Sources and sinks of aquatic carbon in a peatland stream continuum

Streams draining peatland systems contain a number of different C-species, all of which are linked either directly or indirectly to the cycling of C in the terrestrial environment. Concentrations and fluxes of dissolved, particulate and gaseous forms of carbon were measured along a network of streams draining an acidic peatland catchment (46.3km2) in NE Scotland. The main aim was to identify sources and sinks of all the major forms of C in the drainage network and use this to develop a conceptual understanding of the evolution of streamwater chemistry along a peatland stream continuum. The investigation included a small-scale intra-catchment study of three contiguous sites in a 1.3km2 headwater catchment (Brocky Burn) and a larger scale integrated study of seven sites. Mean annual fluxes of the main carbon species varied from 115–215 (DOC), 8.15–97.0 (POC), 0.32–6.90 (HCO3--C) and 2.62–10.4 (free CO2-C)kgCha–1year–1; all contributed to the overall carbon flux to varying degrees. Methane-C was only measurable at sites within areas of deep peat (<0.01–0.09kgCha–1year–1). Downstream spatial changes in the intra-catchment study (Brocky Burn) were characterised by a decrease in DOC, CO2-C and CH4-C and an increase in POC fluxes over a distance of 1.1km from the Upper to the Lower sites. In the context of the integrated catchment study estimated losses and gains of carbon from the water column showed no net change in DOC, a large decrease in POC (–55%) and a slight increase in (HCO3--C) (+7.7%) and CO2-C (+4.5%). A significant decrease in the CO2-C flux: HCO3-C flux ratio with distance downstream from the stream source, illustrates the importance of outgassing of CO2 from streams draining peatland C reservoirs. These data are interpreted in the context of losses and gains of the various components of the aquatic C flux along the peatland stream continuum.     

Effects of microtopography and water table on Sphagnum palustre L. in subtropical high mountains and implications for peatland restoration

Introduction. Human disturbance has recently led to increasingly serious destruction of Sphagnum L. wetlands in subtropical high mountains, resulting in an urgent need for wetland restoration.

Methods. Through a field experiment conducted in western Hubei Province, China, the effects of four different microtopographic types [concave surface, convex surface, concave and convex surface (CC surface), and flat surface] and water table depth (0 to ?30?cm) on three growth indicators (number of capitula, coverage and biomass) of Sphagnum palustre L. were examined. The objective was to identify the optimal hydrological conditions for S. palustre growth and thus facilitate its rapid recolonisation and restoration of these wetlands.

Key results. The results showed that different microtopographic conditions significantly influenced S. palustre growth. Among them, S. palustre in the CC surface showed the worst growth, while no significant differences existed among the other three microtopographic types. Additionally, as the water table increased, the growth of S. palustre increased, but long-term flooding impeded growth. The water table affected S. palustre growth via effects on its tissue water content.

Conclusions. Microtopographic reshaping was not essential for the success of S. palustre recolonisation, and microtopography that maintained the water table to within ?10?cm of the surface without flooding were best, independent of the microtopographic types. In addition, the growth patterns of S. palustre changed with changes in the environment, which may be related to its long-term adaptation to conditions of a lower water table.     

