西南喀斯特地区轮作旱地土壤CO2通量

中国已承诺大幅降低单位GDP碳排放,农业正面临固碳减排的重任.西南喀斯特地区环境独特,旱地面积占据优势比例,土壤碳循环认识亟待加强.以贵州省开阳县玉米-油菜轮作旱地为研究对象,采用密闭箱-气相色谱法对整个轮作期土壤CO2释放通量进行了观测研究,结果表明:(1)整个轮作期旱地均表现为CO2的释放源.其中油菜生长季土壤CO2通量为(178.8±104.8)mg CO2·m-2·h-1,玉米生长季为(403.0±178.8) mg CO2·m-2·h-1,全年平均通量为(271.1±176.4) mg CO2·m-2·h-1,高于纬度较高地区的农田以及同纬度的次生林和松林;(2)CO2通量日变化同温度呈现显著正相关关系,季节变化与温度呈现显著指数正相关关系,并受土壤湿度的影响,基于大气温度计算得出的Q10为2.02,高于同纬度松林以及低纬度的常绿阔叶林;(3)CO2通量与土壤pH存在显著线性正相关关系,显示出土壤pH是研究区旱地土壤呼吸影响因子之一.     

若尔盖高原沼泽湿地CO2排放时空变化特征

为了更好理解若尔盖高原不同微生境下沼泽湿地生态系统CO₂排放通量的变化特征,以若尔盖高原湿地自然保护区为研究对象,2013和2014年生长季期间,采用了静态箱和快速温室气体法原位观测了3种湿地5种微生境下沼泽湿地CO₂排放通量时空变化规律。结果表明：长期淹水微地貌草丘区湿地（PHK）和洼地区湿地（PHW） CO₂排放通量变化范围分别为38.99-1731.74 mg m^-2 h^-1和46.69-335.22 mg m^-2 h^-1,季节性淹水区微地貌草丘区湿地（SHK）和洼地区湿地（SHW） CO₂排放通量变化范围分别为193.90-2575.60 mg m^-2 h^-1和49.93-1467.45 mg m^-2 h^-1,而两者过渡区的无淹水区沼泽湿地（Lawn） CO₂排放通量变化范围194.20-898.75 mg m^-2 h^-1。相关性分析表明5种微地貌区沼泽湿地CO₂排放通量季节性变化与不同深度土壤温度均存在显著正相关,与水位存在显著负相关（PHW、SHW、SHK、Lawn）或不相关（PHK）,并且水位和温度（5 cm）共同解释了CO₂排放通量季节性变化的87%。3种湿地5种微生境下沼泽湿地CO₂排放通量存在空间变化规律,主要受水位影响,但植物也影响沼泽湿地CO₂排放通量空间变化规律,并且表明沼泽湿地CO₂排放通量与水位平均值存在显著负相关。     

Precipitation pulse size effects on Sonoran Desert soil microbial crusts

Deserts are characterized by low productivity and substantial unvegetated space, which is often covered by soil microbial crust communities. Microbial crusts are important for nitrogen fixation, soil stabilization and water infiltration, but their role in ecosystem production is not well understood. This study addresses the following questions: what are the CO₂ exchange responses of crusts to pulses of water, does the contribution of crusts to ecosystem flux differ from the soil respiratory flux, and is this contribution pulse size dependent? Following water application to crusts and soils, CO₂ exchange was measured and respiration was partitioned through mixing model analysis of Keeling plots across treatments. Following small precipitation pulse sizes, crusts contributed 80% of soil-level CO₂ fluxes to the atmosphere. However, following a large pulse event, roots and soil microbes contributed nearly 100% of the soil-level flux. Rainfall events in southern Arizona are dominated by small pulse sizes, suggesting that crusts may frequently contribute to ecosystem production. Carbon cycle studies of arid land systems should consider crusts as important contributors because of their dynamic responses to different pulse sizes as compared to the remaining ecosystem components.     

