Lower‐than‐expected CH4 emissions from rice paddies with rising CO2 concentrations

Elevated atmospheric CO₂ (eCO₂) generally increases carbon input in rice paddy soils and stimulates the growth of methane‐producing microorganisms. Therefore, eCO₂ is widely expected to increase methane (CH₄) emissions from rice agriculture, a major source of anthropogenic CH₄. Agricultural practices strongly affect CH₄ emissions from rice paddies as well, but whether these practices modulate effects of eCO₂ is unclear. Here we show, by combining a series of experiments and meta‐analyses, that whereas eCO₂ strongly increased CH₄ emissions from paddies without straw incorporation, it tended to reduce CH₄ emissions from paddy soils with straw incorporation. Our experiments also identified the microbial processes underlying these results: eCO₂ increased methane‐consuming microorganisms more strongly in soils with straw incorporation than in soils without straw, with the opposite pattern for methane‐producing microorganisms. Accounting for the interaction between CO₂ and straw management, we estimate that eCO₂ increases global CH₄ emissions from rice paddies by 3.7%, an order of magnitude lower than previous estimates. Our results suggest that the effect of eCO₂ on CH₄ emissions from rice paddies is smaller than previously thought and underline the need for judicious agricultural management to curb future CH₄ emissions.     

Effects of free-air CO2 enrichment (FACE) on CH4 emission from a rice paddy field

Methane (CH₄) is a particularly potent greenhouse gas with a radiative forcing 23 times that of CO₂ on a per mass basis. Flooded rice paddies are a major source of CH₄ emissions to the Earth's atmosphere. A free‐air CO₂ enrichment (FACE) experiment was conducted to evaluate changes in crop productivity and the crop ecosystem under enriched CO₂ conditions during three rice growth seasons from 1998 to 2000 in a rice paddy at Shizukuishi, Iwate, Japan. To understand the influence of elevated atmospheric CO₂ concentrations on CH₄ emission, we measured methane flux from FACE rice fields and rice fields with ambient levels of CO₂ during the 1999 and 2000 growing seasons. Methane production and oxidation potentials of soil samples collected when the rice was at the tillering and flowering stages in 2000 were measured in the laboratory by the anaerobic incubation and alternative propylene substrates methods, respectively. The average tiller number and root dry biomass were clearly larger in the plots with elevated CO₂ during all rice growth stages. No difference in methane oxidation potential between FACE and ambient treatments was found, but the methane production potential of soils during the flowering stage was significantly greater under FACE than under ambient conditions. When free‐air CO₂ was enriched to 550 ppmv, the CH₄ emissions from the rice paddy field increased significantly, by 38% in 1999 and 51% in 2000. The increased CH₄ emissions were attributed to accelerated CH₄ production potential as a result of more root exudates and root autolysis products and to increased plant‐mediated CH₄ emissions because of the larger rice tiller numbers under FACE conditions.     

Impact of root exudates of different cultivars and plant development stages of rice (Oryza sativa L.) on methane production in a paddy soil

The impact of root exudates, collected from five rice cultivars, on methane (CH₄) production was studied in a paddy soil under anaerobic conditions. Root exudates of the cultivars Dular, IR72 and IR65598 collected at four growth stages and of B40 and IR65600 collected at two growth stages showed that (a) CH₄ production was commenced rapidly within 2 h upon exudate addition and reached a maximum within a day of addition, and (b) 7-d incubation periods were sufficient to study exudate-induced CH₄ production potentials. Among different cultivars, high C releases from roots, increased the methanogenic source strength of the soil, which finally controlled CH₄ production. The relationship of the amount of CH₄ produced was stronger for the amount of total organic C (r = 0.920) than for the amount of organic acids (r = 0.868) added through exudates. Apparently, CH₄ production and CH₄ emission are more closely related to the release pattern of root exudate-C than to its individual components. The proportion of exudate-C converted to CH₄ ranged between 61 and 83% and remained unaffected by cultivars and growth stages suggesting that the majority of exudate-C served as a methanogenic substrate in the anoxic rice soils. These observations indicate that the use of high-yielding cultivars with lowest excretion (for example IR65598, IR65600) would result in lowest exudate-induced CH₄ production. Therefore, cultivar choice could greatly influence regional and global CH₄ emissions and screening/selection of exiting rice cultivars, and/or breeding new cultivars with low exudation rates could offer an important methane mitigation option as long as yields are not compromised.     