Carbon balance of a boreal patterned peatland

Measurements of the spatial variability of methane (CH₄) emissions, net CO₂ ecosystem exchange (NEE), and dissolved carbon (CH₄, CO₂, and DOC) were made in a boreal patterned peatland in northern Sweden in the summers (May to September) of 1992 and 1993. Carbon balance terms were measured and the carbon balance inferred at different peatland surface topography features (e.g. ridges, lawns, and pools) and at different positions within the peatland (e.g. plateau, margin). Combining these data permits a comparison of the carbon balance at the peatland scale for the two field seasons. Trends in the spatial variability of the net carbon storage, as determined by the difference between inputs and outputs, suggest that carbon storage decreased in lawns from the margin of the peatland to the central plateau, while the reverse trend occurred in ridges. This indicates a difference in carbon exchange processes between sites with different surface topography due to differences in soil moisture and temperature. Total carbon storage for the peatland, weighted for topographic variability, indicates that the peatland gained carbon in 1992 (2.0 g C m^{? 2}), but lost carbon in 1993 ( ? 7.6 g C m^{? 2}). There was little variation in mean seasonal air temperature and total precipitation between the two years suggesting that the timing and magnitude of temperature and precipitation variation within the growing season are important for the season carbon balance. Because the carbon storage differences were small relative to the potential errors we conclude that the peatland was neither a net sink nor source of atmospheric carbon. This research demonstrates the importance of position in a peatland for the inference of long‐term carbon accumulation and the assessment of contemporary exchange rates.     

On the relationship between water table depth and water vapor and carbon dioxide fluxes in a minerotrophic fen

The focus of this study is the relationship between water table depth (WTD) and water vapor [evapotranspiration (ET)] and carbon dioxide [CO₂; net ecosystem exchange (NEE)] fluxes in a fen in western Canada. We analyzed hydrological and eddy covariance measurements from four snow‐free periods (2003–2006) with contrasting meteorological conditions to establish the link between daily WTD and ET and gross ecosystem CO₂ exchange (GEE) and ecosystem respiration (R_eco; NEE=R_eco?GEE), respectively: 2003 was warm and dry, 2004 was cool and wet, and 2005 and 2006 were both wet. In 2003, the water table (WT) was below the ground surface. In 2004, the WT rose above the ground surface, and in 2005 and 2006, the WT stayed well above the ground surface. There were no significant differences in total ET (～316 mm period^?1), but total NEE was significantly different (2003: 8 g C m^?2 period^?1; 2004: ?139 g C m^?2 period^?1; 2005: ?163 g C m^?2 period^?1; 2006: ?195 g C m^?2 period^?1), mostly due to differences in total GEE (2003: 327 g C m^?2 period^?1; 2004: 513 g C m^?2 period^?1; 2005: 411 g C m^?2 period^?1; 2006: 556 g C m^?2 period^?1). Variation in ET is mostly explained by radiation (67%), and the contribution of WTD is only minor (33%). WTD controls the compensating contributions of different land surface components, resulting in similar total ET regardless of the hydrological conditions. WTD and temperature each contribute about half to the explained variation in GEE up to a threshold ponding depth, below which temperature alone is the key explanatory variable. WTD is only of minor importance for the variation in R_eco, which is mainly controlled by temperature. Our study implies that future peatland modeling efforts explicitly consider topographic and hydrogeological influences on WTD.     

Microbial activity and enzymic decomposition processes following peatland water table drawdown

Microbial activity and enzymic decomposition processes were followed during a field-based experimental lowering of the water table in a Welsh peatland. Respiration was not significantly affected by the treatment. However, the enzymes sulphatase, -glucosidase and phosphatase were stimulated by between 31 and 67% upon water table drawdown. A further enzyme, phenol oxidase, was not significantly affected. The observation of elevated enzyme activities without an associated increase in microbial respiratory activity suggests that drought conditions influence peatland mineralisation rates through a direct stimulation of existing enzymes, rather than through a generalised stimulation of microbial metabolism (with associated de-novo enzyme synthesis). Hydrochemical data suggest that the stimulation may have been caused by a reduction in the inhibitory action of iron and phenolics in the peat pore waters. Overall, the findings support the recent hypothesis that drier conditions associated with climate change could stimulate mineralisation within wetlands. ei]R Merckx     

Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO2

Atmospheric CO₂ enrichment may stimulate plant growth directly through (1) enhanced photosynthesis or indirectly, through (2) reduced plant water consumption and hence slower soil moisture depletion, or the combination of both. Herein we describe gas exchange, plant biomass and species responses of five native or semi-native temperate and Mediterranean grasslands and three semi-arid systems to CO₂ enrichment, with an emphasis on water relations. Increasing CO₂ led to decreased leaf conductance for water vapor, improved plant water status, altered seasonal evapotranspiration dynamics, and in most cases, periodic increases in soil water content. The extent, timing and duration of these responses varied among ecosystems, species and years. Across the grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO₂ enrichment were relatively greater in dry years. In contrast, CO₂-induced aboveground biomass increases in the Texas C₃/C₄ grassland and the New Zealand pasture seemed little or only marginally influenced by yearly variation in soil water, while plant growth in the Mojave Desert was stimulated by CO₂ in a relatively wet year. Mediterranean grasslands sometimes failed to respond to CO₂-related increased late-season water, whereas semiarid Negev grassland assemblages profited. Vegetative and reproductive responses to CO₂ were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to functional groups. Results suggest that the indirect effects of CO₂ on plant and soil water relations may contribute substantially to experimentally induced CO₂-effects, and also reflect local humidity conditions. For landscape scale predictions, this analysis calls for a clear distinction between biomass responses due to direct CO₂ effects on photosynthesis and those indirect CO₂ effects via soil moisture as documented here.     

Diurnal variation and interrelations of ecophysiological parameters in three peatland woody species under different weather and soil moisture conditions

Summary The diurnal patterns of twig xylem water potential, net photosynthesis rate, water use efficiency of photosynthesis, and stomatal and mesophyll conductance to CO₂ in tamarack, black spruce and swamp birch growing in a natural peatland in central Alberta, Canada, were examined. The relationships of photosynthesis to other ccophysiological parameters were investigated. Data were collected on three days with different weather and soil moisture conditions in the 1988 growing season. Day 1 was clear and warm and the ground water table was 7 cm above the average peat surface. Day 2 was clear and hot. Day 3 was cloudy but warm. On day 2 and day 3, the water tables were in the normal range for that season. Major findings were: 1) Soil flooding depressed photosynthesis in tamarack and black spruce. 2) Swamp birch was better adapted to flooding than tamarack or black spruce. 3) The trees experienced water stress in the afternoons of the two days with lower water table. 4) Changes in photosynthesis of the three species were primarily affected by changes in mesophyll conductance (g_m) and the response of photosynthesis to changes in g_m was similar for all three species.     

Photosynthetic performance in Sphagnum transplanted along a latitudinal nitrogen deposition gradient

Increased N deposition in Europe has affected mire ecosystems. However, knowledge on the physiological responses is poor. We measured photosynthetic responses to increasing N deposition in two peatmoss species (Sphagnum balticum and Sphagnum fuscum) from a 3-year, north–south transplant experiment in northern Europe, covering a latitudinal N deposition gradient ranging from 0.28 g N m⁻² year⁻¹ in the north, to 1.49 g N m⁻² year⁻¹ in the south. The maximum photosynthetic rate (NP_max) increased southwards, and was mainly explained by tissue N concentration, secondly by allocation of N to the photosynthesis, and to a lesser degree by modified photosystem II activity (variable fluorescence/maximum fluorescence yield). Although climatic factors may have contributed, these results were most likely attributable to an increase in N deposition southwards. For S. fuscum, photosynthetic rate continued to increase up to a deposition level of 1.49 g N m⁻² year⁻¹, but for S. balticum it seemed to level out at 1.14 g N m⁻² year⁻¹. The results for S. balticum suggested that transplants from different origin (with low or intermediate N deposition) respond differently to high N deposition. This indicates that Sphagnum species may be able to adapt or physiologically adjust to high N deposition. Our results also suggest that S. balticum might be more sensitive to N deposition than S. fuscum. Surprisingly, NP_max was not (S. balticum), or only weakly (S. fuscum) correlated with biomass production, indicating that production is to a great extent is governed by factors other than the photosynthetic capacity.     

