Photosynthetic parameters in seedlings of Eucalyptus grandis as affected by rate of nitrogen supply

Seedlings of Eucalyptus grandis were grown at five different rates of nitrogen supply. Once steady‐state growth rates were established, a detailed set of CO₂ and water vapour exchange measurements were made to investigate the effects of leaf nitrogen content (N), as determined by nitrogen supply rate, on leaf structural, photosynthetic, respiratory and stomatal properties. Gas exchange data were used to parametrize the Farquhar–von Caemmerer photosynthesis model. Leaf mass per area (LMA) was negatively correlated to N. A positive correlation was observed between both day (R_d) and night respiration (R_n) and N when they were expressed on a leaf mass basis, but no correlation was found on a leaf area basis. An R_d/R_n ratio of 0·59 indicated a significant inhibition of dark respiration by light. The maximum net CO₂ assimilation rate at ambient CO₂ concentration (A_max), the maximum rate of potential electron transport (J_max) and the maximum rate of carboxylation (V_cmax) significantly increased with N, particularly when expressed on a mass basis. Although the maximum stomatal conductance to CO₂ (g_scmax) was positively correlated with A_max, there was no relationship between g_scmax and N. Leaf N content influenced the allocation of nitrogen to photosynthetic processes, resulting in a decrease of the J_max/V_cmax ratio with increasing N. It was concluded that leaf nitrogen concentration is a major determinant of photosynthetic capacity in Eucalyptus grandis seedlings and, to a lesser extent, of leaf respiration and nitrogen partitioning among photosynthetic processes, but not of stomatal conductance.     

Partitioning net ecosystem carbon exchange with isotopic fluxes of CO2

Because biological and physical processes alter the stable isotopic composition of atmospheric CO₂, variations in isotopic content can be used to investigate those processes. Isotopic flux measurements of ¹³CO₂ above terrestrial ecosystems can potentially be used to separate net ecosystem CO₂ exchange (NEE) into its component fluxes, net photosynthetic assimilation (F_A) and ecosystem respiration (F_R). In this paper theory is developed to partition measured NEE into F_A and F_R, using measurements of fluxes of CO₂ and ¹³CO₂, and isotopic composition of respired CO₂ and forest air. The theory is then applied to fluxes measured (or estimated, for ¹³CO₂) in a temperate deciduous forest in eastern Tennessee (Walker Branch Watershed). It appears that there is indeed enough additional information in ¹³CO₂ fluxes to partition NEE into its photosynthetic and respiratory components. Diurnal patterns in F_A and F_R were obtained, which are consistent in magnitude and shape with patterns obtained from NEE measurements and an exponential regression between night‐time NEE and temperature (a standard technique which provides alternate estimates of F_R and F_A). The light response curve for photosynthesis (F_A vs. PAR) was weakly nonlinear, indicating potential for saturation at high light intensities. Assimilation‐weighted discrimination against ¹³CO₂ for this forest during July 1999 was 16.8–17.1‰, depending on canopy conductance. The greatest uncertainties in this approach lie in the evaluation of canopy conductance and its effect on whole‐canopy photosynthetic discrimination, and thus the indirect methods used to estimate isotopic fluxes. Direct eddy covariance measurements of ¹³CO₂ flux are needed to assess the validity of the assumptions used and provide defensible isotope‐based estimates of the component fluxes of net ecosystem exchange.     

