Estimation of autotrophic and heterotrophic components of soil respiration by trenching is sensitive to corrections for root decomposition and changes in soil water content

This study aims to assess the effects of corrections for disturbances such as an increased amount of dead roots and an increase in volumetric soil water content on the calculation of soil CO₂ efflux partitioning. Soil CO₂ efflux, soil temperature and superficial soil water content were monitored in two young beech sites (H1 and H2) during a trenching experiment. Trenching induced a significant input of dead root mass that participated in soil CO₂ efflux and reduced the soil dissolved organic carbon content, while it increased superficial soil water content within the trenched plot. Annual soil CO₂ efflux in control plots was 528 g C m⁻² year⁻¹ at H1 and 527 g C m⁻² year⁻¹ at H2. The annual soil CO₂ efflux in trenched plots was 353 g C m⁻² year⁻¹ at H1 and 425 g C m⁻² year⁻¹ at H2. By taking into account annual CO₂ efflux from decaying trenched roots, the autotrophic contribution to total soil CO₂ efflux reached 69% at H1 and 54% at H2. The partitioning calculation was highly sensitive to the initial root mass estimated within the trenched plots. Uncertainties in the remaining root mass, the fraction of root C that is incorporated into soil organic matter during root decomposition, and the root decomposition rate constant had a limited impact on the partitioning calculation. Corrections for differences in superficial soil water content had a significant impact on annual respired CO₂ despite a limited effect on partitioning.     

Ponderosa Pine Responses to Elevated CO2and Nitrogen Fertilization

The effects of elevated CO₂ (ambient, +175, and +350 μl l⁻¹) and nitrogen fertilization (0, 100, and 200 kg N ha⁻¹ yr⁻¹ as ammonium sulfate) on C and N accumulations in biomass and soils planted with ponderosa pine (Pinus ponderosa Laws) over a 6-year study period are reported. Both nitrogen fertilization and elevated CO₂ caused increases in C and N contents of vegetation over the study period. The pattern of responses varied over time. Responses to CO₂ decreased in the +175 μl l⁻¹ and increased in the +350 μl l⁻¹ after the first year, whereas responses to N decreased after the first year and became non-significant by year six. Foliar N concentrations were lower and tree C:N ratios were higher with elevated CO₂ in the early years, but this was offset by the increases in biomass, resulting in substantial increases in N uptake with elevated CO₂. Nitrogen budget estimates showed that the major source of the N for unfertilized trees, with or without elevated CO₂, was likely the soil organic N pool. There were no effects of elevated CO₂ on soil C, but a significant decrease in soil N and an increase in soil C:N ratio in year six. Nitrogen fertilization had no significant effect on tree C:N ratios, foliar N concentrations, soil C content, soil N content, or soil C:N ratios. There were no significant interactions between CO₂ and N treatments, indicating that N fertilization had no effect on responses to CO₂ and that CO₂ treatments had no effect on responses to N fertilization. These results illustrate the importance of long-term studies involving more than one level of treatment to assess the effects of elevated CO₂.     

Stimulated N2O flux from intact grassland monoliths after two growing seasons under elevated atmospheric CO2

Long-term exposure of native vegetation to elevated atmospheric CO₂ concentrations is expected to increase C inputs to the soil and, in ecosystems with seasonally dry periods, to increase soil moisture. We tested the hypothesis that these indirect effects of elevated CO₂ (600 μl l⁻¹ vs 350 μl l⁻¹) would improve conditions for microbial activity and stimulate emissions of nitrous oxide (N₂O), a very potent and long-lived greenhouse gas. After two growing seasons, the mean N₂O efflux from monoliths of calcareous grassland maintained at elevated CO₂ was twice as high as that measured from monoliths maintained at current ambient CO₂ (70 ± 9 vs 37 ± 4 μg N₂O m⁻² h⁻¹ in October, 27 ± 5 vs 13 ± 3 μg N₂O m⁻² h⁻¹ in November after aboveground harvest). The higher N₂O emission rates at elevated CO₂ were associated with increases in soil moisture, soil heterotrophic respiration, and plant biomass production, but appear to be mainly attributable to higher soil moisture. Our results suggest that rising atmospheric CO₂ may contribute more to the total greenhouse effect than is currently estimated because of its plant-mediated effects on soil processes which may ultimately lead to increased N₂O emissions from native grasslands. Received: 11 September 1997 / Accepted: 20 March 1998     

