic methanogenesis were higher for soil from cultivated rice (α = 1.054) versus wild rice (α = 1.039). However, the percentage contribution of hydrogenotrophic methanogenesis to total CH₄ production was similar in both soils (27-35%). In conclusion, although the two soils exhibited different δ¹³C values of soil organic matter and derived products, they were similar with respect to rates and composition of the methanogenic communities.     

Dynamics of methanogenesis and methanotrophy in tropical paddy soils as influenced by elevated CO2 and temperature interaction

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.     

Wheat straw management affects CH4 and N2O emissions from rice fields

A 3-year field experiment was conducted in Jiangsu Province, China from 2004 to 2006 to investigate CH₄ and N₂O emissions from paddy fields as affected by various wheat straw management practices prior to rice cultivation. Five methods of returning wheat straw, no straw, evenly incorporating, burying straw, ditch mulching and strip mulching, were adopted in the experiment. Evenly incorporating is the most common management practice in the region. Results showed that compared with no straw, evenly incorporating increased CH₄ emission significantly by a factor of 3.9-10.5, while decreasing N₂O emission by 1-78%. Methane emission from burying straw was comparable with that from evenly incorporating, while N₂O emission from burying straw was 94-314% of that from evenly incorporating. Compared with evenly incorporating, CH₄ emission was decreased by 23-32% in ditch mulching and by 32% in strip mulching, while N₂O emission was increased by a factor of 1.4-3.7 in ditch mulching and by a factor of 5.1 in strip mulching. During the rice-growing season, the emitted N₂O was negligible compared to that of emitted CH₄. No significant difference in grain yield was observed between ditch mulching, burying straw, evenly incorporating and no straw. Compared with no straw, the grain yield was increased by 27% in strip mulching. Based on these results, the best management practice for returning wheat straw to the soil is strip mulching wheat straw partially or completely onto the field surface, as the method reduced CH₄ emission from rice fields with no decrease in rice yield.     

Is alternate wetting and drying irrigation technique enough to reduce methane emission from a tropical rice paddy?

Combination of a pre-season wet soil condition and rice straw incorporation just before transplanting, which is typical for a tropical rice double cropping, can induce a flash of methane (CH₄) emission shortly after the transplanting. The conventional practice of alternate wetting and drying (AWD) irrigation technique that typically starts at 21 days after transplanting (DAT) can hardly reduce this emission because the soil become methanogenic before the onset of AWD treatment. Field experiments were conducted in Central Luzon, Philippines, during the 2014–2017 dry rice seasons to examine the effects of the timing of rice straw/stubble incorporation on the efficacy of AWD in reducing the CH₄ emission. Two treatments of the timing of stubble incorporation were stubbles incorporated during the start of wet land preparation (S1) and stubbles incorporated during the dry fallow tillage (S2). For the water management, we compared two treatments: continuous flooding (CF) and AWD with – 15 cm threshold for irrigation. The AWD under S2 was implemented earlier at 10 DAT. We observed a significant interaction (p < 0.01) between effects of AWD and straw management on CH₄ emissions; the seasonal total CH₄ emission was reduced by AWD compared with CF by 73% under S2, while the reduction was <20% under S1. The AWD significantly (p < 0.05) increased the nitrous oxide (N₂O) emissions by 47 and 48% relative to CF under S1 and S2, respectively. The global warming potential (GWP, CH₄ + N₂O) and yield-scaled GWP were still substantially lower by 62 and 59%, respectively, in AWD than in CF under S2, but the reduction was not realized under S1 due to the relatively smaller CH₄ reduction and increased N₂O emission. The results confirm that pre-season aerobic stubble decomposition and earlier implementation of AWD enhanced AWD’s mitigation potential in reducing substantially the CH₄ emission from the tropical rice double-cropping system.     

Effect of the number of tillages in fallow season and fertilizer type on greenhouse gas emission from a rice (Oryza sativa L.) paddy field in Ehime,southwestern Japan

Agricultural fields, including rice (Oryza sativa L.) paddy fields, constitute one of the major sources of atmospheric methane (CH₄) and nitrous oxide (N₂O). Organic matter application, such as straw and organic fertilizer, enhances CH₄ emission from paddy fields. In addition, rice straw management after harvest regulates CH₄ emissions in the growing season. The interaction of tillage times and organic fertilizer application on CH₄ and N₂O emissions is largely unknown. Therefore, we studied the effects of fallow-season tillage times and fertilizer types on CH₄ and N₂O emissions in paddy fields in Ehime, southwestern Japan. From November 2011 to October 2013, four treatments, two (autumn and spring) or one (spring) in the first year, or two (autumn and spring) or three (autumn, winter, and spring) in the second year times of tillage with chemical or organic fertilizer application, were established. Gas fluxes were measured by the closed-chamber method. Increasing the number of tillage times from one to two decreased succeeding CH₄ emission and the emission factor for CH₄ (EF_CH4) in the rice-growing season, suggesting that the substrate for CH₄ production was reduced by autumn and spring tillage in the fallow season. Higher EF_CH4 [1.8–2.0 kg carbon (C) ha^?1 d^?1] was observed when more straw was applied (6.9–7.2 Mg ha^?1) in the second year. Organic fertilizer application induced higher CH₄ emission just after the application as basal and supplemental fertilizers, especially at a lower straw application rate. This indicated that EF_CH4 in the organically managed fields should be determined individually. Organic fertilizer application with two tillage times induced N₂O efflux during the rice-growing season in the second year, but N₂O emissions were not affected by winter tillage. Although paddy fields can act as an N₂O sink because of reduced soil conditions when straw application was high, application of organic C and nitrogen as fertilizer can enhance N₂O production by the denitrification process during the growing season, especially in the ripening stage when soil anaerobic conditions became moderate. These results suggest that negative emission factors for N₂O (EF_N2O) can be applied, and EF_N2O of organic fertilizer should be considered during the estimation of N₂O emission in the paddy field.