Steel slag amendment reduces methane emission and increases rice productivity in subtropical paddy fields in China

Paddy field, being a man-made wetland, is recognized as one of the major sources of global methane (CH₄) emission. Since China has the second-largest area of rice cultivation in the world, it is important to develop valid and reliable strategies to reduce CH₄ emission and sustain rice productivity in Chinese paddy fields. In this study, we applied steel slag fertilizer, a by-product of steel industry with a high concentration of active iron (Fe), at rates of 0, 2, 4, and 8 Mg ha^?1 in subtropical rice (Oryza sativa L.) paddy fields in China to assess the effect of steel slag amendment on CH₄ emissions as well as rice growth and yield characteristics. Results showed that the Fe concentrations in paddy soils significantly increased with the application levels of steel slag fertilizer. Steel slag amendment in paddy fields largely reduced the CH₄ production rate, resulting in a decrease in the overall CH₄ emission rate. In response to the applications of steel slag at a rate of 2, 4 and 8 Mg ha^?1, total CH₄ emission during rice cultivation decreased by 26.6, 43.3 and 49.3 %, respectively. Furthermore, steel slag amendment had a significant, positive effect on the rice grain yield and the percentage of ripened grain, most probably due to the increased availability of inorganic nutrients such as silicate and manganese. Our results suggest that steel slag can be an effective soil amendment for reducing CH₄ emissions as well as increasing rice productivity in subtropical paddy fields in China.     

Microbial mechanism for rice variety control on methane emission from rice field soil

Rice variety is one of the key factors regulating methane (CH₄) production and emission from the paddy fields. However, the relationships between rice varieties and populations of microorganisms involved in CH₄ dynamics are poorly understood. Here we investigated CH₄ dynamics and the composition and abundance of CH₄‐producing archaea and CH₄‐oxidizing bacteria in a Chinese rice field soil planted with three types of rice. Hybrid rice produced 50–60% more of shoot biomass than Indica and Japonica cultivars. However, the emission rate of CH₄ was similar to Japonica and lower than Indica. Furthermore, the dissolved CH₄ concentration in the rhizosphere of hybrid rice was markedly lower than Indica and Japonica cultivars. The rhizosphere soil of hybrid rice showed a similar CH₄ production potential but a higher CH₄ oxidation potential compared with the conventional varieties. Terminal restriction fragment length polymorphism analysis of the archaeal 16S rRNA genes showed that the hydrogenotrophic methanogens dominated in the rhizosphere whereas acetoclastic methanogens mainly inhabited the bulk soil. The abundance of total archaea as determined by quantitative (real‐time) PCR increased in the later stage of rice growth. However, rice variety did not significantly influence the structure and abundance of methanogenic archaea. The analysis of pmoA gene fragments (encoding the α‐subunit of particulate methane monooxygenase) revealed that rice variety also did not influence the structure of methanotrophic proteobacteria, though variable effects of soil layer and sampling time were observed. However, the total copy number of pmoA genes in the rhizosphere of hybrid rice was approximately one order of magnitude greater than the two conventional cultivars. The results suggest that hybrid rice stimulates the growth of methanotrophs in the rice rhizosphere, and hence enhances CH₄ oxidation which attenuates CH₄ emissions from the paddy soil. Hybrid rice is becoming more and more popular in Asian countries. The present study demonstrated that planting of hybrid rice will not enhance CH₄ emissions albeit a higher grain production than the conventional varieties.     