Partitioning net ecosystem carbon exchange and the carbon isotopic disequilibrium in a subalpine forest

We investigate the utility of an improved isotopic method to partition the net ecosystem exchange of CO₂ (F) into net photosynthesis (F_A) and nonfoliar respiration (F_R). Measurements of F and the carbon isotopic content in air at a high‐elevation coniferous forest (the Niwot Ridge AmeriFlux site) were used to partition F into F_A and F_R. Isotopically partitioned fluxes were then compared with an independent flux partitioning method that estimated gross photosynthesis (GEE) and total ecosystem respiration (TER) based on statistical regressions of night‐time F and air temperature. We compared the estimates of F_A and F_R with expected canopy physiological relationships with light (photosynthetically active radiation) and air temperature. Estimates of F_A and GEE were dependent on light as expected, and TER, but not F_R, exhibited the expected dependence on temperature. Estimates of the isotopic disequilibrium D , or the difference between the isotopic signatures of net photosynthesis (δ_A, mean value ?24.6‰) and ecosystem respiration (δ_R, mean value ?25.1‰) were generally positive (δ_A>δ_R). The sign of D observed here is inconsistent with many other studies. The key parameters of the improved isotopic flux partitioning method presented here are ecosystem scale mesophyll conductance (g_m) and maximal vegetative stomatal conductance (g_cmax). The sensitivity analyses of F_A, F_R, and D to g_cmax indicated a critical value of g_cmax (0.15 mol m^?2 s^?1) above which estimates of F_A and F_R became larger in magnitude relative to GEE and TER. The value of D decreased with increasing values of g_m and g_cmax, but was still positive across all values of g_m and g_cmax. We conclude that the characterization of canopy‐scale mesophyll and stomatal conductances are important for further progress with the isotope partitioning method, and to confirm our anomalous isotopic disequilibrium findings.     

Overestimation of respiration rates in commercially available clamp-on leaf chambers. Complications with measurement of net photosynthesis

The possible interference when measuring gas exchange with respiratory CO₂ produced under the gasket of commercially available clamp‐on leaf chambers was investigated. Two of these chambers were compared with a leaf chamber that accommodated an entire leaf without clamping it under a gasket. An overestimation of dark respiration rate (R_D) by 55% was found with Plantago major leaves, a species with homobaric leaves that have high resistance for lateral gaseous transport. The percentage was similar in the heterobaric Ficus benjamina, but was 32% in the highly porous homobaric Nicotiana tabacum. Net photosynthetic rate at low photon flux density was underestimated by 35% in the clamp‐on chamber. However, the gasket effect was not detectable at light saturation because the error was small in comparison with the high photosynthetic rates. Estimation of respiration in the light (R_L) in Nicotiana as derived from CO₂ exchange at low CO₂ concentrations was complicated by three factors. The inward diffusion of respiratory CO₂ from under the gasket was added to a diffusion of CO₂ from outside through the gasket material and through the leaf, which produced an even larger error in R_L in comparison with R_D at ambient CO₂. These errors are significant for estimations of carbon gain at whole plant and canopy level and also at the leaf level when photosynthetic rates are low. Possible improvements in gasket design and corrections of CO₂ exchange measurements for the gasket effect are discussed.     

Integration of Process-based Soil Respiration Models with Whole-Ecosystem CO2 Measurements

We integrated soil models with an established ecosystem process model (SIPNET, simplified photosynthesis and evapotranspiration model) to investigate the influence of soil processes on modelled values of soil CO₂ fluxes (R _Soil). Model parameters were determined from literature values and a data assimilation routine that used a 7-year record of the net ecosystem exchange of CO₂ and environmental variables collected at a high-elevation subalpine forest (the Niwot Ridge AmeriFlux site). These soil models were subsequently evaluated in how they estimated the seasonal contribution of R _Soil to total ecosystem respiration (TER) and the seasonal contribution of root respiration (R _Root) to R _Soil. Additionally, these soil models were compared to data assimilation output of linear models of soil heterotrophic respiration. Explicit modelling of root dynamics led to better agreement with literature values of the contribution of R _Soil to TER. Estimates of R _Soil/TER when root dynamics were considered ranged from 0.3 to 0.6; without modelling root biomass dynamics these values were 0.1–0.3. Hence, we conclude that modelling of root biomass dynamics is critically important to model the R _Soil/TER ratio correctly. When soil heterotrophic respiration was dependent on linear functions of temperature and moisture independent of soil carbon pool size, worse model-data fits were produced. Adding additional complexity to the soil pool marginally improved the model-data fit from the base model, but issues remained. The soil models were not successful in modelling R _Root/R _Soil. This is partially attributable to estimated turnover parameters of soil carbon pools not agreeing with expected values from literature and being poorly constrained by the parameter estimation routine. We conclude that net ecosystem exchange of CO₂ alone cannot constrain specific rhizospheric and microbial components of soil respiration. Reasons for this include inability of the data assimilation routine to constrain soil parameters using ecosystem CO₂ flux measurements and not considering the effect of other resource limitations (for example, nitrogen) on the microbe biomass. Future data assimilation studies with these models should include ecosystem-scale measurements of R _Soil in the parameter estimation routine and experimentally determine soil model parameters not constrained by the parameter estimation routine.     