川西贡嘎山峨眉冷杉成熟林生态系统CO2通量特征

成熟森林的碳收支对陆地生态系统碳循环研究具有重要意义。目前,我国关于西南亚高山暗针叶林成熟林碳通量的研究还相对较少,尚不明确对碳循环的作用。以涡度相关技术为基础,对川西贡嘎山东坡峨眉冷杉成熟林生态系统尺度的CO_2通量进行长期定位观测。利用2015年6月至2016年5月观测数据,分析了峨眉冷杉成熟林净生态系统CO_2交换量(NEE)、生态系统呼吸(Re)和总生态系统生产力(GPP)的季节变异特征及其源汇状况,并结合环境因子,分析CO_2通量的主要控制因子。结果表明:(1)峨眉冷杉成熟林NEE具有明显的日变化特征,呈现"U"形变化,白天为负值,夜间为正值,中午前后CO_2通量达到最大;各月间日平均NEE变化差异显著,NEE峰值最大出现在2015年6月(-0.64 mg CO_2m~(-2)s~(-1)),峰值最小出现在2016年1月(-0.08 mg CO_2m~(-2)s~(-1));日平均NEE由正值变为负值的时间夏季最早,冬季最晚,NEE由负值变为正值的时间冬季最早,夏季最晚。(2)峨眉冷杉成熟林NEE、Re和GPP具有明显的月变化。2015年6月和12月NEE分别达到最大值(-46.02 g C m~(-2)月~(-1))和最小值(-1.42 g C m~(-2)月~(-1));Re呈现单峰变化,最大和最小值分别出现在2015年6月(84.78 g C m~(-2)月~(-1))和2016年1月(12.82 g C m~(-2)月~(-1));GPP最大值和最小值分别出现在2015年6月(130.81 g C m~(-2)月~(-1))与2016年1月(16.15 g C m~(-2)月~(-1))。(3)空气温度(T_a)、5 cm土壤温度(T_(s5))和光合有效辐射(PAR)是影响峨眉冷杉成熟林CO_2通量的主要环境因子。T_a与CO_2通量呈指数相关(R~2=0.5283,P0.01);白天CO_2通量与PAR显著相关(R~2=0.4373,P0.01);夜晚CO_2通量与T_(s5)显著相关(R~2=0.4717,P0.01)。(4)全年NEE、Re和GPP分别为-241.87、564.81 g C m~(-2)和806.68 g C m~(-2),表明川西贡嘎山峨眉冷杉成熟林具有较强的碳汇功能。     

Multichannel automated chamber system for continuous monitoring of CO2 exchange between the agro-ecosystem or soil and the atmosphere

A multichannel automated chamber system was developed for continuous monitoring of CO₂ exchange at multiple points between agro-ecosystem or soil and atmosphere. This system consisted of an automated chamber subsystem with a CO₂ concentration analyzer and a data logging subsystem. Both subsystems were under the control of a programmable logic controller (PLC). The automated chamber subsystem contained 18 chambers (50 cm × 50 cm × 50 cm) and a compressor. The chamber lids were closed and can be automatically opened. During measurement, one of the 18 chambers was kept closed for three minutes for measuring and the other chambers were kept open to maintain the natural soil conditions to the maximum extent. Environmental variables were simultaneously measured using sensors and recorded by the data logger. The reliability of the multichannel automated chamber system was tested and the results showed that the turbulence of the fans had no significant effect on the CO₂ exchange. The changes in the air and the temperature of soil and soil moisture inside the chambers, caused by the enclosure of the chambers, were not significant. The net ecosystem CO₂ exchange for the wheat ecosystem was ?2.35 μmol·m^?2·s^>?1 and the soil respiration was 3.87 μmol·m^?2·s^>?1 in the wheat field, and 6.61 μmol·m^?2·s^>?1 in the apple orchard.     

CO2 flux in alpine wetland ecosystem on the Qinghai-Tibetan Plateau, China

Using the CO₂ flux data measured by the eddy covariance method in the northeast of Qinghai-Tibetan Plateau in 2005, we analyzed the carbon flux dynamics in relation to meteorological and biotic factors. The results showed that the alpine wetland ecosystem was the carbon source, and it emitted 316.02 gCO₂ · m⁻² to atmosphere in 2005 with 230.16 gCO₂ · m⁻² absorbed in the growing season from May to September and 546.18 gCO₂ · m⁻² released in the non-growing season from January to April and from October to December. The maximum of the averaged daily CO₂ uptake rates and release rates was (0.45 ± 0.0012) mgCO₂ · m⁻² · s⁻¹ (Mean ± SE) in July and (0.22 ± 0.0090) mgCO₂ · m⁻² · s⁻¹ in August, respectively. The averaged diurnal variation showed a single-peaked pattern in the growing season, but exhibited very small fluctuation in the non-growing season. Net ecosystem exchange (NEE) and gross primary production (GPP) were all correlated with some meteorological factors, and they showed a negatively linear correlation with aboveground biomass, while a positive correlation existed between the ecosystem respiration (R_es) and those factors.     