Limited potential of harvest index improvement to reduce methane emissions from rice paddies

Rice is a staple food for nearly half of the world's population, but rice paddies constitute a major source of anthropogenic CH₄ emissions. Root exudates from growing rice plants are an important substrate for methane‐producing microorganisms. Therefore, breeding efforts optimizing rice plant photosynthate allocation to grains, i.e., increasing harvest index (HI), are widely expected to reduce CH₄ emissions with higher yield. Here we show, by combining a series of experiments, meta‐analyses and an expert survey, that the potential of CH₄ mitigation from rice paddies through HI improvement is in fact small. Whereas HI improvement reduced CH₄ emissions under continuously flooded (CF) irrigation, it did not affect CH₄ emissions in systems with intermittent irrigation (II). We estimate that future plant breeding efforts aimed at HI improvement to the theoretical maximum value will reduce CH₄ emissions in CF systems by 4.4%. However, CF systems currently make up only a small fraction of the total rice growing area (i.e., 27% of the Chinese rice paddy area). Thus, to achieve substantial CH₄ mitigation from rice agriculture, alternative plant breeding strategies may be needed, along with alternative management.     

Characterization of Root Exudates at Different Growth Stages of Ten Rice (Oryza sativa L.) Cultivars

Abstract: Plant root exudates play important roles in the rhizosphere. We tested three media (nutrient solution, deionized water and CaSO₄ solution) for three periods of time (2, 4 and 6 h) for collecting root exudates of soil‐grown rice plants. Nutrient culture solution created complications in the analyses of exudates for total organic C (TOC) by the wet digestion method and of organic acids by HPLC due to the interference by its components. Deionized water excluded such interference in analytical analyses but affected the turgor of root cells; roots of four widely different rice cultivars excreted 20 to 60 % more TOC in deionized water than in 0.01 M CaSO₄. Furthermore, the proportion of carbohydrates in TOC was also enhanced. Calcium sulfate solution maintained the osmotic environment for root cells and did not interfere in analytical procedures. Collection for 2 h avoided under‐estimation of TOC and its components exuded by rice roots, which occurred during prolonged exposure. By placing plants in 0.01 M CaSO₄ for 2 h, root exudates of soil‐grown traditional, tall rice cultivars (Dular, B40 and Intan), high‐yielding dwarf cultivars (IR72, IR52, IR64 and PSBRc 20), new plant type cultivars (IR65598 and IR65600) and a hybrid (Magat) were collected at seedling, panicle initiation, flowering and maturity and characterized for TOC and organic acids. The exudation rates were, in general, lowest at seedling stage, increased until flowering but decreased at maturity. Among organic acids, malic acid showed the highest concentration followed by tartaric, succinic, citric and lactic acids. With advancing plant growth, exudation of organic acids substituted exudation of sugars. Root and shoot biomass were positively correlated with carbon exudation suggesting that it is driven by plant biomass. As root exudates provide substrates for methanogenesis in rice fields, large variations in root exudation by cultivars and at different growth stages could greatly influence CH₄ emissions. Therefore, the use of high‐yielding cultivars with lowest root excretions, for example IR65598 and IR65600, would mediate low exudate‐induced CH₄ production. The screening of exciting rice cultivars and breeding of new cultivars with low exudation rates could offer an important option for mitigation of CH₄ emission from rice agriculture to the atmosphere.     

Effects of elevated ozone concentration on methane emission from a rice paddy in Yangtze River Delta,China

