Long-term growth at elevated carbon dioxide stimulates methane emission in tropical paddy rice

Recent anthropogenic emissions of key atmospheric trace gases (e.g. CO₂ and CH₄) which absorb infra-red radiation may lead to an increase in mean surface temperatures and potential changes in climate. Although sources of each gas have been evaluated independently, little attention has focused on potential interactions between gases which could influence emission rates. In the current experiment, the effect of enhanced CO₂ (300 μL L^–1 above ambient) and/or air temperature (4 °C above ambient) on methane generation and emission were determined for the irrigated tropical paddy rice system over 3 consecutive field seasons (1995 wet and dry seasons 1996 dry season). For all three seasons, elevated CO₂ concentration resulted in a significant increase in dissolved soil methane relative to the ambient control. Consistent with the observed increases in soil methane, measurements of methane flux per unit surface area during the 1995 wet and 1996 dry seasons also showed a significant increase at elevated carbon dioxide concentration relative to the ambient CO₂ condition (+49 and 60% for each season, respectively). Growth of rice at both increasing CO₂ concentration and air temperature did not result in additional stimulation of either dissolved or emitted methane compared to growth at elevated CO₂ alone. The observed increase in methane emissions were associated with a large, consistent, CO₂-induced stimulation of root growth. Results from this experiment suggest that as atmospheric CO₂ concentration increases, methane emissions from tropical paddy rice could increase above current projections.     

Influence of soil temperature on methane emission from rice paddy fields

Methane emission rates from an Italian rice paddy field showed diel and seasonal variations. The seasonal variations were not closely related to soil temperatures. However, the dieL changes of CH₄ fluxes were significantly correlated with the diel changes of the temperature in a particular soil depth. The soil depths with the best correlations between CH4 flux and temperature were shallow (1–5cm) in May and June, deep (10–15cm) in June and July, and again shallow (1–5 cm) in August. Apparent activation energies (E_a) calculated from these correlations using the Arrhenius model were relatively low (50–150 kJ mol^–1) in May and June, but increased to higher values (80–450 kJ mol^–1) in August. In the laboratory, CH₄ emission from two rice cultures incubated at temperatures between 20 and 38°C showed E . values of 41 and 53 kJ mol^–1) Methane production in anoxic paddy soil suspensions incubated between 7 and 43°C showed E values between 53 and 132 kJ mol^–1 with an average value of 85 kJ mol^–1) and in pure cultures of hydrogenotrophic methanogenic bacteria E _a values between 77 and 173 (average 126) kJ mol^–1. It is suggested that diel changes of soil properties other than temperature affect CH₄ emission rates, e.g. diel changes in root exudation or in efficiency of CH⁴ oxidation in the rhizosphere.     

The effect of elevated atmospheric carbon dioxide levels on soil bacterial communities

The effect of elevated carbon dioxide levels on total bacterial communities was studied in a series of controlled and replicated model terrestrial ecosystems over a period of 38 weeks. The bacterial community was profiled using Denaturing Gradient Gel Electrophoresis (DGGE) analysis of bacterial 16S rRNA gene fragments amplified by the Polymerase Chain Reaction from DNA extracted directly from soil. Bacterial community DGGE profiles provided three major findings: (i) there was a high degree of profile similarity after ≈ 12 weeks (one plant generation); (ii) whilst overall DGGE profile was maintained over the 38 weeks (three plant generations), the banding patterns became more diverse with time; (iii) DGGE data provided no evidence for a shift in bacterial community structure resulting from exposure of the ecosystem to an increased atmospheric CO₂ level.     

Influence of rice cultivar on methane emission from paddy fields

Influence of rice cultivars on CH₄ emissions from a paddy field was studied using four Japonica types, two Indica types, and two Japonica/Indica F₁ hybrids. In addition, the suppression of CH₄ emission by interrupting irrigation at the flowering stage was investigated. Patterns of seasonal variation in CH₄ emission rates were similar among the eight cultivars. Two of the Japonica types showed the maximum and minimum CH₄ emissions among the cultivars investigated. Neither the number of tillers, shoot length, shoot weight, and root weight correlated with the CH₄ emission rates at the tillering and reproductive growth stages. Following temporary interruption of irrigation at the flowering stage, CH₄ emission rates decreased drastically and remained at very low levels until the harvesting stage, indicating its great effectiveness for the suppression of CH₄ emission from rice paddies.     