Soil O2 and CO2 effects on root respiration of cacti

Through use of a recently developed technique that can measure CO₂ exchange by individual attached roots, the influences of soil O₂ and CO₂ concentrations on root respiration were determined for two species of shallow-rooted cacti that typically occur in porous, well-drained soils. Although soil O₂ concentrations in the rooting zone in the field were indistinguishable from that in the ambient air (21% by volume), the CO₂ concentrations 10 cm below the soil surface averaged 540 μLL⁻¹ for the barrel cactusFerocactus acanthodes under dry conditions and 2400 μLL⁻¹ under wet conditions in a loamy sand. For the widely cultivated platyopuntiaOpuntia ficus-indica in a sandy clay loam, the CO₂ concentration at 10 cm averaged 1080 μLL⁻¹ under dry conditions and 4170 μLL⁻¹ under wet conditions. For both species, the respiration rate in the laboratory was zero at 0% O₂ and increased to its maximum value at 5% O₂ for rain roots (roots induced by watering) and 16% O₂ for established roots. Established roots ofO. ficus-indica were slightly more tolerant of elevated CO₂ than were those ofF. acanthodes, 5000 μLL⁻¹ inhibiting respiration by 35% and 46%, respectively. For both species, root respiration was reduced to zero at 20,000 μLL⁻¹ (2%) CO₂. In contrast to the reversible effects of 0% O₂, inhibition by 2% CO₂ was irreversible and led to the death of cortical cells in established roots in 6 h. Although the restriction of various cacti and other CAM plants to porous soils has generally been attributed to their requirement for high O₂ concentrations, the present results indicate that susceptibility of root respiration to elevated soil CO₂ concentrations may be more important.     

Characteristics of soil CO2 efflux variability in an aseasonal tropical rainforest in Borneo Island

Although soil carbon dioxide (CO₂) efflux from tropical forests may play an important role in global carbon (C) balance, our knowledge of the fluctuations and factors controlling soil CO₂ efflux in the Asian tropics is still poor. This study characterizes the temporal and spatial variability in soil CO₂ efflux in relation to temperature/moisture content and estimates annual efflux from the forest floor in an aseasonal intact tropical rainforest in Sarawak, Malaysia. Soil CO₂ efflux varied widely in space; the range of variation averaged 17.4 μmol m⁻² s⁻¹ in total. While most CO₂ flux rates were under 10 μmol m⁻² s⁻¹, exceptionally high fluxes were observed sporadically at several sampling points. Semivariogram analysis revealed little spatial dependence in soil CO₂ efflux. Temperature explained nearly half of the spatial heterogeneity, but the effect varied with time. Seasonal variation in CO₂ efflux had no fixed pattern, but was significantly correlated with soil moisture content. The correlation coefficient with soil moisture content (SMC) at 30 and 60 cm depth was higher than at 10 cm depths. The annual soil CO₂ efflux, estimated from the relationship between CO₂ efflux and SMC at 30 cm depth, was 165 mol m⁻² year⁻¹ (1,986 g C m⁻² year⁻¹). As this area is known to suffer severe drought every 4–5 years caused by the El Nino-Southern Oscillation, the results suggest that an unpredictable dry period might affect soil CO₂ efflux, leading an annual variation in soil C balance.     

Temporal variation in CO<Subscript>2</Subscript> efflux from soil and snow surfaces in a Japanese cedar (<Emphasis Type="Italic">Cryptomeria japonica</Emphasis>) plantation,central Japan