Effects of root diameter and root nitrogen concentration on in situ root respiration among different seasons and tree species

Knowledge of root respiration is a prerequisite for a better understanding of ecosystem carbon budget and carbon allocation. However, there are not many relevant data in the literature on direct measurements of in situ root respiration by root chamber method. Furthermore, few studies have been focused on the effects of root diameter (D _r) and root nitrogen concentration (N _r) on in situ root respiration among different seasons and tree species. To address these goals, we used a simplified root-chamber system to measure in situ root respiration rates of Acacia crassicarpa and Eucalyptus urophylla in subtropical plantations of south China. We found that the species and season variation in root respiration were affected by D _r and N _r. Also, the root respiration per unit dry mass (R _r, nmol CO₂ g⁻¹ s⁻¹) and root respiration per unit N (R _n, nmol CO₂ g N⁻¹ s⁻¹) were affected by D _r and N _r. The R _r, R _n, N _r and soil temperature sensitivity (Q ₁₀) of R _r for the two species significantly decreased with an increase of D _r. The R _r of the two species showed significant an inter-seasonal and diurnal pattern, and this trend decreased with increasing D _r. Both the R _r and Q ₁₀ of the two species increased with increasing N _r. The D _r and N _r explained 54 and 52% of the observed variation in R _r for A. crassicarpa, and 65 and 70% for E. urophylla. The R _r, N _r, and Q ₁₀ of A. crassicarpa were significantly higher than those of E. urophylla. Our results indicated that root respiration was dependent on D _r and N _r, and this dependence varied with season and plant species.     

Different Pathways are Involved in the Enhancement of Photosynthetic Rate by Sodium Bisulfite and Benzyladenine, a Case Study with Strawberry (Fragaria×Ananassa Duch) Plants

In order to understand the pathway involved in the chemical enhancement of photosynthetic rate, sodium bisulfite (NaHSO₃) and benzyladenine (BA), a growth regulator, were applied to strawberry plants. The influence of these compounds on gas exchange and millisecond delayed light emission (ms-DLE) was investigated using 2-month-old plants. Results showed the net photosynthetic rate (A) in leaves was promoted by both NaHSO₃ and BA. Stomatal conductance (g) and transpiration rate (E) were significantly increased only by BA, while intercellular CO₂ concentration (C_i) was significantly decreased by NaHSO₃. The enhancement of A by NaHSO₃ and BA was only a short-term effect, lasting approximately 5 days for NaHSO₃ and 30 h for BA. Plants treated with NaHSO₃, BA or NaHSO₃ + BA, showed no significant fluctuations in carboxylation efficiency (CE), photorespiration (R_P) or dark respiration (R_D). These results suggest that the influences of NaHSO₃ and BA on gas exchange particularly A, could be via different mechanisms: the enhancement of A by the application of low concentrations of NaHSO₃ appears to be associated with increased cyclic electron flow, while BA enhancement of A is at least partially due to increased g and/or E.     