Effect of the replacement of a native savanna by an African Brachiaria decumbens pasture on the CO2 exchange in the Orinoco lowlands,Venezuela

In the Orinoco lowlands, savannas have been often replaced by pastures composed of the C₄ grass, Brachiaria decumbens Stapf. We addressed following questions: (1) How does the replacement of the native vegetation affect CO₂ exchange on seasonal and annual scales? (2) How do biophysical constraints change when the landscape is transformed? To assess how these changes affect carbon exchange, we determined simultaneously the CO₂ fluxes by eddy covariance, and the soil CO₂ efflux by a chamber-based system in B. decumbens and herbaceous savanna stands. Measurements covered a one-year period from the beginning of the dry season (November 2008) to the end of the wet season (November 2009). During the wet season, the net ecosystem CO₂ exchange reached maximum values of 23 and 10 μmol(CO₂) m^?2 s^?1 in the B. decumbens field and in the herbaceous savanna stand, respectively. The soil CO₂ efflux for both stands followed a temperature variation during the dry and wet seasons, when the soil water content (SWC) increased above 0.087 m³ m^?3 in the latter case. Bursts of CO₂ emissions were evident when the dry soil experienced rehydration. The carbon source/sink dynamics over the two canopies differed markedly. Annual measurements of the net ecosystem production indicated that the B. decumbens field constituted a strong carbon sink of 216 g(C) m^?2 y^?1. By contrast, the herbaceous savanna stand was found to be only a weak sink [36 g(C) m^?2 y^?1]. About 53% of the gross primary production was lost as the ecosystem respiration. Carbon uptake was limited by SWC in the herbaceous savanna stand as evident from the pattern of water-use efficiency (WUE). At the B. decumbens stand, WUE was relatively insensitive to SWC. Although these results were specific to the studied site, the effect of land use changes and the physiological response of the studied stands might be applicable to other savannas.     

Effect of photon flux density on inorganic carbon accumulation and net CO2 exchange in a high-CO2-requiring mutant of Chlamydomonas reinhardtii

The effect of photon flux density on inorganic carbon accumulation and photosynthetic CO₂ assimilation was determined by CO₂ exchange studies at three, limiting CO₂ concentrations with a ca-1 mutant of Chlamydomonas reinhardiii. This mutant accumulates a large internal inorganic carbon pool in the light which apparently is unavailable for photosynthetic assimilation. Although steady-state photosynthetic CO₂ assimilation did not respond to the varying photon flux densities because of CO₂ limitation, components of inorganic-carbon accumulation were not clearly light saturated even at 1100 mol photons m^-2 s^-1, indicating a substantial energy requirement for inorganic carbon transport and accumulation. Steady-state photosynthetic CO₂ assimilation responded to external CO₂ concentrations but not to changing internal inorganic carbon concentrations, confirming that diffusion of CO₂ into the cells supplies most of the CO₂ for photosynthetic assimilation and that the internal inorganic carbon pool is essentially unavailable for photosynthetic assimilation. The estimated concentration of the internal inorganic carbon pool was found to be relatively insensitive to the external CO₂ concentration over the small range tested, as would be expected if the concentration of this pool is limited by the internal to external inorganic carbon gradient. An attempt to use this CO₂ exchange method to determine whether inorganic carbon accumulation and photosynthetic CO₂ assimilation compete for energy at low photon flux densities proved inconclusive.     