土壤有机碳和外源添加碳对水稻土土体呼吸的影响

土体呼吸输出碳来源于土壤固有有机碳和外源添加碳,而以往关于不同施肥措施对水稻土碳排放的研究少有区分碳的来源。本试验利用一个长达30年的水稻土定位试验,在保证原有定位试验继续正常开展的前提下变更部分施肥处理,得到继续施用高量有机肥(HOM)、施用常量有机肥30年后改施高量有机肥(N-H)、继续施用常量有机肥(NOM)、施用化肥30年后改施常量有机肥(C-N)、施用高量有机肥30年后改施化肥(H-C)、施用常量有机肥30年后改施化肥(N-C)、继续施用化肥(CF)等7种施肥处理。通过观测早稻生长期间原有施肥和改施肥处理土体CO2排放通量(FCO2),研究不同后续施肥对水稻土FCO2的影响,以期探讨土壤原始有机碳和外源添加碳对土壤FCO2的影响。结果表明:7种不同施肥处理土体CO2平均排放通量(F珔CO2)分别为85.34、69.10、51.27、49.15、14.89、12.92和11.59 mg C.m-2.h-1;对施用无机肥料和常量有机肥料的土体而言,土壤本身有机碳含量对F珔CO2无显著影响,但对施用高量有机肥的土体而言,土壤本身的高有机碳含量会增强F珔CO2;CO2排放通量(Y)与添加外源碳量(x)之间符合指数方程:Y=13.33e1.719 x(R2=0.967,n=21),施入的外源有机碳对土体FCO2产生极显著影响;当季外源添加碳以CO2-C矿化分解释放的碳占其总碳量的14%左右,且该分解率受土壤有机碳含量和有机物料添加量的影响较小。     

生物质炭化还田对稻田温室气体排放及土壤理化性质的影响

通过水稻种植田间试验,研究了水稻秸秆直接还田、水稻秸秆与生活垃圾炭化后还田对稻田温室气体CH4、CO2和N2O排放及土壤理化性质和水稻产量的影响.结果表明:与直接还田相比,秸秆炭化后还田可显著降低稻田CH4和N2O的累积排放量,降幅分别为64.2%～78.5%和16.3%～18.4%.与不添加生物炭相比,无论种植水稻与否,添加秸秆炭和垃圾炭均显著降低了稻田N2O的累积排放量;不种植水稻情况下,添加垃圾炭显著降低了稻田CO2的累积排放量,降幅为25.3%.秸秆炭对提高稻田土壤pH和速效钾含量的作用优于垃圾炭.两种生物炭均能显著提高稻田土壤有机碳含量,但对土壤容重、全氮、有效磷、阳离子交换量及水稻籽粒产量均未产生显著影响.与秸秆直接还田相比,秸秆炭化后还田对水稻增产的效果更佳.     

Seasonal variation in methane flux from rice paddies associated with methane concentration in soil water,rice biomass and temperature,and its modelling

To attempt to develop physicochemical and physiological modelling for methane transport from the rhizosphere to the atmosphere through rice plants, methane flux, methane concentration in the soil water, and the biomass of rice were measured in lysimeter rice paddies (2.5 × 4 m, depth 2.0 m) once per week throughout the entire growing season in 1992 at Tsukuba, Japan. The addition of exogenous organic matter (rice straw) or soil amendments with the presence or absence of vegetation were also examined for their influence on methane emissions. The total methane emission over the growing season varied from 3.2 g CH₄ m^-2 y^-1 without the addition of rice straw to 49.7 g CH₄ m^-2 y^-1 with rice straw and microbiological amendment. In the unvegetated plot with the addition of rice straw, there was much ebullition of gas bubbles, particularly in the summer. The annual methane emission due to the ebullition of gas bubbles,from the unvegetated plot with the addition of rice straw was estimated to be almost the same as that from the vegetated site with the addition of rice straw. In the early growth stage, the methane flux can be analyzed by the diffusion model (Flux=Methane concentration × Conductance of rice body) using parameters for methane concentration in the soil water as a difference in concentration between the atmosphere and the rhizosphere, and for the biomass of rice as a conductance of rice body. On the other hand, although the diffusion model was inapplicable to a large extent from the middle to late growth stage, methane flux could be estimated by air temperature and concentration in the soil water. Thus, methane transport from the rhizosphere to the atmosphere through rice plants consisted of two phases: one was an explainable small part by diffusion in rice body; the other was a large part strongly, governed by air temperature. The existence of gas bubbles in the soil may be related to the transition between the two phases     