Coupling between carbon cycling and climate in a high-elevation,subalpine forest: a model-data fusion analysis

Fundamental questions exist about the effects of climate on terrestrial net ecosystem CO₂ exchange (NEE), despite a rapidly growing body of flux observations. One strategy to clarify ecosystem climate–carbon interactions is to partition NEE into its component fluxes, gross ecosystem CO₂ exchange (GEE) and ecosystem respiration (R _E), and evaluate the responses to climate of each component flux. We separated observed NEE into optimized estimates of GEE and R _E using an ecosystem process model combined with 6 years of continuous flux data from the Niwot Ridge AmeriFlux site. In order to gain further insight into the processes underlying NEE, we partitioned R _E into its components: heterotrophic (R _H) and autotrophic (R _A) respiration. We were successful in separating GEE and R _E, but less successful in accurately partitioning R _E into R _A and R _H. Our failure in the latter was due to a lack of adequate contrasts in the assimilated data set to distinguish between R _A and R _H. We performed most model runs at a twice-daily time step. Optimizing on daily-aggregated data severely degraded the model’s ability to separate GEE and R _E. However, we gained little benefit from using a half-hourly time step. The model-data fusion showed that most of the interannual variability in NEE was due to variability in GEE, and not R _E. In contrast to several previous studies in other ecosystems, we found that longer growing seasons at Niwot Ridge were correlated with less net CO₂ uptake, due to a decrease of available snow-melt water during the late springtime photosynthetic period. Warmer springtime temperatures resulted in increased net CO₂ uptake only if adequate moisture was available; when warmer springtime conditions led into mid-summer drought, the annual net uptake declined.     

Elevated CO2 does not affect stem CO2 efflux nor stem respiration in a dry Eucalyptus woodland,but it shifts the vertical gradient in xylem [CO2]

To quantify stem respiration (R_S) under elevated CO₂ (eCO₂), stem CO₂ efflux (E_A) and CO₂ flux through the xylem (F_T) should be accounted for, because part of respired CO₂ is transported upwards with the sap solution. However, previous studies have used E_A as a proxy of R_S, which could lead to equivocal conclusions. Here, to test the effect of eCO₂ on R_S, both E_A and F_T were measured in a free‐air CO₂ enrichment experiment located in a mature Eucalyptus native forest. Drought stress substantially reduced E_A and R_S, which were unaffected by eCO₂, likely as a consequence of its neutral effect on stem growth in this phosphorus‐limited site. However, xylem CO₂ concentration measured near the stem base was higher under eCO₂, and decreased along the stem resulting in a negative contribution of F_T to R_S, whereas the contribution of F_T to R_S under ambient CO₂ was positive. Negative F_T indicates net efflux of CO₂ respired below the monitored stem segment, likely coming from the roots. Our results highlight the role of nutrient availability on the dependency of R_S on eCO₂ and suggest stimulated root respiration under eCO₂ that may shift vertical gradients in xylem [CO₂] confounding the interpretation of E_A measurements.     

A new viewpoint to understand the response of leaf dark respiration to elevated CO<Subscript>2</Subscript> concentration

By investigating the R _D-C _a (dark respiration rate-atmospheric CO₂ concentration) and P _N (net photosynthetic rate)-C _a curves of bamboo (Fargesia denudata) and poplar (Populus cathayanna), we found that: (1) the minimal R _D was close to ambient CO₂ concentration, and the elevated or decreased atmospheric CO₂ concentration enhanced the R _D of both species; (2) the response curves of R _D-C _a were simulated well by quadratic function. This phenomenon might be an inherent property of leaf R _D of F. denudata and P. cathayanna. If this was true, it implies that effect of CO₂ on R _D could be interpreted with the relationship of R _D-C _a curves and the quadratic function.     