Seasonal patterns in soil surface CO<Subscript>2</Subscript> flux under snow cover in 50 and 300 year old subalpine forests

Soil CO₂ flux can contribute as much as 60–80% of total ecosystem respiration in forests. Although considerable research has focused on quantifying this flux during the growing season, comparatively little effort has focused on non-growing season fluxes. We measured soil CO₂ efflux through snow in 50 and ~300 year old subalpine forest stands near Fraser CO. Our objectives were to quantify seasonal patterns in wintertime soil CO₂ flux; determine if differences in soil CO₂ flux between the two forest ages during the growing season persist during winter; and to quantify the sample size necessary to discern treatment differences. Soil CO₂ flux during the 2002–2003 and 2003–2004 snow season averaged 0.31 and 0.35 μmols m⁻² s⁻¹ for the young and old forests respectively; similar to the relative difference observed during summer. There was a significant seasonal pattern of soil CO₂ flux during the winter with fluxes averaging 0.22 μmols m⁻² s⁻¹ in December and January and increasing to an average of 0.61 μmols m⁻² s⁻¹ in May. Within-plot variability for measurements used in calculating flux was low. The coefficients of variation (CV) for CO₂ concentration, snowpack density, and snow depth were 17, 8 and 14%, respectively, yielding a CV for flux measurements within-plot of 29%. A within plot CV of 29% requires 8 sub-samples per plot to estimate the mean flux with a standard error of ±10% of the mean. Variability in CO₂ flux estimates among plots (size = 400 m²) was similar to that within plot and was also low (CV = ~28%). With a CV of 28% among plots, ten plots per treatment would have a 50% probability of detecting a 25% difference in treatment means for α = 0.05.     

High rates of net ecosystem carbon assimilation by Brachiara pasture in the Brazilian Cerrado

To investigate the consequences of land use on carbon and energy exchanges between the ecosystem and atmosphere, we measured CO₂ and water vapour fluxes over an introduced Brachiara brizantha pasture located in the Cerrado region of Central Brazil. Measurements using eddy covariance technique were carried out in field campaigns during the wet and dry seasons. Midday CO₂ net ecosystem exchange rates during the wet season were ?40 μmol m^?2 s^?1, which is more than twice the rate found in the dry season (?15 μmol m^?2 s^?1). This was observed despite similar magnitudes of irradiance, air and soil temperatures. During the wet season, inferred rates of canopy photosynthesis did not show any tendency to saturate at high solar radiation levels, with rates of around 50 μmol m^?2 s^?1 being observed at the maximum incoming photon flux densities of 2200 μmol m^?2 s^?1. This contrasted strongly to the dry period when light saturation occurred with 1500 μmol m^?2 s^?1 and with maximum canopy photosynthetic rates of only 20 μmol m^?2 s^?1. Both canopy photosynthetic rates and night‐time ecosystem CO₂ efflux rates were much greater than has been observed for cerrado native vegetation in both the wet and dry seasons. Indeed, observed CO₂ exchange rates were also much greater than has previously been reported for C₄ pastures in the tropics. The high rates in the wet season may have been attributable, at least in part, to the pasture not being grazed. Higher than expected net rates of carbon acquisition during the dry season may also have been attributable to some early rain events. Nevertheless, the present study demonstrates that well‐managed, productive tropical pastures can attain ecosystem gas exchange rates equivalent to fertilized C₄ crops growing in the temperate zone.     

Photosynthetic characteristics of Cymbidium plantlet in vitro

The photosynthetic characteristics of the Cymbidium plantlet in vitro cultured on Hyponex-agar medium with 2% sucrose were determined based on the measurements of CO₂ concentration inside and outside of the culture vessels. The CO₂ measurements were made with a gas chromatograph at a PPF (photosynthetic photon flux) of 35, 102 and 226 mol m^-2 s^-1, a chamber air temperature of 15, 25 and 35°C and a CO₂ concentration outside the vessel of approximately 350, 1100 and 3000 ppm. The net photosynthetic rates were determined on individual plantlets and were expressed on a dry weight basis. The steady-state CO₂ concentration during the photoperiod was lower inside the vessel than outside the vessel at any PPF greater than 35 mol m^-2s^-1 and at any chamber air temperature. The photosynthetic response curves relating the net photosynthetic rate, PPF, and CO₂ concentration in the vessel and chamber air temperature were similar to those for Cymbidium plants grown outside and other C₃ plants grown outside under shade. The results indicate that CO₂ enrichment for the plantlets in vitro at a relatively high PPF would promote photosynthesis and hence the growth of chlorophyllous shoots/plantlets in vitro and that the plantlets in vitro would make photoautotrophic growth under environmental conditions favorable for photosynthesis.Abbreviations C_in CO₂ concentration in the culture vessel - C_out CO₂ concentration outside the vessel (in the culture room) - PPF photosynthetic photon flux     