Direct effects of atmospheric carbon dioxide concentration on whole canopy dark respiration of rice

The purpose of this study was to test for direct inhibition of rice canopy apparent respiration by elevated atmospheric carbon dioxide concentration ([CO₂]) across a range of short‐term air temperature treatments. Rice (cv. IR‐72) was grown in eight naturally sunlit, semiclosed, plant growth chambers at daytime [CO₂] treatments of 350 and 700 μmol mol^?1. Short‐term night‐time air temperature treatments ranged from 21 to 40 °C. Whole canopy respiration, expressed on a ground area basis (R_d), was measured at night by periodically venting the chambers with ambient air. This night‐time chamber venting and resealing procedure produced a range of increasing chamber [CO₂] which we used to test for potential inhibitory effects of rising [CO₂] on R_d. A nitrous oxide leak detection system was used to correct R_d measurements for chamber leakage rate (L) and also to determine if apparent reductions in night‐time R_d with rising [CO₂] could be completely accounted for by L. The L was affected by both CO₂ concentration gradient between the chamber and ambient air and the inherent leakiness of each individual chamber. Nevertheless, after correcting R_d for L, we detected a rapid and reversible, direct inhibition of R_d with rising chamber [CO₂] for air temperatures above 21 °C. This effect was larger for the 350 compared with the 700 μmol mol^?1 daytime [CO₂] treatment and was also increased with increasing short‐term air temperature treatments. However, little difference in R_d was found between the two daytime [CO₂] treatments when night‐time [CO₂] was at the respective daytime [CO₂]. These results suggest that naturally occurring diurnal changes in both ambient [CO₂] and air temperature can affect R_d. Because naturally occurring diurnal changes in both [CO₂] and air temperature can be expected in a future higher CO₂ world, short‐term direct effects of these environmental variables on rice R_d can also be expected.     

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.     

Estimation of the increase in CH4 emission from paddy soils by rice straw application

The relationship between the amount of CH₄ emission to the atmosphere from submerged paddy soils with rice plants and the application level (0–8 g kg^-1) of rice straw (RS) in soil was investigated in a pot experiment. Amounts of CH₄ emitted from pots with respective RS levels differed between a clayey yellow soil and a silty gray lowland soil. However, the increase in cumulative amounts of CH₄ emission with the increase in the application level of RS was similar in pattern between the two soils, and the increase (Y) was formulated with a logistic curve: x, application level of RS; k, a coefficient for relative CH₄ emission.Since the seasonal variations in coefficients a, b, and c in the logistic equation were also formulated as the function of the sum of effective temperature (E, (T–15); T, daily average temperature), the increase in cumulative amounts of CH₄ emission from any paddy soil by any level of RS application was known to be estimated by the following equation:     

Seasonal patterns of photosynthetic response and acclimation to elevated carbon dioxide in field-grown strawberry