CO₂ efflux from soil and snow surfaces was measured continuously in a Japanese cedar (Cryptomeria japonica D. Don) forest in central Japan using an open dynamic chamber system. The chamber opens and closes automatically and records measurements based on an open-flow dynamic method. Between May and December, mean soil CO₂ efflux ranged from 1,529 mg CO₂ m⁻² h⁻¹ in September to 255 mg CO₂ m⁻² h⁻¹ in December. The seasonal change in CO₂ efflux from the soil paralleled the seasonal pattern of soil temperature. No marked diurnal trends in soil CO₂ efflux were observed on days without rainfall, whereas significant pulses in soil CO₂ efflux were observed on days with rainfall. In this plantation, soil CO₂ efflux frequently responded to rainfall. Measurements of changes from litter-covered soil to snow-covered surfaces revealed that CO₂ efflux decreased from values of ca. 250 mg CO₂ m⁻² h⁻¹ above soil to less than 33 mg CO₂ m⁻² h⁻¹ above snow. Soil temperature alone explained 66% of the overall variation in soil CO₂ efflux, but explained approximately 85% of the variation when data from two anomalous periods were excluded. Moreover, we found a significant correlation between soil CO₂ efflux and soil moisture (which explained 44% of the overall variation) using a second-order polynomial function. Our results suggest that the seasonality of CO₂ efflux is affected not only by soil temperature and moisture, but also by drying and rewetting cycles and by litterfall pulses.     

Contribution of Lolium perenne rhizodeposition to carbon turnover of pasture soil

Carbon rhizodeposition and root respiration during eight development stages of Lolium perenne were studied on a loamy Gleyic Cambisol by ¹⁴CO₂ pulse labelling of shoots in a two compartment chamber under controlled laboratory conditions. Total ¹⁴CO₂ efflux from the soil (root respiration, microbial respiration of exudates and dead roots) in the first 8 days after ¹⁴C pulse labelling decreased during plant development from 14 to 6.5% of the total ¹⁴C input. Root respiration accounted for was between 1.5 and 6.5% while microbial respiration of easily available rhizodeposits and dead root remains were between 2 and 8% of the ¹⁴C input. Both respiration processes were found to decline during plant development, but only the decrease in root respiration was significant. The average contribution of root respiration to total ¹⁴CO₂ efflux from the soil was approximately 41%. Close correlation was found between cumulative ¹⁴CO₂ efflux from the soil and the time when maximum ¹⁴CO₂ efflux occurred (r=0.97). The average total of CO₂ Defflux from the soil with Lolium perenne was approximately 21 μg C-CO₂ d⁻¹ g⁻¹. It increased slightly during plant development. The contribution of plant roots to total CO₂ efflux from the soil, calculated as the remainder from respiration of bare soil, was about 51%. The total ¹⁴C content after 8 days in the soil with roots ranged from 8.2 to 27.7% of assimilated carbon. This corresponds to an underground carbon transfer by Lolium perenne of 6–10 g C m⁻² at the beginning of the growth period and 50–65 g C m⁻² towards the end of the growth period. The conventional root washing procedure was found to be inadequate for the determination of total carbon input in the soil because 90% of the young fine roots can be lost. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Acclimation of the summer annual species, Lolium temulentum, to CO2 enrichment

Lolium temulentum L. Ba 3081 was grown hydroponically in air (350 μmol mol⁻¹ CO₂) and elevated CO₂ (700 μmol mol⁻¹ CO₂) at two irradiances (150 and 500 μmol m⁻² s⁻¹) for 35 days at which point the plants were harvested. Elevated CO₂ did not modify relative growth rate or biomass at either irradiance. Foliar carbon-to-nitrogen ratios were decreased at elevated CO₂ and plants had a greater number of shorter tillers, particularly at the lower growth irradiance. Both light-limited and light-saturated rates of photosynthesis were stimulated. The amount of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) protein was increased at elevated CO₂, but maximum extractable Rubisco activities were not significantly increased. A pronounced decrease in the Rubisco activation state was found with CO₂ enrichment, particularly at the higher growth irradiance. Elevated-CO₂-induced changes in leaf carbohydrate composition were small in comparison to those caused by changes in irradiance. No CO₂-dependent effects on fructan biosynthesis were observed. Leaf respiration rates were increased by 68% in plants grown with CO₂ enrichment and low light. We conclude that high CO₂ will only result in increased biomass if total light input favourably increases the photosynthesis-to-respiration ratio. At low irradiances, biomass is more limited by increased rates of respiration than by CO₂-induced enhancement of photosynthesis. Received: 23 February 1999 / Accepted: 15 June 1999     