Few investigations have been made on the impact of elevated ozone (O₃) concentration on methane (CH₄) emission from rice paddies. Using open‐top chambers in situ with different O₃ treatments, CH₄ emissions were measured in a rice paddy in Yangtze River Delta, China in 2007 and 2008. There were four treatments applied: charcoal‐filtered air (CF), nonfiltered air (NF), and charcoal‐filtered air with different O₃ additions (O₃‐1 and O₃‐2). The mean O₃ concentrations during the O₃ fumigation were 19.7, 22.6, 69.6 and 118.6 ppb in 2007 and 7.0, 17.4, 82.2 and 138.3 ppb in 2008 for treatments CF, NF, O₃‐1 and O₃‐2, respectively. The rice yields, as compared with CF, were reduced by 32.8% and 37.1%, 58.3% and 52.1% in treatments O₃‐1 and O₃‐2 in 2007 and 2008, respectively. The diurnal patterns of CH₄ emission varied temporally with treatments and there was inconsistence in diurnal variations in CH₄ emissions from the paddy field. The daily mean CH₄ emissions were significantly lower in treatments O₃‐1 and O₃‐2 than those in treatments CF and NF. Compared with CF treatment, CH₄ emissions from the paddy field were decreased to 46.5% and 38.3%, 50.6% and 46.8% under treatments O₃‐1 and O₃‐2 in the whole growing seasons of 2007 and 2008, respectively. The seasonal mean CH₄ emissions were negatively related with AOT40 (accumulative O₃ concentration above 40 ppb; P < 0.01 in both years), but positively related to the relative rice yield (reference to CF; P < 0.01 in 2007 and P < 0.001 in 2008), aboveground biomass (P < 0.01 in both years) and underground biomass (P < 0.01 in 2007 and P < 0.05 in 2008). The decreased CH₄ emission from the rice paddy due to an increased O₃ exposure might partially mitigate the global warming potential induced by soil carbon loss under elevated O₃ concentrations.     

Agronomic aspects of wetland rice cultivation and associated methane emissions

Wetland rice cultivation is one of the major sources of atmospheric methane (CH₄). Global rice production may increase by 65% between 1990 and 2025, causing an increase of methane emissions from a 92 Tg CH₄ y^–1 now to 131 Tg in 2025.Methane production depends strongly on the ratio oxidizing: reducing capacity of the soil. It can be influenced by e.g. addition of sulphate, which inhibits methanogenesis. The type and application mode of mineral fertilizers may also affect methane emissions. Addition of organic matter in the form of compost or straw causes an increase of methane emissions, but methane production is lower for materials with a low C/N ratio.High percolation rates in wetland rice soils and occasional drying up of the soil during the cultivation period depresses methane release. Water management practices aimed at reducing emissions are only feasible during specific periods in the rice growing season in flat lowland irrigated areas with high security of water availability and good control of the water supply. Intermittent drying of soils may not be possible on terraced rice lands.Assuming a 10 to 30% reduction in emission rates per unit harvested area, the global emission may amount to 93 Tg CH₄ y^– in 2025. A reduction of global emissions seems very difficult. To develop techniques for reducing CH₄ emissions from wetland rice fields, research is required concerning interactions between soil chemical and physical properties, and soil, water and crop management and methanogenesis. Such techniques should not adversely affect rice yields.     

Nitrogen-regulated effects of free-air CO2 enrichment on methane emissions from paddy rice fields

Using the free‐air CO₂ enrichment (FACE) techniques, we carried out a 3‐year mono‐factorial experiment in temperate paddy rice fields of Japan (1998–2000) and a 3‐year multifactorial experiment in subtropical paddy rice fields in the Yangtze River delta in China (2001–2003), to investigate the methane (CH₄) emissions in response to an elevated atmospheric CO₂ concentration (200±40 mmol mol^?1 higher than that in the ambient atmosphere). No significant effect of the elevated CO₂ upon seasonal accumulative CH₄ emissions was observed in the first rice season, but significant stimulatory effects (CH₄ increase ranging from 38% to 188%, with a mean of 88%) were observed in the second and third rice seasons in the fields with or without organic matter addition. The stimulatory effects of the elevated CO₂ upon seasonal accumulative CH₄ emissions were negatively correlated with the addition rates of decomposable organic carbon (P<0.05), but positively with the rates of nitrogen fertilizers applied in either the current rice season (P<0.05) or the whole year (P<0.01). Six mechanisms were proposed to explain collectively the observations. Soil nitrogen availability was identified as an important regulator. The effect of soil nitrogen availability on the observed relation between elevated CO₂ and CH₄ emission can be explained by (a) modifying the C/N ratio of the plant residues formed in the previous growing season(s); (b) changing the inhibitory effect of high C/N ratio on plant residue decomposition in the current growing season; and (c) altering the stimulatory effects of CO₂ enrichment upon plant growth, as well as nitrogen uptake in the current growing season. This study implies that the concurrent enrichment of reactive nitrogen in the global ecosystems may accelerate the increase of atmospheric methane by initiating a stimulatory effect of the ongoing dramatic atmospheric CO₂ enrichment upon methane emissions from nitrogen‐poor paddy rice ecosystems and further amplifying the existing stimulatory effect in nitrogen‐rich paddy rice ecosystems.     