Year-round observations of the net ecosystem exchange of carbon dioxide in a native tallgrass prairie

An Ameriflux site was established in mid 1996 to study the exchange of CO₂ in a native tallgrass prairie of north‐central Oklahoma, USA. Approximately the first 20 months of measurements (using eddy covariance) are described here. This prairie, dominated by warm season C4 grasses, is typical of the central Kansas/northern Oklahoma region. During the first three weeks of the measurement period (mid‐July–early August 1996), moisture‐stress conditions prevailed. For the remainder of the period (until March 1998), however, soil moisture was nonlimiting. Mid‐day net ecosystem CO₂ exchange (NEE), under well‐watered conditions, reached a maximum magnitude of 1.4 mg CO₂ m^?2 s^?1 (flux toward the surface is positive) during peak growth (mid‐July 1997), with green leaf area index of 2.8. In contrast, under moisture‐stress conditions in the same growth stage in 1996, mid‐day NEE was reduced to near‐zero. Average night NEE ranged from near‐zero, during winter dormancy, to ? 0.50 mg CO₂ m^?2 s^?1, during peak growth. Most of the variance in average night NEE was explained by changes in soil temperature (0.1 m depth) and green leaf area. The daytime NEE measurements were examined in terms of a rectangular hyperbolic relationship with incident photosynthetically active radiation. The analysis showed that the quantum yield during peak growth was similar to those measured in other prairies and the y‐intercept, so obtained, can be potentially used as an estimate of night‐time CO₂ emissions when eddy covariance data are unavailable. Daily integrated NEE reached its peak magnitude of 30.8 g CO₂ m^?2 d^?1 (8.4 g C m^?2 d^?1) in mid‐July when the green LAI was the largest (about 2.8). In general, the seasonal trend of daily NEE (on relatively clear days) followed that of green LAI. Annually integrated carbon exchange, between prescribed burns in 1997 and 1998, was 268 g C m^?2 y^?1. After incorporating carbon loss during the prescribed burn , the net annual carbon exchange in this prairie was near‐zero in 1998.     

Seasonal CO<Subscript>2</Subscript>-exchange variations of temperate semi-desert grassland in Hungary

CO₂ exchange components of a temperate semi-desert sand grassland ecosystem in Hungary were measured 21 times in 2000–2001 using a closed IRGA system. Stand CO₂ uptake and release, soil respiration rate (R _s), and micrometeorological values were determined with two types of closed system chambers to investigate the daily courses of gas exchange. The maximum CO₂ uptake and release were –3.240 and 1.903 mol m^–2 s^–1, respectively, indicating a relatively low carbon sequestration potential. The maximum and the minimum R _s were 1.470 and 0.226 mol(CO₂) m^–2 s^–1, respectively. Water shortage was probably more effective in decreasing photosynthetic rates than R _s, indicating water supply as the primary driving variable for the sink-source relations in this ecosystem type.     

Photosynthetic characteristics of the leaves of 'Golden Delicious' appletrees

Abstract Using an open-system leaf chamber, gas exchange measurements on attached leaves of 3-4-year-old Golden Delicious apple trees, made through two seasons, provided data from which the parameters of a leaf photosynthesis model could be derived. The equation is: where C₁ is internal CO₂ concentration and Q_p is the incident quantum flux. There was considerable leaf to leaf variation in the values of the parameters but no clear seasonal trends were established. The initial slope (a) had an average value of about 2.5 × 10^?3 mg μmol^?1? (i.e. quantum yield ～ 0.057); the mesophyll conductance (g_m) was about 3.5 mm s^?1 in extension leaves of trees carrying fruit and 2.5 mm s^?1 in extension leaves of defruited trees. Differences between the values of g_m for spur leaves with and without subtending fruits were not significant; 2.5 mm s^?1 may be used. Dark respiration (R_d, mg m^?2 s^?1) increased exponentially with temperature (T°C); R_d～ 0.006 exp (0.09 T). At saturating photon flux density P_n was linearly related to C_i, up to C_i～ 250 mg m^?3. Optimum temperatures for P_n were slightly different in the two years and were in the range 16-26°C.     