水位影响泥炭沼泽土壤有机碳分解的生物化学机制研究进展

在人类活动和气候变化影响下,泥炭沼泽生态系统急剧退化,其独特的氧化还原过程使得退化泥炭沼泽及其恢复过程中土壤有机碳（SOC）分解与存储机制成为研究的热点问题。泥炭沼泽排水/再湿过程会显著改变土壤的氧化还原条件,进而改变土壤微生物群落和酶活性,驱动铁氧化还原过程,影响SOC分解。已有研究对"缺氧是维持泥炭地碳存储的关键"的传统理论提出了质疑,而土壤酶及铁（Fe）在土壤SOC分解与存储过程中分别扮演着"酶锁"和"铁门"的作用,二者同时受到氧化还原条件的影响。然而,有关退化泥炭沼泽及其恢复过程中酶-土壤SOC-Fe相互作用及微生物驱动机制还有待深入。总结了干旱/排水/再湿对泥炭沼泽土壤SOC组分、分子结构、碳排放的影响,并从微生物、酶、Fe化学的角度归纳总结了泥炭沼泽土壤SOC分解的生物化学机制。未来研究中应将土壤水分与土壤SOC分解的生物地球化学机制联系起来,探寻水位变化过程中生物及非生物要素对土壤SOC分子结构变化的调控机制及土壤氧化酶-酚类物质/SOC分子结构-水解酶之间的作用机制。同时,关注Fe的氧化和还原过程,评估Fe-SOC在泥炭沼泽土壤有机碳中的地位,利用分子生物学手段探究水位变化过程中酶-SOC分解/碳排放-铁之间的权衡机制。     

Increased temperature sensitivity of net DOC production from ombrotrophic peat due to water table draw-down

The production and release of dissolved organic carbon (DOC) from peat soils is thought to be sensitive to changes in climate, specifically changes in temperature and rainfall. However, little is known about the actual rates of net DOC production in response to temperature and water table draw‐down, particularly in comparison to carbon dioxide (CO₂) fluxes. To explore these relationships, we carried out a laboratory experiment on intact peat soil cores under controlled temperature and water table conditions to determine the impact and interaction of each of these climatic factors on net DOC production. We found a significant interaction (P < 0.001) between temperature, water table draw‐down and net DOC production across the whole soil core (0 to −55 cm depth). This corresponded to an increase in the Q₁₀ (i.e. rise in the rate of net DOC production over a 10 °C range) from 1.84 under high water tables and anaerobic conditions to 3.53 under water table draw‐down and aerobic conditions between −10 and − 40 cm depth. However, increases in net DOC production were only seen after water tables recovered to the surface as secondary changes in soil water chemistry driven by sulphur redox reactions decreased DOC solubility, and therefore DOC concentrations, during periods of water table draw‐down. Furthermore, net microbial consumption of DOC was also apparent at − 1 cm depth and was an additional cause of declining DOC concentrations during dry periods. Therefore, although increased temperature and decreased rainfall could have a significant effect on net DOC release from peatlands, these climatic effects could be masked by other factors controlling the biological consumption of DOC in addition to soil water chemistry and DOC solubility. These findings highlight both the sensitivity of DOC release from ombrotrophic peat to episodic changes in water table draw‐down, and the need to disentangle complex and interacting controls on DOC dynamics to fully understand the impact of environmental change on this system.     