Annual emissions of dissolved CO2, CH4, and N2O in the subsurface drainage from three cropping systems

Indirect emission of nitrous oxide (N₂O), associated with nitrogen (N) leaching and runoff from agricultural lands is a major source of atmospheric N₂O. Recent studies have shown that carbon dioxide (CO₂) and methane (CH₄) are also emitted via these pathways. We measured the concentrations of three dissolved greenhouse gases (GHGs) in the subsurface drainage from field lysimeter that had a shallow groundwater table. Aboveground fluxes of CH₄ and N₂O were monitored using an automated closed‐chamber system. The annual total emissions of dissolved and aboveground GHGs were compared among three cropping systems; paddy rice, soybean and wheat, and upland rice. The annual drainage in the paddy rice, the soybean and wheat, and the upland rice plots was 1435, 782, and 1010 mm yr^?1, respectively. Dissolved CO₂ emissions were highest in the paddy rice plots, and were equivalent to 1.05–1.16% of the carbon storage in the topsoil. Dissolved CH₄ emissions were also higher in the paddy rice plots, but were only 0.03–0.05% of the aboveground emissions. Dissolved N₂O emissions were highest in the upland rice plots, where leached N was greatest due to small crop biomass. In the soybean and wheat plots, large crop biomass, due to double cropping, decreased the drainage volume, and thus decreased dissolved GHG emissions. Dissolved N₂O emissions from both the soybean and wheat plots and the upland rice plots were equivalent to 50.3–67.3% of the aboveground emissions. The results indicate that crop type and rotation are important factors in determining dissolved GHG emissions in the drainage from a crop field.     

CH4 emission with differences in atmospheric CO2 enrichment and rice cultivars in a Japanese paddy soil

A pot experiment was conducted to investigate CH₄ emissions from a sandy paddy soil as influenced by rice cultivars and atmospheric CO₂ elevation. The experiment with two CO₂ levels, 370 μL L⁻¹ (ambient) and 570 μL L⁻¹ (elevated), was performed in a climatron, located at the National Institute for Agro‐Environmental Sciences, Tsukuba, Japan. Four rice cultivars were tested in this experiment, including IR65598, IR72, Dular and Koshihikari. Tiller number, root length and grain yield were clearly larger under elevated CO₂ than under ambient CO₂. IR72 and Dular showed significantly higher tiller number, root length and grain yield than Koshihikari and IR65598. Average daily CH₄ fluxes under elevated CO₂ were significantly larger by 10.9–23.8% than those under ambient CO₂, and varied with the cultivars in the sequence Dular ≧ IR72>IR65598 ≧ Koshihikari. Dissolved organic C (DOC) content in the soil was obviously higher under elevated CO₂ than under ambient CO₂ and differed among the cultivars, in the sequence IR72>Dular>Koshihikari>IR65598. The differences in average daily CH₄ fluxes between CO₂ levels and among the cultivars were related to different root exudation as DOC content, root length and tiller number. This study indicated that Koshihikari should be a potential cultivar for mitigating CH₄ emission and simultaneously keeping stable grain yield, because this cultivar emitted lowest CH₄ emission and produced medium grain yield.     

Modeling methane emissions from irrigated rice cultivation in China from 1960 to 2050