Increased photosynthetic rates following gibberellic acid treatments to the roots of tomato plants

This study was conducted to evaluate the effects of root applications of gibberellic acid (GA₃) on photosynthesis in tomato plants grown hydroponically. Photosynthetic rates (mg CO₂/dm²/hr) determined using an open infrared CO₂ gas exchange system showed a 40–50% increase within 5 hr after treatment with a 1.4 µM gibberellic acid (GA₃) to their roots. The effect was shown to persist for the duration of the experiment (9 days). Plants receiving pulses of 1.4 µM GA₃ to the roots for 1, 4, 8 or 12 hr exhibited significantly higher photosynthetic rates than the control for 6 days following treatment. By day 9 however, there was no significant difference. Continual treatments with 1.4 µM GA₃ to the roots maintained the photosynthetic rate significantly higher than the control for the duration of the experiment. Interestingly, at the lower light levels the percent stimulation was more dramatic. There was approximately a 90% increase in the photosynthetic rate at 80 µE m^-2 s^-1 while at saturating light conditions (560 µE m^-2 s^-1) there was approximately a 40% increase over the control rate. The light saturation point for both treated and control plants was 240 µE m^-2 s^-1. Applications of physiologically relevant concentrations of GA₃ to the roots of tomato plants stimulates photosynthesis more consistently than that achieved by previous studies involving foliar absorption.Approved for publication on May 28, 1981 as paper number 6242 in the Journal series of the Pennsylvania Agricultural Experiment Station.     

Process-level controls on CO<Subscript>2</Subscript> fluxes from a seasonally snow-covered subalpine meadow soil,Niwot Ridge,Colorado

Fluxes of CO₂ during the snow-covered season contribute to annual carbon budgets, but our understanding of the mechanisms controlling the seasonal pattern and magnitude of carbon emissions in seasonally snow-covered areas is still developing. In a subalpine meadow on Niwot Ridge, Colorado, soil CO₂ fluxes were quantified with the gradient method through the snowpack in winter 2006 and 2007 and with chamber measurements during summer 2007. The CO₂ fluxes of 0.71 μmol m⁻² s⁻¹ in 2006 and 0.86 μmol m⁻² s⁻¹ in 2007 are among the highest reported for snow-covered ecosystems in the literature. These fluxes resulted in 156 and 189 g C m⁻² emitted over the winter, ~30% of the annual soil CO₂ efflux at this site. In general, the CO₂ flux increased during the winter as soil moisture increased. A conceptual model was developed with distinct snow cover zones to describe this as well as the three other reported temporal patterns in CO₂ flux from seasonally snow-covered soils. As snow depth and duration increase, the factor controlling the CO₂ flux shifts from freeze–thaw cycles (zone I) to soil temperature (zone II) to soil moisture (zone III) to carbon availability (zone IV). The temporal pattern in CO₂ flux in each zone changes from periodic pulses of CO₂ during thaw events (zone I), to CO₂ fluxes reaching a minimum when soil temperatures are lowest in mid-winter (zone II), to CO₂ fluxes increasing gradually as soil moisture increases (zone III), to CO₂ fluxes decreasing as available carbon is consumed. This model predicts that interannual variability in snow cover or directional shifts in climate may result in dramatically different seasonal patterns of CO₂ flux from seasonally snow-covered soils.     

Effect of CO2 concentration on the PIC/POC ratio in the coccolithophore Emiliania huxleyi grown under light-limiting conditions and different daylengths

We compared the effect of CO₂ concentration ([CO₂], ranging from ∼5 to ∼34 μmol l⁻¹) at four different photon flux densities (PFD=15, 30, 80 and 150 μmol m⁻² s⁻¹) and two light/dark (L/D) cycles (16/8 and 24/0 h) on the coccolithophore Emiliania huxleyi. With increasing [CO₂], a decrease in the particulate inorganic carbon to particulate organic carbon (PIC/POC) ratio was observed at all light intensities and L/D cycles tested. The individual response in cellular PIC and POC to [CO₂] depended strongly on the PFD. POC production increased with rising [CO₂], irrespective of the light intensity, and PIC production decreased with increasing [CO₂] at a PFD of 150 μmol m⁻² s⁻¹, whereas below this light level it was unaffected by [CO₂]. Cell growth rate decreased with decreasing PFD, but was largely independent of ambient [CO₂]. The diurnal variation in PIC and POC content, monitored over a 38-h period (16/8 h L/D, PFD=150 μmol m⁻² s⁻¹), exceeded the difference in carbon content between cells grown at high (∼29 μmol l⁻¹) and low (∼4 μmol l⁻¹) [CO₂]. However, consistent with the results described above, cellular POC content was higher and PIC content lower at high [CO₂], compared to the values at low [CO₂], and the offset was observed throughout the day. It is suggested that the observed sensitivity of POC production for ambient [CO₂] may be of importance in regulating species-specific primary production and species composition.     