Sensitivity of CO<Subscript>2</Subscript> Exchange of Fen Ecosystem Components to Water Level Variation

Abstract Climate change is predicted to bring about a water level (WL) draw-down in boreal peatlands. This study aimed to assess the effect of WL on the carbon dioxide (CO₂) dynamics of a boreal oligotrophic fen ecosystem and its components; Sphagnum mosses, sedges, dwarf shrubs and the underlying peat. We measured CO₂ exchange with closed chambers during four growing seasons in a study site that comprised different vegetation treatments. WL gradient in the site was broadened by surrounding half of the site with a shallow ditch that lowered the WL by 10–25 cm. We modeled gross photosynthesis (P _G) and ecosystem respiration (R _ECO) and simulated the CO₂ exchange in three WL conditions: prevailing and WL draw-down scenarios of 14 and 22 cm. WL draw-down both reduced the P _G and increased the R _ECO, thus leading to a smaller net CO₂ uptake in the ecosystem. Of the different components, Sphagnum mosses were most sensitive to WL draw-down and their physiological activities almost ceased. Vascular plant CO₂ exchange, en bloc, hardly changed but whereas sedges contributed twice as much to the CO₂ exchange as shrubs in the prevailing conditions, the situation was reversed in the WL draw-down scenarios. Peat respiration was the biggest component in R _ECO in all WL conditions and the increase in R _ECO following the WL draw-down was due to the increase in peat respiration. The results imply that functional diversity buffers the ecosystem against environmental variability and that in the long term, WL draw-down changes the vegetation composition of boreal fens.     

Seasonal changes in the contribution of root respiration to total soil respiration in a cool-temperate deciduous forest

A trenching method was used to determine the contribution of root respiration to soil respiration. Soil respiration rates in a trenched plot (R _trench) and in a control plot (R _control) were measured from May 2000 to September 2001 by using an open-flow gas exchange system with an infrared gas analyser. The decomposition rate of dead roots (R _D) was estimated by using a root-bag method to correct the soil respiration measured from the trenched plots for the additional decaying root biomass. The soil respiration rates in the control plot increased from May (240–320 mg CO₂ m^–2 h^–1) to August (840–1150 mg CO₂ m^–2 h^–1) and then decreased during autumn (200–650 mg CO₂ m^–2 h^–1). The soil respiration rates in the trenched plot showed a similar pattern of seasonal change, but the rates were lower than in the control plot except during the 2 months following the trenching. Root respiration rate (R _r) and heterotrophic respiration rate (R _h) were estimated from R _control, R _trench, and R _D. We estimated that the contribution of R _r to total soil respiration in the growing season ranged from 27 to 71%. There was a significant relationship between R _h and soil temperature, whereas R _r had no significant correlation with soil temperature. The results suggest that the factors controlling the seasonal change of respiration differ between the two components of soil respiration, R _r and R _h.     

A branch bag technique for simultaneous CO2 enrichment and assimilation measurements on beech (Fagus sylvatica L.)

A cheap CO₂ enrichment system was designed to perform continuous gas exchange measurements of branches of mature European beech trees (Fagus sylvatica L.). Branches were grown at ambient (350 cm³ m^-3) and elevated CO₂ (700cm³ m^-3) during the whole 1992 leafy period. Leaks resulting from airtightness defaults in the system appeared to be low enough to measure accurately net CO₂ assimilation and transpiration rates during the day. However, the CO₂ exchange rates during the night (respiration) were too low to allow accurate measurements. Elevated CO₂ had a great effect on the net assimilation rate of branches via its influence on both the C₃ photosynthetic pathway and the shade-tolerance of beech trees (85% increase). The A/Ca curves showed no acclimation effect to high CO₂, both control and enriched branches increasing their net assimilation in the same way. The decrease of net assimilation rates in mature leaves was similar for both control and enriched branches. The pattern of daily transpiration rates remained the same for both control and enriched branches, hence we can assume that there was no visible CO₂ effect on stomata.     