Subsurface CO<Subscript>2</Subscript> Dynamics in Temperate Beech and Spruce Forest Stands

Rates of soil respiration (CO₂ effluxes), subsurface pore gas CO₂/O₂ concentrations, soil temperature and soil water content were measured for 15 months in two temperate and contrasting Danish forest ecosystems: beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] Karst.). Soil CO₂ effluxes showed a distinct seasonal trend in the range of 0.48–3.3 μmol CO₂ m⁻² s⁻¹ for beech and 0.50–2.92 μmol CO₂ m⁻² s⁻¹ for spruce and were well-correlated with near-surface soil temperatures. The soil organic C-stock (upper 1 m including the O-horizon) was higher in the spruce stand (184±23 Mg C ha⁻¹) compared to the beech stand (93±19 Mg C ha⁻¹) and resulted in a faster turnover time as calculated by mass/flux in soil beneath the beech stand (28 years) compared to spruce stand (60 years). Observed soil CO₂ concentrations and effluxes were simulated using a Fickian diffusion-reaction model based on vertical CO₂ production rates and soil diffusivity. Temporal trends were simulated on the basis of observed trends in the distribution of soil water, temperature, and live roots as well as temperature and water content sensitivity functions. These functions were established based on controlled laboratory incubation experiments. The model was successfully validated against observed soil CO₂ effluxes and concentrations and revealed that temporal trends generally could be linked to variations in subsurface CO₂ production rates and diffusion over time and with depths. However, periods with exceptionally high CO₂ effluxes (> 20 μmol CO₂ m⁻² s⁻¹) were noted in March 2000 in relation to drying after heavy rain and after the removal of snow from collars. Both cases were considered non-steady state and could not be simulated.     

Soil CO2 efflux in a beech forest: comparison of two closed dynamic systems

The aim of this study was to understand why two closed dynamic systems with a very similar design gave large differences in soil CO₂ efflux measurements (PP systems and LI-COR). Both in the field (forest beech stand) and in the laboratory, the PPsystems gave higher estimations of soil CO₂ efflux than the LI-COR system (ranging from 30% to 50%). The difference in wind speed occurring within the soil respiration chambers (0.9 m s⁻¹ within the SRC-1 and 0.4 m s⁻¹ within the LI-6000-09 chambers) may account for the discrepancy between the two systems. An excessive air movement inside the respiration chamber is thought to disrupt the high laminar boundary layer over the forest floor. This would promote an exhaust of the CO₂ accumulated into the upper soil layers into the chamber and a lateral diffusion of CO₂ in the soil towards the respiration chamber. The discrepancy between the two systems was reduced (i) by decreasing fan speed within the SRC-1, (ii) by increasing wind speed over the soil surface outside the respiration chamber, or (iii) by using an artificial soil design without high CO₂ concentration in soil pores. We show that wind speed is an important component of soil CO₂ diffusion which must be taken into account when measuring soil CO₂ efflux, even on very fine textured soil like silt-loam soil. Proper measurement can be achieved by maintaining wind speed inside the chamber below 0.4 m s⁻¹ since low wind speed conditions predominate under forest canopies. However, more accurate measurements will be obtained by regulating wind speeds within the chamber at a velocity representative of the wind speed recorded simultaneously at the floor surface. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of elevated atmospheric CO2 concentration on soil and root respiration in winter wheat by using a respiration partitioning chamber