Strawberry (Fragaria × ananassa) plants were grown in field plots at the current ambient [CO₂], and at ambient + 300 and ambient + 600 μmol mol⁻¹ [CO₂]. Approximately weekly measurements were made of single leaf gas exchange of upper canopy leaves from early spring through fall of two years, in order to determine the temperature dependence of the stimulation of photosynthesis by elevated [CO₂], whether growth at elevated [CO₂] resulted in acclimation of photosynthesis, and whether any photosynthetic acclimation was reduced when fruiting created additional demand for the products of photosynthesis. Stimulation of photosynthetic CO₂ assimilation by short-term increases in [CO₂] increased strongly with measurement temperature. The stimulation exceeded that predicted from the kinetic characteristics of ribulose-1,5-bisphosphate carboxylase at all temperatures. Acclimation of photosynthesis to growth at elevated [CO₂] was evident from early spring through summer, including the fruiting period in early summer, with lower rates under standard measurement conditions in plants grown at elevated [CO₂]. The degree of acclimation increased with growth [CO₂]. However, there were no significant differences between [CO₂] treatments in total nitrogen per leaf area, and photosynthetic acclimation was reversed one day after switching the [CO₂] treatments. Tests showed that acclimation did not result from a limitation of photosynthesis by triose phosphate utilization rate at elevated [CO₂]. Photosynthetic acclimation was not evident during dry periods in midsummer, when the elevated [CO₂] treatments conserved soil water and photosynthesis declined more at ambient than at elevated [CO₂]. Acclimation was also not evident during the fall, when plants were vegetative, despite wet conditions and continued higher leaf starch content at elevated [CO₂]. Stomatal conductance responded little to short-term changes in [CO₂] except during drought, and changed in parallel with photosynthetic acclimation through the seasons in response to the long-term [CO₂] treatments. The data do not support the hypothesis that source-sink balance controls the seasonal occurrence of photosynthetic acclimation to elevated [CO₂] in this species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Assessing the effect of elevated carbon dioxide on soil carbon: a comparison of four meta-analyses

Soil is the largest reservoir of organic carbon (C) in the terrestrial biosphere and soil C has a relatively long mean residence time. Rising atmospheric carbon dioxide (CO₂) concentrations generally increase plant growth and C input to soil, suggesting that soil might help mitigate atmospheric CO₂ rise and global warming. But to what extent mitigation will occur is unclear. The large size of the soil C pool not only makes it a potential buffer against rising atmospheric CO₂, but also makes it difficult to measure changes amid the existing background. Meta‐analysis is one tool that can overcome the limited power of single studies. Four recent meta‐analyses addressed this issue but reached somewhat different conclusions about the effect of elevated CO₂ on soil C accumulation, especially regarding the role of nitrogen (N) inputs. Here, we assess the extent of differences between these conclusions and propose a new analysis of the data. The four meta‐analyses included different studies, derived different effect size estimates from common studies, used different weighting functions and metrics of effect size, and used different approaches to address nonindependence of effect sizes. Although all factors influenced the mean effect size estimates and subsequent inferences, the approach to independence had the largest influence. We recommend that meta‐analysts critically assess and report choices about effect size metrics and weighting functions, and criteria for study selection and independence. Such decisions need to be justified carefully because they affect the basis for inference. Our new analysis, with a combined data set, confirms that the effect of elevated CO₂ on net soil C accumulation increases with the addition of N fertilizers. Although the effect at low N inputs was not significant, statistical power to detect biogeochemically important effect sizes at low N is limited, even with meta‐analysis, suggesting the continued need for long‐term experiments.     

The effect of carbon dioxide concentration on respiration of growing and mature soybean leaves

Soybean plants were grown continuously at 350 and 700cm³m^?3 CO₂ at constant temperature. Respiration rates of third trifoliolate leaves were measured at the growth CO₂ concentration for the whole dark period from 5d before through to 5d after full area expansion. The short-term response of respiration rate to the measurement CO₂ concentration was also determined at each age. Respiration rates per unit of dry mass declined with age and were significantly less at a given age or RGR in leaves grown and measured at the elevated CO₂. The difference in respiration rate was largest in mature leaves and resulted from the different measurement CO₂ concentrations. The respiratory costs of the tissue synthesis, estimated from the elemental composition of the tissue, did not differ substantially between CO₂ treatments. The response of respiration rate to carbon dioxide concentration was not strongly affected by the form of nitrogen supplied. Maintenance respiration calculated by subtracting growth respiration from total respiration was negative in rapidly growing leaves for both CO₂ treatments. This indicates that CO₂ efflux in the dark does not accurately reflect the average 24 h rate of energy expenditure on growth and maintenance for soybean leaves.     