Rice paddy is a major source of anthropogenic terrestrial methane (CH₄). China has the second‐largest area of rice cultivation in the world, accounting for ca. 19% of the world's rice‐producing area. Recognizing the significance of China's rice cultivation in the global CH₄ budget, we estimated the CH₄ emissions resulting from irrigated rice cultivation in China from 1960 to 2050 using a CH4MOD model. The model estimates suggest that the annual CH₄ emissions decreased from 5.62 Tg yr^?1 in 1960 to 4.13 Tg yr^?1 in 1970, and this decrease was attributed to changes in water management from continuous flooding to mid‐season drainage irrigation. Since the early 1970s, the amount of CH₄ emissions gradually increased to 6.85 Tg yr^?1 by 2009 because of significant improvements in crop production that led to high‐crop residue retention. Higher levels of CH₄ emissions occurred in southern China, where double rice cropping systems are most common. For the A1B and B1 scenarios of the IPCC Special Report on Emissions Scenarios (SRES), the amount of CH₄ emissions from 2010 to 2050 is predicted to increase at an average rate of 1.2 kg ha^?1 yr^?1 in response to global warming. Compared to 2009, the CH₄ flux is predicted to increase by ca. 14% by the late 2040s, and the increase in these emissions in northeastern China is estimated to become more significant than in the other rice‐growing regions of the country. Under the assumptions that the rice‐producing land area will remain the same, decrease by 25% or increase by 38% by the late 2040s, the CH₄ emissions are projected to be 7.8, 5.6 or 11.7 Tg yr^?1, respectively.     

Benefits and trade‐offs of replacing synthetic fertilizers by animal manures in crop production in China: A meta‐analysis

Recycling of livestock manure to agricultural land may reduce the use of synthetic fertilizer and thereby enhance the sustainability of food production. However, the effects of substitution of fertilizer by manure on crop yield, nitrogen use efficiency (NUE), and emissions of ammonia (NH₃), nitrous oxide (N₂O) and methane (CH₄) as function of soil and manure properties, experimental duration and application strategies have not been quantified systematically and convincingly yet. Here, we present a meta‐analysis of these effects using results of 143 published studies in China. Results indicate that the partial substitution of synthetic fertilizers by manure significantly increased the yield by 6.6% and 3.3% for upland crop and paddy rice, respectively, but full substitution significantly decreased yields (by 9.6% and 4.1%). The response of crop yields to manure substitution varied with soil pH and experimental durations, with relatively large positive responses in acidic soils and long‐term experiments. NUE increased significantly at a moderate ratio (<40%) of substitution. NH₃ emissions were significantly lower with full substitution (62%–77%), but not with partial substitution. Emissions of CH₄ from paddy rice significantly increased with substitution ratio (SR), and varied by application rates and manure types, but N₂O emissions decreased. The SR did not significantly influence N₂O emissions from upland soils, and a relative scarcity of data on certain manure characteristic was found to hamper identification of the mechanisms. We derived overall mean N₂O emission factors (EF) of 0.56% and 0.17%, as well as NH₃ EFs of 11.1% and 6.5% for the manure N applied to upland and paddy soils, respectively. Our study shows that partial substitution of fertilizer by manure can increase crop yields, and decrease emissions of NH₃ and N₂O, but depending on site‐specific conditions. Manure addition to paddy rice soils is recommended only if abatement strategies for CH₄ emissions are also implemented.     

控释氮肥对抗除草剂转基因水稻田土壤甲烷排放的影响

采用温室盆栽和静态箱-气相色谱法,研究了控释氮肥对抗除草剂转基因水稻和亲本常规水稻稻田土壤甲烷(CH4)排放的影响。供试土壤为潴育型水稻土,氮肥种类为尿素和控释氮肥。结果表明,与对照(尿素)相比,控释氮肥提高了水稻分蘖数、株高、生物量及产量。水稻品种对CH4季节性排放规律没有明显影响,CH4排放通量基本表现为,自水稻移栽后逐渐升高,移栽后62—92 d出现峰值,而后逐渐降低至水稻收获。与对照相比,控释氮肥可显著降低CH4排放通量和全生育期累积排放量。抗除草剂转基因水稻稻田土壤CH4排放通量和累积排放量均显著低于亲本常规水稻。研究认为,一次性基施控释氮肥和种植抗除草剂转基因水稻对有效减缓稻田甲烷排放具有重要意义。     

Impact of gas transport through rice cultivars on methane emission from rice paddy fields