Soil surface CO2 flux in a Minnesota peatland

Soil surface CO₂ flux was measured in hollow and hummock microhabitats in a peatland in north central Minnesota from June to October in 1991. We used a closed infrared gas exchange system to measure soil CO₂ flux. The rates of CO₂ evolution from hummocks (9.8 ± 3.5 g m⁻² d⁻¹, [mean ± SE]) were consistently higher than those from hollows (5.4 ± 2.9 g m⁻² d⁻¹) (the hummock values included the contribution of moss dark respiration, which may account for 10–20% of the total measured flux). The soil CO₂ flux was strongly temperature-dependent (Q₁₀ ≈ 3.7) and appeared to be linearly related to changes in water table depth. An empirical multiplicative model, using peat temperature and water table depth as independent variables, explained about 81% of the variance in the CO₂ flux data. Using the empirical model with measurements of peat temperature and estimates of hollow/hummock microtopographic distribution (relative to water table elevation), daily rates of “site-averaged” CO₂ evolution were calculated. For the six-month period (May–October), the total soil CO₂ released from this ecosystem was estimated to be about 1340 g CO₂ m⁻². Published as Paper No. 9950, Journal Series, Nebraska Agricultural Research Division, University of Nebraska, Lincoln, NE, USA.     

Carbon Dioxide Emission in Relation to the Growth of Geotrichum candidum in Solid Cultures

Geotrichum candidum plays an important role in the ripening of Camembert‐type cheeses. However, the direct measurement of the biomass concentration is rather tedious and, therefore, the development of alternative methods for monitoring the growth on solid media would be very useful. For this purpose, a non‐structured model was previously developed to describe the CO₂ emission during the growth of G. candidum in liquid cultures. The CO₂ production was assumed to be partially associated with growth: a part resulted from growth and the remaining from cellular maintenance. This model has also been validated in solid cultures on peptone‐lactate based medium. The coefficients for growth‐associated and non‐growth‐associated production were found to be 0.301 and 0.123 per day, respectively. Therefore, the CO₂ production may be a non‐destructive and useful tool to monitor fungal growth in solid cultures. In the case of mixed cultures of both fungi (Geotrichum candidum and Penicillium camembertii) involved in the ripening of Camembert cheeses, CO₂ emission can be related to the total viable biomass, while ammonia and volatile sulphur compounds can be linked to G. candidum biomass. Indeed, it was previously shown that Penicillium camembertii released only very low amounts of both compounds.     

Fluxes of nitrous oxide from boreal peatlands as affected by peatland type,water table level and nitrification capacity

Peat soils with high nitrogen content are potential sources of nitrous oxide (N₂O). Fluxes of nitrous oxide were measuredin situ on nine virgin and ten drained peatlands of different hydrology and nutrient status. Numbers of nitrifying bacteria were estimated in different layers of the peat profiles with a most-probable-number technique. Nitrification potentials were determined in soil slurries of pH 4 and 6 from the profiles of six peat soils. Many virgin peatlands showed low N₂O uptake. Lowering of the water table generally increased the average fluxes of N₂O from the soils, although more in minerotrophic (nutrient rich) than in ombrotrophic (nutrient poor) sites. Ammonium oxidizing bacteria were found on only two sites but nitrite oxidizers were detected in almost all peat profiles. More nitrite oxidizers were found in drained than in virgin peat profiles. Nitrification was enhanced after lowering of the water table in minerotrophic peat but not in ombrotrophic peat. The N₂O fluxes correlated positively with the numbers of nitrite oxidizers, nitrification potential, N, P and Ca content and pH of the soil and negatively with the level of water table (expressed as negative values) and K content of the soil.     

Progress in the studies on the greenhouse gas emissions from reservoirs

The green credentials of hydroelectricity in terms of greenhouse-gas (GHG) emissions have been tarnished with the finding of the researches on GHG emissions from hydroelectric reservoirs in the last two decades. Substantial amounts of GHGs release from the tropical reservoirs, especially methane (CH₄) from Brazil’s Amazonian areas. CH₄ contributes strongly to climate change because it has a global warming potential (GWP) 24 times higher than carbon dioxide (CO₂) on a per molecule basis over a 100-year time horizon. GHGs may emit from reservoirs through four different pathways to the atmosphere: (1) diffusive flux at the reservoir surface, (2) gas bubble flux in the shallow zones of a reservoir, (3) water degassing flux at the outlet of the powerhouse downstream of turbines and spillways, and (4) flux across the air–water interface in the rivers downstream of the dams. This paper reviewed the productions and emissions of CH₄, CO₂, and N₂O in reservoirs, and the environmental variables influencing CH₄ and CO₂ emissions were also summarized. Moreover, the paper combined with the progress of GHG emissions from Three Gorges Reservoir and proposed three crucial problems to be resolved on GHG emissions from reservoirs at present, which would be benefit to estimate the total GHG emissions from Three Gorges Reservoir accurately.     