Effect of soil characteristics on sequential reduction and methane production in sixteen rice paddy soils from China, the Philippines, and Italy

The potentials for sequential reduction of inorganic electron acceptors and production of methane have been examined in sixteen rice soils obtained from China, the Philippines, and Italy. Methane, CO₂, Fe(II), NO ₃ ^- , SO ₄ ² , pH, E_h, H₂ and acetate were monitored during anaerobic incubation at 30 °C for 120 days. Based on the accumulation patterns of CO₂ and CH₄, the reduction process was divided into three distinct phases: (1) an initial reduction phase during which most of the inorganic electron acceptors were depleted and CO₂ production was at its maximum, (2) a methanogenic phase during which CH₄ production was initiated and reached its highest rate, and (3) a steady state phase with constant production rates of CH₄ and CO₂. The reduction phases lasted for 19 to 75 days with maximum CO₂ production of 2.3 to 10.9 mol d^-1 g^-1 dry soil. Methane production started after 2 to 87 days and became constant after about 38--68 days (one soil >120 days). The maximum CH₄ production rates ranged between 0.01 and 3.08 mol d^-1 g^-1. During steady state the constant CH₄ and CO₂ production rates varied from 0.07 to 0.30 mol d^-1 g^-1 and 0.02 and 0.28 mol d^-1 g^-1, respectively. Within the 120 d of anaerobic incubation only 6--17% of the total soil organic carbon was released into the gas phase. The gaseous carbon released consisted of 61--100% CO₂, <0.1--35% CH₄, and <5% nonmethane hydrocarbons. Associated with the reduction of available Fe(III) most of the CO₂ was produced during the reduction phase. The electron transfer was balanced between total CO₂ produced and both CH₄ formed and Fe(III), sulfate and nitrate reduced. Maximum CH₄ production rate (r = 0.891) and total CH₄ produced (r = 0.775) correlated best with the ratio of soil nitrogen to electron acceptors. Total nitrogen content was a better indicator for available organic substrates than the total organic carbon content. The redox potential was not a good predictor of potential CH₄ production. These observations indicate that the availability of degradable organic substrates mainly controls the CH₄ production in the absence of inorganic electron acceptors.     

Interactive effects of elevated carbon dioxide and growth temperature on photosynthesis in cotton leaves

Cotton (Gossypium hirsutum L., cv DPL 5415) plants were grown in naturally lit environment chambers at day/night temperature regimes of 26/18 (T-26/18), 31/23 (T-31/23) and 36/28 °C (T-36/28) and CO2 concentrations of 350 (C-350), 450 (C-450) and 700 L L-1 (C-700). Net photosynthesis rates, stomatal conductance, transpiration, RuBP carboxylase activity and the foliar contents of starch and sucrose were measured during different growth stages. Net CO2 assimilation rates increased with increasing CO2 and temperature regimes. The enhancement of photosynthesis was from 24 mol CO2 m-2 s-1 (with C-350 and T-26/18) to 41 mol m-2 s-1 (with C-700 and T-36/28). Stomatal conductance decreased with increasing CO2 while it increased up to T-31/23 and then declined. The interactive effects of CO2 and temperature resulted in a 30% decrease in transpiration. Although the leaves grown in elevated CO2 had high starch and sucrose concentrations, their content decreased with increasing temperature. Increasing temperature from T-26/18 to 36/28 increased RuBP carboxylase activity in the order of 121, 172 and 190 mol mg-1 chl h-1 at C-350, C-450 and C-700 respectively. Our data suggest that leaf photosynthesis in cotton benefited more from CO_2 enrichment at warm temperatures than at low growth temperature regimes.     

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.     