Two Italian rice (Oryza sativa var. japonica) cultivars, Lido and Roma, were tested in the field for methane production, oxidation and emission. In two consecutive years, fields planted with the rice cultivar Lido showed methane emissions 24–31% lower than fields planted with the cultivar Roma. This difference was observed irrespective of fertilizer treatment. In contrast to methane emissions, differences in methane production or oxidation were not observed between fields planted with the two cultivars. Plant-mediated transport of methane from the sediment to the atmosphere was the dominating pathway of methane emission. During the entire vegetation period, the contribution of this pathway to total methane emission amounted to c. 90%, whereas the contribution of gas bubble release and of diffusion through the water column to total methane emission was of minor significance. Results obtained from transport studies of tracer gas through the aerenchyma system of rice plants demonstrated that the root–shoot transition zone is the main site of resistance to plant-mediated gas exchange between the soil and the atmosphere. The cultivar Lido, showing relatively low methane emissions in the field, had a significantly lower gas transport capacity through the aerenchyma system than the cultivar Roma. Thus, the observed differences in methane emissions in the field between the cultivars Lido and Roma can be explained by different gas transport capacities. Apparently, these differences in gas transport capacities are a consequence of differences in morphology of the aerenchyma systems, especially in the root–shoot transition zone. It is, therefore, concluded that identification and use of high-yielding rice cultivars which have a low gas transport capacity represent an economically feasible, environmentally sound and promising approach to mitigating methane emissons from rice paddy fields.     

Revising a process-based biogeochemistry model (DNDC) to simulate methane emission from rice paddy fields under various residue management and fertilizer regimes

A comprehensive biogeochemistry model, DNDC, was revised to simulate crop growth and soil processes more explicitly and improve its ability to estimate methane (CH₄) emission from rice paddy fields under a wide range of climatic and agronomic conditions. The revised model simulates rice growth by tracking photosynthesis, respiration, C allocation, tillering, and release of organic C and O₂ from roots. For anaerobic soil processes, it quantifies the production of electron donors [H₂ and dissolved organic carbon (DOC)] by decomposition and rice root exudation, and simulates CH₄ production and other reductive reactions based on the availability of electron donors and acceptors (NO₃^?, Mn⁴⁺, Fe³⁺, and SO₄^2?). Methane emission through rice is simulated by a diffusion routine based on the conductance of tillers and the CH₄ concentration in soil water. The revised DNDC was tested against observations at three rice paddy sites in Japan and China with varying rice residue management and fertilization, and produced estimates consistent with observations for the variation in CH₄ emission as a function of residue management. It also successfully predicted the negative effect of (NH₄)₂SO₄ on CH₄ emission, which the current model missed. Predicted CH₄ emission was highly sensitive to the content of reducible soil Fe³⁺, which is the dominant electron acceptor in anaerobic soils. The revised DNDC generally gave acceptable predictions of seasonal CH₄ emission, but not of daily CH₄ fluxes, suggesting the model's immaturity in describing soil heterogeneity or rice cultivar‐specific characteristics of CH₄ transport. It also overestimated CH₄ emission at one site in a year with low temperatures, suggesting uncertainty in root biomass estimates due to the model's failure to consider the temperature dependence of leaf area development. Nevertheless, the revised DNDC explicitly reflects the effects of soil electron donors and acceptors, and can be used to quantitatively estimate CH₄ emissions from rice fields under a range of conditions.     

Effect of CO2 enrichment and elevated temperature on methane emissions from rice, Oryza sativa

Methane emissions from rice grown within Temperature Gradient Greenhouse Tunnels under doubled CO₂ concentrations were 10–45 times less than emissions from control plants grown under ambient CO₂. For two cultivars of rice (cvs. Lemont and IR-72), methane emissions increased with a temperature increase of 2°, from outdoor ambient temperatures to the first cell of the ambient CO₂ tunnel (ambient temperature + 2 °C). Within both tunnels and for both cultivars methane emissions decreased with further temperature increases (from 2° to 5 °C above ambient). Carbon dioxide enrichment stimulated both above- and below-ground production. Our original hypothesis was that increased CO₂ would stimulate plant productivity and therefore stimulate methane emission, since direct linkages between these parameters have been observed. We hypothesize that CO₂ enrichment led to the attenuation of methane production due to increased delivery of oxygen to the rhizosphere because of increased root biomass and porosity. The increased root biomass due to elevated CO₂ may have more effectively aerated the soil, suppressing methane production. However, this study may be unique because the low organic content (< 1%) of the sandy soils in which the rice was grown created very little oxygen demand.     