Soil respiration in a cropland is the sum of heterotrophic (mainly microorganisms) and autotrophic (root) respiration. The contribution of both these types to soil respiration needs to be understood to evaluate the effects of environmental change on soil carbon cycling and sequestration. In this paper, the effects of free-air CO₂ enrichment (FACE) on hetero- and autotrophic respiration in a wheat field were differentiated and evaluated by a novel split-root growth and gas collection system. Elevated atmospheric pCO₂ of approximately 200 μmol mol⁻¹ above the ambient pCO₂ significantly increased soil respiration by 15.1 and 14.8% at high nitrogen (HN) and low nitrogen (LN) application rates, respectively. The effect of elevated atmospheric pCO₂ on root respiration was not consistent across the wheat growth stages. Elevated pCO₂ significantly increased and decreased root respiration at the booting-heading stage (middle stage) and the late-filling stage (late stage), respectively, in HN and LN treatments; however, no significant effect was found at the jointing stage (early stage). Thus, the effect of increased pCO₂ on cumulative root respiration for the entire wheat growing season was not significant. Cumulative root respiration accounted for approximately 25–30% of cumulative soil respiration in the entire wheat growing season. Consequently, cumulative microbial respiration (soil respiration minus root respiration) increased by 22.5 and 21.1% due to elevated pCO₂ in HN and LN, respectively. High nitrogen application significantly increased root respiration at the late stage under both elevated pCO₂ and ambient pCO₂; however, no significant effects were found on cumulative soil respiration, root respiration, and microbial respiration. These findings suggest that heterotrophic respiration, which is influenced by increased substrate supplies from the plant to the soil, is the key process to determine C emission from agro-ecosystems with regard to future scenarios of enriched pCO₂.     

Long-term Effects of Free Air CO2 Enrichment (FACE) on Soil Respiration

Emissions of CO₂ from soils make up one of the largest fluxes in the global C cycle, thus small changes in soil respiration may have large impacts on global C cycling. Anthropogenic additions of CO₂ to the atmosphere are expected to alter soil carbon cycling, an important component of the global carbon budget. As part of the Duke Forest Free-Air CO₂ Enrichment (FACE) experiment, we examined how forest growth at elevated (+200 ppmv) atmospheric CO₂ concentration affects soil CO₂ dynamics over 7 years of continuous enrichment. Soil respiration, soil CO₂ concentrations, and the isotopic signature of soil CO₂ were measured monthly throughout the 7 years of treatment. Estimated annual rates of soil CO₂ efflux have been significantly higher in the elevated plots in every year of the study, but over the last 5 years the magnitude of the CO₂ enrichment effect on soil CO₂ efflux has declined. Gas well samples indicate that over 7 years fumigation has led to sustained increases in soil CO₂ concentrations and depletion in the δ¹³C of soil CO₂ at all but the shallowest soil depths.     

Carbon balance and allocation of assimilated CO2 in Scots pine, Norway spruce, and Silver birch seedlings determined with gas exchange measurements and 14C pulse labelling

Carbon dioxide is released from the soil to the atmosphere in heterotrophic respiration when the dead organic matter is used for substrates for soil micro-organisms and soil animals. Respiration of roots and mycorrhiza is another major source of carbon dioxide in soil CO₂ efflux. The partitioning of these two fluxes is essential for understanding the carbon balance of forest ecosystems and for modelling the carbon cycle within these ecosystems. In this study, we determined the carbon balance of three common tree species in boreal forest zone, Scots pine, Norway spruce, and Silver birch with gas exchange measurements conducted in laboratory in controlled temperature and light conditions. We also studied the allocation pattern of assimilated carbon with ¹⁴C pulse labelling experiment. The photosynthetic light responses of the tree species were substantially different. The maximum photosynthetic capacity (P _max) was 2.21 μg CO₂ s⁻¹ g⁻¹ in Scots pine, 1.22 μg CO₂ s⁻¹ g⁻¹ in Norway spruce and 3.01 μg CO₂ s⁻¹ g⁻¹ in Silver birch seedlings. According to the pulse labelling experiments, 43–75% of the assimilated carbon remained in the aboveground parts of the seedlings. The amount of carbon allocated to root and rhizosphere respiration was about 9–26%, and the amount of carbon allocated to root and ectomycorrhizal biomass about 13–21% of the total assimilated CO₂. The ¹⁴CO₂ pulse reached the root system within few hours after the labelling and most of the pulse had passed the root system after 48 h. The transport rate of carbon from shoot to roots was fastest in Silver birch seedlings.     