Photosynthesis and dark respiration of leaves of terrestrial carnivorous plants

Using CO₂ gasometry, net photosynthetic (P _N) and dark respiration rates (R _D) were measured in leaves or traps of 12 terrestrial carnivorous plant species usually grown in the shade. Generally, mean maximum P _N (60 nmol CO₂ g⁻¹(DM) s⁻¹ or 2.7 μmol m⁻² s⁻¹) was low in comparison with that of vascular non-carnivorous plants but was slightly higher than that reported elsewhere for carnivorous plants. After light saturation, the facultatively heliophytic plants behaved as shade-adapted plants. Mean R _D in leaves and traps of all species reached about 50% of maximum P _N and represents the high photosynthetic (metabolic) cost of carnivory.     

Exposure to an enriched CO2 atmosphere alters carbon assimilation and allocation in a pine forest ecosystem

We linked a leaf-level CO₂ assimilation model with a model that accounts for light attenuation in the canopy and measurements of sap-flux-based canopy conductance into a new canopy conductance-constrained carbon assimilation (4C-A) model. We estimated canopy CO₂ uptake (A_nC) at the Duke Forest free-air CO₂ enrichment (FACE) study. Rates of A_nC estimated from the 4C-A model agreed well with leaf gas exchange measurements (A_net) in both CO₂ treatments. Under ambient conditions, monthly sums of net CO₂ uptake by the canopy (A_nC) were 13% higher than estimates based on eddy-covariance and chamber measurements. Annual estimates of A_nC were only 3% higher than carbon (C) accumulations and losses estimated from ground-based measurements for the entire stand. The C budget for the Pinus taeda component was well constrained (within 1% of ground-based measurements). Although the closure of the C budget for the broadleaf species was poorer (within 20%), these species are a minor component of the forest. Under elevated CO₂, the C used annually for growth, turnover, and respiration balanced only 80% of the A_nC. Of the extra 700 g C m⁻² a⁻¹ (1999 and 2000 average), 86% is attributable to surface soil CO₂ efflux. This suggests that the production and turnover of fine roots was underestimated or that mycorrhizae and rhizodeposition became an increasingly important component of the C balance. Under elevated CO₂, net ecosystem production increased by 272 g C m⁻² a⁻¹: 44% greater than under ambient CO₂. The majority (87%) of this C was sequestered in a moderately long-term C pool in wood, with the remainder in the forest floor–soil subsystem.     

Physiological and environmental regulation of interannual variability in CO2 exchange on rangelands in the western United States

For most ecosystems, net ecosystem exchange of CO₂ (NEE) varies within and among years in response to environmental change. We analyzed measurements of CO₂ exchange from eight native rangeland ecosystems in the western United States (58 site‐years of data) in order to determine the contributions of photosynthetic and respiratory (physiological) components of CO₂ exchange to environmentally caused variation in NEE. Rangelands included Great Plains grasslands, desert shrubland, desert grasslands, and sagebrush steppe. We predicted that (1) week‐to‐week change in NEE and among‐year variation in the response of NEE to temperature, net radiation, and other environmental drivers would be better explained by change in maximum rates of ecosystem photosynthesis (A_max) than by change in apparent light‐use efficiency (α) or ecosystem respiration at 10 °C (R₁₀) and (2) among‐year variation in the responses of NEE, A_max, and α to environmental drivers would be explained by changes in leaf area index (LAI). As predicted, NEE was better correlated with A_max than α or R₁₀ for six of the eight rangelands. Week‐to‐week variation in NEE and physiological parameters correlated mainly with time‐lagged indices of precipitation and water‐related environmental variables, like potential evapotranspiration, for desert sites and with net radiation and temperature for Great Plains grasslands. For most rangelands, the response of NEE to a given change in temperature, net radiation, or evaporative demand differed among years because the response of photosynthetic parameters (A_max, α) to environmental drivers differed among years. Differences in photosynthetic responses were not explained by variation in LAI alone. A better understanding of controls on canopy photosynthesis will be required to predict variation in NEE of rangeland ecosystems.     