Ecosystem respiration in a heterogeneous temperate peatland and its sensitivity to peat temperature and water table depth

Background and aims

Ecosystem respiration (R _eco ) is controlled by thermal and hydrologic regimes, but their relative importance in defining the CO₂ emissions in peatlands seems to be site specific. The aim of the paper is to investigate the sensitivity of R _eco to variations in temperature and water table depth (WTD) in a wet, geogenous temperate peatland with a wide variety of vegetation community groups.

Methods

The CO₂ fluxes were measured using chambers. Measurements were made at four microsites with different vegetation communities and peat moisture and temperature conditions every 3 to 4 weeks during the period 2008–2009, 2 years with contrasting WTD patterns. Models were used to examine the relative response of each microsite to variations in peat temperature and WTD and used to estimate annual total R _eco .

Results

Temporal variations in R _eco were strongly related to peat temperature at the 5 cm depth. However, two of the microsites did not show any significant change in this relationship while two others showed contrasting responses including an increase and decrease in temperature sensitivity with deeper WTD. Average R _eco varied among the microsites and tended to be greatest for those with greatest leaf area which also positively correlated with deeper WTD, ash content and degree of peat decomposition at 20 cm. A combined temperature and WTD model explained up to 94 % of the temporal variation in daily average R _eco and was used to show that on an annual basis, R _eco was between 5 and 18 % greater in the warmer year with deeper WTD.

Conclusion

Microsite-specific responses were related to differences in vegetation and peat characteristics among microsites. R _eco may have remained insensitive to WTD variations at one microsite due to the dominance of autotrophic respiration from abundant sedge biomass. At a Sphagnum-dominated microsite, a lack of response may have been due to relatively small variations in WTD that did not greatly influence microbial respiration or due to offsets between decreasing and increasing respiration rates in near-surface and deeper peat. The microsite with the most recalcitrant peat had reduced R _eco sensitivity to temperature under more aerobic conditions while another microsite showed the opposite response, perhaps due to less nutrient availability during the wet year. Ultimately, micro-site specific models with both soil temperature and WTD as explanatory variables described temporal variations in R _eco and highlighted the significant spatial variations in respiration rates that may occur within a single wetland.     

Carbon dioxide biofixation by Chlorella vulgaris at different CO2 concentrations and light intensities

In order to develop an effective CO₂ mitigation process using microalgae for potential industrial application, the growth and physiological activity of Chlorella vulgaris in photobioreactor cultures were studied. C. vulgaris was grown at two CO₂ concentrations (2 and 13% of CO₂ v/v) and at three incident light intensities (50, 120 and 180 μmol m^?2 s^?1) for 9 days. The measured specific growth rate was similar under all conditions tested but an increase in light intensity and CO₂ concentration affected the biomass and cell concentrations. Although carbon limitation was observed at 2% CO₂, similar cellular composition was measured in both conditions. Light limitation induced a net change in the growth behavior of C. vulgaris. Nitrogen limitation seemed to decrease the nitrogen quota of the cells and rise the intracellular carbon:nitrogen ratio. Exopolysaccharide production per cell appeared to be affected by light intensity. In order to avoid underestimation of the CO₂ biofixation rate of the microalgae, exopolysaccharide production was taken into account. The maximum CO₂ removal rate (0.98 g CO₂ L^?1 d^?1) and the highest biomass concentration (4.14 g DW L^?1) were determined at 13% (v/v) CO₂ and 180 μmol m^?2 s^?1. Our results show that C. vulgaris has a real potential for industrial CO₂ remediation.     