Biomass allocation and canopy development in spruce model ecosystems under elevated CO2 and increased N deposition

Ecosystem-level experiments on the effects of atmospheric CO₂ enrichment and N deposition on forest trees are urgently needed. Here we present data for nine model ecosystems of spruce (Picea abies) on natural nutrient-poor montane forest soil (0.7 m² of ground and 350 kg weight). Each system was composed of six 7-year-old (at harvest) trees each representing a different genotype, and a herbaceous understory layer (three species). The model ecosystems were exposed to three different CO₂ concentrations (280, 420, 560 μl l⁻¹) and three different rates of wet N deposition (0, 30, 90 kg ha⁻¹ year⁻¹) in a simulated annual course of Swiss montane climate for 3 years. The total ecosystem biomass was not affected by CO₂ concentration, but increased with increasing N deposition. However, biomass allocation to roots increased with increasing CO₂ leading to significantly lower leaf mass ratios (LMRs) and leaf area ratios (LARs) in trees grown at elevated CO₂. In contrast to CO₂ enrichment, N deposition increased biomass allocation to the aboveground plant parts, and thus LMR and LAR were higher with increasing N deposition. We observed no CO₂ ×  N interactions on growth, biomass production, or allocation, and there were also no genotype × treatment interactions. The final leaf area index (LAI) of the spruce canopies was 19% smaller at 420 and 27% smaller at 560 than that measured at 280 μl CO₂ l⁻¹, but was not significantly altered by increasing N deposition. Lower LAIs at elevated CO₂ largely resulted from shorter branches (less needles per individual tree) and partially from increased needle litterfall. Independently of N deposition, total aboveground N content in the spruce communities declined with increasing CO₂ (−18% at 420 and −31% at 560 compared to 280 μl CO₂ l⁻¹). N deposition had the opposite effect on total above ground N content (+18% at 30 and +52% at 90 compared to 0 kg N ha⁻¹ year⁻¹). Our results suggest that under competitive conditions on natural forest soil, atmospheric CO₂ enrichment may not lead to higher ecosystem biomass production, but N deposition is likely to do so. The reduction in LAI under elevated CO₂ suggests allometric down-regulation of photosynthetic carbon uptake at the canopy level. The strong decline in the tree nitrogen mass per unit ground area in response to elevated CO₂ may indicate CO₂-induced reductions of soil N availability. Received: 11 May 1997 / Accepted: 4 August 1997     

Soil organic matter dynamics under soybean exposed to elevated [CO2]

It is unclear how changing atmospheric composition will influence the plant–soil interactions that determine soil organic matter (SOM) levels in fertile agricultural soils. Positive effects of CO₂ fertilization on plant productivity and residue returns should increase SOM stocks unless mineralization or biomass removal rates increase in proportion to offset gains. Our objectives were to quantify changes in SOM stocks and labile fractions in prime farmland supporting a conventionally managed corn–soybean system and the seasonal dynamics of labile C and N in soybean in plots exposed to elevated [CO₂] (550 ppm) under free-air concentration enrichment (FACE) conditions. Changes in SOM stocks including reduced C/N ratios and labile N stocks suggest that SOM declined slightly and became more decomposed in all plots after 3 years. Plant available N (>273 mg N kg⁻¹) and other nutrients (Bray P, 22–50 ppm; extractable K, 157–237 ppm; Ca, 2,378–2,730 ppm; Mg, 245–317 ppm) were in the high to medium range. Exposure to elevated [CO₂] failed to increase particulate organic matter C (POM-C) and increased POM-N concentrations slightly in the surface depth despite known increases (≈30%) in root biomass. This, and elevated CO₂ efflux rates indicate accelerated decay rates in fumigated plots (2001: elevated [CO₂]: 10.5 ± 1.2 μmol CO₂ m⁻² s⁻¹ vs. ambient: 8.9 ± 1.0 μmol CO₂ m⁻² s⁻¹). There were no treatment-based differences in the within-season dynamics of SOM. Soil POM-C and POM-N contents were slightly greater in the surface depth of elevated than ambient plots. Most studies attribute limited ability of fumigated soils to accumulate SOM to N limitation and/or limited plant response to CO₂ fertilization. In this study, SOM turnover appears to be accelerated under elevated [CO₂] even though soil moisture and nutrients are non-limiting and plant productivity is consistently increased. Accelerated SOM turnover rates may have long-term implications for soil’s productive potential and calls for deeper investigation into C and N dynamics in highly-productive row crop systems.     