Net Ecosystem Exchange of Carbon dioxide in a Temperate Poor Fen: a Comparison of Automated and Manual Chamber Techniques

We used five analytical approaches to compare net ecosystem exchange (NEE) of carbon dioxide (CO₂) from automated and manual static chambers in a peatland, and found the methods comparable. Once per week we sampled manually from 10 collars with a closed chamber system using a LiCor 6200 portable photosynthesis system, and simulated four photosynthetically active radiation (PAR) levels using shrouds. Ten automated chambers sampled CO₂ flux every 3 h with a LiCor 6252 infrared gas analyzer. Results of the five comparisons showed (1) NEE measurements made from May to August, 2001 by the manual and automated chambers had similar ranges: −10.8 to 12.7 μmol CO₂ m⁻² s⁻¹ and −17.2 to 13.1 μmol CO₂ m⁻² s⁻¹, respectively. (2) When sorted into four PAR regimes and adjusted for temperature (respiration was measured under different temperature regimes), mean NEE did not differ significantly between the chambers (p < 0.05). (3) Chambers were not significantly different in regression of ln( − respiration) on temperature. (4) But differences were found in the PAR vs. NEE relationship with manual chambers providing higher maximum gross photosynthesis estimates (GP_max), and slower uptake of CO₂ at low PAR (α) even after temperature adjustment. (5) Due to the high variability in chamber characteristics, we developed an equation that includes foliar biomass, water table, temperature, and PAR, to more directly compare automated and manual NEE. Comparing fitted parameters did not identify new differences between the chambers. These complementary chamber techniques offer a unique opportunity to assess the variability and uncertainty in CO₂ flux measurements.     

Photosynthetic characteristics of Amaranthus tricolor,a C4 tropical leafy vegetable

The gas exchange characteristics are reported for Amaranthus tricolor, a C₄ vegetable amaranth of southeastern Asia. Maximum photosynthetic capacity was 48.3±1.0 μmol CO₂ m^-2 s^-1 and the temperature optimum was 35°C. The calculated intercellular CO₂ concentration at this leaf temperature and an incident photon flux (400–700 mm) of 2 mmol m^-2 s^-1 averaged 208±14 μl l^-1, abnormally high for a C₄ species. The photosynthetic rate, intercellular CO₂ concentration, and leaf conductance all decreased with an increase in water vapor pressure deficit. However, the decrease in leaf conductance which resulted in a decrease in intercellular CO₂ concentration accounted for only one fourth of the observed decrease in photosynthetic rate as water vapor pressure deficit was increased. Subsequent measurements indicated that the dependence of net photosynthesis on intercellular CO₂ concentration changed with water vapor pressure deficit.     

Fluxes of carbon dioxide and water vapour over an undisturbed tropical forest in south-west Amazonia

• 1 Carbon dioxide and water vapour fluxes were measured for 55 days by eddy covariance over an undisturbed tropical rain forest in Rondonia, Brazil. Profiles of CO₂ inside the canopy were also measured.
• 2 During the night, CO₂ concentration frequently built up to 500 ppm throughout the canopy as a result of low rates of exchange with the atmosphere. In the early morning hours, ventilation of the canopy occurred.
• 3 Ecosystem gas exchange was calculated from a knowledge of fluxes above the canopy and changes of CO₂ stored inside the canopy. Typically, uptake by the canopy was 15 μmol m^?2 s^?1 in bright sunlight and dark respiration was 6-7 μmol m^?2 s^?1 The quantum requirement at low irradiance was: 40 mol photons per mol of CO₂.
• 4 Bulk stomatal conductance of the ecosystem was maximal in the early morning (0.4-1.0 mol m^?2 s^?1) and declined over the course of the day as leaf-to-air vapour pressure difference increased.