Stimulation of N2O emission by manure application to agricultural soils may largely offset carbon benefits: a global meta‐analysis

Animal manure application as organic fertilizer does not only sustain agricultural productivity and increase soil organic carbon (SOC) stocks, but also affects soil nitrogen cycling and nitrous oxide (N₂O) emissions. However, given that the sign and magnitude of manure effects on soil N₂O emissions is uncertain, the net climatic impact of manure application in arable land is unknown. Here, we performed a global meta‐analysis using field experimental data published in peer‐reviewed journals prior to December 2015. In this meta‐analysis, we quantified the responses of N₂O emissions to manure application relative to synthetic N fertilizer application from individual studies and analyzed manure characteristics, experimental duration, climate, and soil properties as explanatory factors. Manure application significantly increased N₂O emissions by an average 32.7% (95% confidence interval: 5.1–58.2%) compared to application of synthetic N fertilizer alone. The significant stimulation of N₂O emissions occurred following cattle and poultry manure applications, subsurface manure application, and raw manure application. Furthermore, the significant stimulatory effects on N₂O emissions were also observed for warm temperate climate, acid soils (pH < 6.5), and soil texture classes of sandy loam and clay loam. Average direct N₂O emission factors (EFs) of 1.87% and 0.24% were estimated for upland soils and rice paddy soils receiving manure application, respectively. Although manure application increased SOC stocks, our study suggested that the benefit of increasing SOC stocks as GHG sinks could be largely offset by stimulation of soil N₂O emissions and aggravated by CH₄ emissions if, particularly for rice paddy soils, the stimulation of CH₄ emissions by manure application was taken into account.     

东北黑土地区稻田甲烷排放时空演变及排放潜力分析

稻田甲烷排放是农业源甲烷排放的主要来源。东北黑土地区是我国最大的粮食生产基地,农业温室气体减排是实现黑土地永续利用的关键议题之一。运用稻田甲烷排放模型(CH4MOD)核算并分析了2009-2018年东北黑土地区稻田甲烷排放的时空演变特征,结合GOSAT卫星遥感数据探究了水稻生产与区域甲烷排放的时空动态联系,进一步量化了稻田甲烷对区域甲烷排放的贡献程度及不同情景下的排放潜力。结果表明,受水稻生产面积扩张和排放强度提高的影响,东北黑土地区稻田甲烷排放总量从2009年的39.05万t增加到2018年的79.53万t。东北黑土地区区域甲烷排放在季节变化和栅格单元上表现出与稻田甲烷排放较为一致的时空动态,大规模的稻田耕作可能会增加水稻生产与区域甲烷排放直接相关的可能性。随着水稻持续扩种稳产,2018年东北黑土地区水稻生产贡献了区域甲烷排放总量的15.04%,其中黑龙江省的贡献率高达31.06%。在基准发展情景下,预计2035年东北黑土地区稻田CH₄排放量较2018年增加19.5%;在粮食供给保障情景下,维持当前稻田耕作面积,水稻生产集约化程度提高,预计其稻田CH₄排放量较2018年减少0.88%;在此基础上,采取促进秸秆还田、增施有机肥、实施节水间歇灌溉等稻田管理措施将使稻田CH₄排放量增加17.8%-63.6%。以满足膳食需求和供给保障为导向,优化水稻种植结构、控制稻田耕作面积,推动技术进步、品种改良以提升单产水平,采取化肥和有机肥搭配施用、节水间歇灌溉等途径能够缓解稻田甲烷排放。研究综合运用自上而下的遥感数据和自下而上的模型运算,刻画了水稻生产与区域甲烷排放的时空联系,进一步评估了稻田甲烷的排放潜力及减排措施的减排效果,为促进东北黑土地区农业甲烷减排和生产布局优化提供了理论依据和决策参考。