Stomatal conductance, photosynthesis and respiration of temperate deciduous tree seedlings grown outdoors at an elevated concentration of carbon dioxide

Seedlings of temperate deciduous tree species were grown outdoors at ambient and at an elevated concentration of carbon dioxide to examine how aspects of their gas exchange would be altered by growth at elevated carbon dioxide concentration. Leaf conductances to water vapour and net carbon dioxide exchange rates were determined periodically near midday. Whole-plant carbon dioxide efflux rates in darkness were also determined. The stomatal conductance of leaves of plants grown and measured at 700 cm³ m^?3 carbon dioxide did not differ from that of plants grown and measured at 350 cm³ m^?3 in Malus domestica, Quercus prinus and Quercus robur at any measurement time. In Acer saccharinum, lower conductances occurred for plants grown and measured at elevated carbon dioxide concentration only at measurement temperatures above 33°C. Photo-synthetic adjustment to elevated carbon dioxide concentration was evident only in Q. robur. All species examined had lower rates of dark respiration per unit of mass when grown and measured at elevated carbon dioxide concentration.     

A comparison of the effects of carbon dioxide concentration and temperature on respiration, translocation and nitrate reduction in darkened soybean leaves

BACKGROUND AND AIMS: Respiration of autotrophs is an important component of their carbon balance as well as the global carbon dioxide budget. How autotrophic respiration may respond to increasing carbon dioxide concentrations, [CO(2)], in the atmosphere remains uncertain. The existence of short-term responses of respiration rates of plant leaves to [CO(2)] is controversial. Short-term responses of respiration to temperature are not disputed. This work compared responses of dark respiration and two processes dependent on the energy and reductant supplied by dark respiration, translocation and nitrate reduction, to changes in [CO(2)] and temperature. METHODS: Mature soybean leaves were exposed for a single 8-h dark period to one of five combinations of air temperature and [CO(2)], and rates of respiration, translocation and nitrate reduction were determined for each treatment. KEY RESULTS: Low temperature and elevated [CO(2)] reduced rates of respiration, translocation and nitrate reduction, while increased temperature and low [CO(2)] increased rates of all three processes. A given change in the rate of respiration was accompanied by the same change in the rate of translocation or nitrate reduction, regardless of whether the altered respiration was caused by a change in temperature or by a change in [CO(2)]. CONCLUSIONS: These results make it highly unlikely that the observed responses of respiration rate to [CO(2)] were artefacts due to errors in the measurement of carbon dioxide exchange rates in this case, and indicate that elevated [CO(2)] at night can affect translocation and nitrate reduction through its effect on respiration.     

Response of respiration of soybean leaves grown at ambient and elevated carbon dioxide concentrations to day-to-day variation in light and temperature under field conditions

BACKGROUND AND AIMS: Respiration is an important component of plant carbon balance, but it remains uncertain how respiration will respond to increases in atmospheric carbon dioxide concentration, and there are few measurements of respiration for crop plants grown at elevated [CO(2)] under field conditions. The hypothesis that respiration of leaves of soybeans grown at elevated [CO(2)] is increased is tested; and the effects of photosynthesis and acclimation to temperature examined. METHODS: Net rates of carbon dioxide exchange were recorded every 10 min, 24 h per day for mature upper canopy leaves of soybeans grown in field plots at the current ambient [CO(2)] and at ambient plus 350 micromol mol(-1) [CO(2)] in open top chambers. Measurements were made on pairs of leaves from both [CO(2)] treatments on a total of 16 d during the middle of the growing seasons of two years. KEY RESULTS: Elevated [CO(2)] increased daytime net carbon dioxide fixation rates per unit of leaf area by an average of 48 %, but had no effect on night-time respiration expressed per unit of area, which averaged 53 mmol m(-2) d(-1) (1.4 micromol m(-2) s(-1)) for both the ambient and elevated [CO(2)] treatments. Leaf dry mass per unit of area was increased on average by 23 % by elevated [CO(2)], and respiration per unit of mass was significantly lower at elevated [CO(2)]. Respiration increased by a factor of 2.5 between 18 and 26 degrees C average night temperature, for both [CO(2)] treatments. CONCLUSIONS: These results do not support predictions that elevated [CO(2)] would increase respiration per unit of area by increasing photosynthesis or by increasing leaf mass per unit of area, nor the idea that acclimation of respiration to temperature would be rapid enough to make dark respiration insensitive to variation in temperature between nights.     

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.