Oxygen consumption during leaf nitrate assimilation in a C3 and C4 plant: the role of mitochondrial respiration

Measurements of net fluxes of CO₂ and O₂ from leaves and chlorophyll a fluorescence were used to determine the role of mitochondrial respiration during nitrate (NO₃^–) assimilation in both a C₃ (wheat) and a C₄ (maize) plant. Changes in the assimilatory quotient (net CO₂ consumed over net O₂ evolved) when the nitrogen source was shifted from NO₃^– to NH₄⁺ (ΔAQ) provided a measure of shoot NO₃^– assimilation. According to this measure, elevated CO₂ inhibited NO₃^– assimilation in wheat but not maize. Net O₂ exchange under ambient CO₂ concentrations increased in wheat plants receiving NO₃^– instead of NH₄⁺, but gross O₂ evolution from the photosynthetic apparatus (J_O2) was insensitive to nitrogen source. Therefore, O₂ consumption within wheat photosynthetic tissue (ΔΟ₂), the difference between J_O2 and net O₂ exchange, decreased during NO₃^– assimilation. In maize, NO₃^– assimilation was insensitive to changes in intercellular CO₂ concentration (C_i); nonetheless, ΔΟ₂ at low C_i values was significantly higher in NO₃^–‐fed than in NH₄⁺‐fed plants. Changes in O₂ consumption during NO₃^– assimilation may involve one or more of the following processes: (a) Mehler ascorbate peroxidase (MAP) reactions; (b) photorespiration; or (c) mitochondrial respiration. The data presented here indicates that in wheat, the last process, mitochondrial respiration, is decreased during NO₃^– assimilation. In maize, NO₃^– assimilation appears to stimulate mitochondrial respiration when photosynthetic rates are limiting.     

Ca<Superscript>2+</Superscript> reduces the effect of hypoxia in mosses <Emphasis Type="Italic">Mnium undulatum</Emphasis> and <Emphasis Type="Italic">Polytrichum commune</Emphasis>

Gametophores of mosses Mnium undulatum and Polytrichum commune were submerged in distilled water or in calcium chloride solution (0.9 mM Ca²⁺) to induce hypoxia. The net photosynthetic (P_N) and dark respiration rate (R_D) were measured in the air containing 300–400 μmol(CO₂)·mol⁻¹(air) and 0.21 mol(O₂)·mol⁻¹(air). P_N of M. undulatum gametophores decreased to 58 % of the control after 1-h submersion in water, whereas to 80 % of the control in P. commune gametophores. A smaller decrease in P_N was observed when the gametophores were immersed in CaCl₂ solution. In hypoxia, R_D in the tested mosses species was a little higher than in the control.     

Diurnal and seasonal variation of carbon dioxide exchange from a former true raised bog

Carbon dioxide exchange was measured, using the eddy covariance technique, during a one and a half year period in 1994 and 1995. The measurements took place over a former true raised bog, characterized by a shallow peat layer and a vegetation dominated by Molinia caerulea. The growing season extended from May until late October, with a maximum LAI in August of 1.7. The carbon balance shows a net release of 97 g C m^–2 y^–1 (265 kg C ha^–1 y^–1) from the peat bog ecosystem to the atmosphere. During June, July and August there is net consumption of CO₂, while during the rest of the year there is net production of CO₂. The average daytime assimilation rates ranged between – 0.2 and – 0.5 mg CO₂ m^–2 s^–1 (– 45 and –11.3 μmol CO₂ m^–2 s^–1), in a period where the LAI ranged between 1 and 1.7. A high vapour pressure deficit (> 15 hPa) corresponding with high temperatures was found to reduce the assimilation rate by on average 50%. Apart from these factors, LAI and the soil temperature codetermine the net exchange of CO₂. The total nocturnal respiration during the growing season lies within the same order as the average daytime net assimilation rate. Temperature was found to be the main factor controlling soil respiration, with a Q₁₀ of 4.8.     

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.