Inhibition by light of CO2 evolution from dark respiration: Comparison of two gas exchange methods

Two approaches to determine the fraction (μ) of mitochondrial respiration sustained during illumination by measuring CO₂ gas exchange are compared. In single leaves, the respiration rate in the light (`day respiration' rate R<sub>d</sub>) is determined as the ordinate of the intersection point of A–c<sub>i</sub> curves at various photon flux densities and compared with the CO<sub>2</sub> evolution rate in darkness (`night respiration' rate R_n). Alternatively, using leaves with varying values of CO₂ compensation concentration (Γ), intracellular resistance (r_i) and R_n, an average number for μ can be derived from the linear regression between Γ and the product r_iċR_n. Both methods also result in a number c* for that intercellular CO₂ concentration at which net CO₂ uptake rate is equal to –R_d. c* is an approximate value of the photocompensation point Γ* (Γ in the absence of mitochondrial respiration), which is related to the CO₂/O₂ specificity factor of Rubisco S_c/o. The presuppositions and limitations for application of both approaches are discussed. In leaves of Nicotiana tabacum, at 22 °C, single leaf measurements resulted in mean values of μ = 0.71 and c_* = 34 μmol mol⁻¹. At the photosynthetically active photon flux density of 960 μmol quanta m⁻² s⁻¹, nearly the same numbers were derived from the linear relationship between Γ and r_iċR_n. c* and R_d determined by single leaf measurements varied between 31 and 41 μmol mol⁻¹ and between 0.37 and 1.22 μmol m⁻² s⁻¹, respectively. A highly significant negative correlation between c* and R_d was found. From the regression equation we obtained estimates for Γ* (39 μmol mol⁻¹), S_c/o (96.5 mol mol⁻¹) and the mesophyll CO₂ transfer resistance (7.0 mol⁻¹ m² s). This revised version was published online in June 2006 with corrections to the Cover Date.     

Carbon balance in tussock tundra under ambient and elevated atmospheric CO2

Summary Whole ecosystem CO₂ flux under ambient (340 l/l) and elevated (680 l/l) CO₂ was measured in situ in Eriophorum tussock tundra on the North Slope of Alaska. Elevated CO₂ resulted in greater carbon acquisition than control treatments and there was a net loss of CO₂ under ambient conditions at this upland tundra site. These measurements indicate a current loss of carbon from upland tundra, possibly the result of recent climatic changes. Elevated CO₂ for the duration of one growing season appeared to delay the onset of dormancy and resulted in approximately 10 additional days of positive ecosystem flux. Homeostatic adjustment of ecosystem CO₂ flux (sum of species' response) was apparent by the third week of exposure to elevated CO₂. Ecosystem dark respiration rates were not significantly higher at elevated CO₂ levels. Rapid homeostatic adjustment to elevated CO₂ may limit carbon uptake in upland tundra. Abiotic factors were evaluated as predictors of ecosystem CO₂ flux. For chambers exposed to ambient and elevated CO₂ levels for the duration of the growing season, seasonality (Julian day) was the best predictor of ecosystem CO₂ flux at both ambient and elevated CO₂ levels. Light (PAR), soil temperature, and air temperature were also predictive of seasonal ecosystem flux, but only at elevated CO₂ levels. At any combination of physical conditions, flux of the elevated CO₂ treatment was greater than that at ambient. In short-term manipulations of CO₂, tundra exposed to elevated CO₂ had threefold greater carbon gain, and had one half the ecosystem level, light compensation point when compared to ambient CO₂ treatments. Elevated CO₂-acclimated tundra had twofold greater carbon gain compared to ambient treatments, but there was no difference in ecosystem level, light compensation point between elevated and ambient CO₂ treatments. The predicted future increases in cloudiness could substantially decrease the effect of elevated atmospheric CO₂ on net ecosystem carbon budget. These analyses suggest little if any long-term stimulation of ecosystem carbon acquisition by increases in atmospheric CO₂.     

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.     