The effect of water table levels and short-term ditch restoration on mountain peatland carbon cycling in the Cordillera Blanca,Peru

Wetlands Ecology and Management - Many tropical mountain peatlands in the Andes are formed by cushion plants. These unique cushion plant peatlands are intensively utilized for grazing and are also...     

Carbon dioxide supersaturation in Florida lakes

We examined data on CO₂ and related limnological and geographic information from a sample of 948 Florida freshwater lakes. The objectives for this study were (1) to determine the partial pressures of carbon dioxide (ρCO₂) in the surface waters of a large sample of Florida lakes, (2) to determine if several limnological or geographic factors are related to levels of ρCO₂ in Florida lakes, and (3) to estimate the net annual rate of loss of CO₂ to the atmosphere from the freshwater lakes of Florida. The calculated ρCO₂ for the lakes in our sample range from 0 to 81,000 μatm, with a mean of 3,550 μatm, a median of 1,030 μatm, and a geometric mean of 1,270 μatm. About 87% of the Florida lakes were supersaturated with CO₂. There were statistically significant correlations between values for ρCO₂ and several water chemistry variables; however, the R ² values were small and accounted for only a small portion of the variance. In general the ρCO₂ values were higher in the lakes with low alkalinities and low contents of dissolved salts. The best predictor of ρCO₂ is pH, with an R ² of 0.82 for a polynomial relationship. The ρCO₂ values tend to decrease from northwest to southeast across the state of Florida, which corresponds to the gradients we found for pH, alkalinity, and specific conductance. The average areal rate of carbon emission from the Florida lakes was 328 g C m⁻² y⁻¹, and the total carbon loss for the lakes and ponds of Florida was 2.0 Tg y⁻¹. This amounts to about 2% of the total carbon emissions from all the lakes of the world as estimated by previous studies. Handling editors: Darren Bade     

Photosynthesis of cotton plants exposed to elevated levels of carbon dioxide in the field

The cotton (Gossypium hirsutum L.) plant responds to a doubling of atmospheric CO₂ with almost doubled yield. Gas exchange of leaves was monitored to discover the photosynthetic basis of this large response. Plants were grown in the field in open-top chambers with ambient (nominally 350 l/l) or enriched (nominally either 500 or 650 l/l) concentrations of atmospheric CO₂. During most of the season, in fully-irrigated plants the relationship between assimilation (A) and intercellular CO₂ concentration (c_i) was almost linear over an extremely wide range of c_i. CO₂ enrichment did not alter this relationship or diminish photosynthetic capacity (despite accumulation of starch to very high levels) until very late in the season, when temperature was somewhat lower than at midseason. Stomatal conductance at midseason was very high and insensitive to CO₂, leading to estimates of c_i above 85% of atmospheric CO₂ concentration in both ambient and enriched chambers. Water stress caused A to show a saturation response with respect to c_i, and it increased stomatal closure in response to CO₂ enrichment. In fully-irrigated plants CO₂ enrichment to 650 l/l increased A more than 70%, but in water-stressed plants enrichment increased A only about 52%. The non-saturating response of A to c_i, the failure of CO₂ enrichment to decrease photosynthetic capacity for most of the season, and the ability of the leaves to maintain very high c_i, form in part the basis for the very large response to CO₂ enrichment.Abbreviations c_a- atmospheric CO₂ concentration - c_i- intercellular CO₂ concentration - A- rate of assimilation of CO₂ - g_s- stomatal conductance to water vapor - g_b- boundary layer conductance to water vapor - g_m- mesophyll conductance to CO₂ - VPD- vapor pressure deficit - _w leaf water potential - L- stomatal limitation to CO₂ uptake