Influence of net ecosystem metabolism in transferring riverine organic carbon to atmospheric CO<Subscript>2</Subscript> in a tropical coastal lagoon (Chilka Lake,India)

Studies on biogeochemical cycling of carbon in the Chilka Lake, Asia’s largest brackish lagoon on the east coast of India, revealed, for the first time, strong seasonal and spatial variability associated with salinity distribution. The lake was studied twice during May 2005 (premonsoon) and August 2005 (monsoon). It exchanges waters with the sea (Bay of Bengal) and several rivers open into the lake. The lake showed contrasting levels of dissolved inorganic carbon (DIC) and organic carbon (DOC) in different seasons; DIC was higher by ∼22% and DOC was lower by ∼36% in premonsoon than in monsoon due to seasonal variations in their supply from rivers and in situ production/mineralisation. The DIC/DOC ratios in the lake during monsoon were influenced by physical mixing of end member water masses and by intense respiration of organic carbon. A strong relationship between excess DIC and apparent oxygen utilisation showed significant control of biological processes over CO₂ production in the lake. Surface partial pressure of CO₂ (pCO₂), calculated using pH–DIC couple according to Cai and Wang (Limnol and Oceanogr 43:657–668, 1998), exhibited discernable gradients during monsoon through northern (1,033–6,522 μatm), central (391–2,573 μatm) and southern (102–718 μatm) lake. The distribution pattern of pCO₂ in the lake seems to be governed by pCO₂ levels in rivers and their discharge rates, which were several folds higher during monsoon than premonsoon. The net CO₂ efflux, based on gas transfer velocity parameterisation of Borges et al. (Limnol and Oceanogr 49(5):1630–1641, 2004), from entire lake during monsoon (141 mmolC m⁻² d⁻¹ equivalent to 2.64 GgC d⁻¹ at basin scale) was higher by 44 times than during premonsoon (9.8 mmolC m⁻² d⁻¹ ≈ 0.06 GgC d⁻¹). 15% of CO₂ efflux from lake in monsoon was contributed by its supply from rivers and the rest was contributed by in situ heterotrophic activity. Based on oxygen and total carbon mass balance, net ecosystem production (NEP) of lake (−308 mmolC m⁻² d⁻¹ ≈ −3.77 GgC d⁻¹) was found to be almost in consistent with the total riverine organic carbon trapped in the lake (229 mmolC m⁻² d⁻¹ ≈ 2.80 GgC d⁻¹) suggesting that the strong heterotrophy in the lake is mainly responsible for elevated fluxes of CO₂ during monsoon. Further, the pelagic net community production represented 92% of NEP and benthic compartment plays only a minor role. This suggests that Chilka lake is an important region in biological transformation of organic carbon to inorganic carbon and its export to the atmosphere.     

Influence of elevated CO2 on canopy development and red:far-red ratios in two-storied stands ofRicinus communis

Vertical structure of plant stands and canopies may change under conditions of elevated CO₂ due to differential responses of overstory and understory plants or plant parts. In the long term, seedling recruitment, competition, and thus population or community structure may be affected. Aside from the possible differential direct effects of elevated CO₂ on photosynthesis and growth, both the quantity and quality of the light below the overstory canopy could be indirectly affected by CO₂-induced changes in overstory leaf area index (LAI) and/or changes in overstory leaf quality. In order to explore such possible interactions, we compared canopy leaf area development, canopy light extinction and the quality of light beneath overstory leaves of two-storied monospecific stands ofRicinus communis exposed to ambient (340 μl l⁻¹) and elevated (610 μl l⁻¹) CO₂. Plants in each stand were grown in a common soil as closed “artificial ecosystems” with a ground area of 6.7 m². LAI of overstory plants in all ecosystems more than doubled during the experiment but was not different between CO₂ treatments at the end. As a consequence, extinction of photosynthetically active radiation (PAR) was also not altered. However, under elevated CO₂ the red to far-red ratio (R:FR) measured beneath overstory leaves was 10% lower than in ecosystems treated with ambient CO₂. This reduction was associated with increased thickness of palisade layers of overstory leaves and appears to be a plausible explanation for the specific enhancement of stem elongation of understory plants (without a corresponding biomass response) under elevated CO₂. CO₂ enrichment led to increased biomass of overstory plants (mainly stem biomass) but had no effect on understory biomass. The results of this study raise the possibility of an important indirect effect of elevated CO₂ at the stand-level. We suggest that, under elevated CO₂, reductions in the R:FR ratio beneath overstory canopies may affect understory plant development independently of the effects of PAR extinction.