Analyses of carbon isotope ratios (δ13C) in soil organic matter (SOM) and soil respired CO2 provide insights into dynamics of the carbon cycle. δ13C analyses do not provide direct measures of soil CO2 efflux rates but are useful as a constraint in carbon cycle models. In many cases, δ13C analyses allow the identification of components of soil CO2 efflux as well as the relative contribution of soil to overall ecosystem CO2 fluxes. δ13C values provide a unique tool for quantifying historical shifts between C3 and C4 ecosystems over decadal to millennial time scales, which are relevant to climate change and land-use change issues. We identify the need to distinguish between δ13C analyses of SOM and those of soil CO2 efflux in carbon cycle studies, because time lags in the turnover rates of different soil carbon components can result in fluxes and stocks that differ in isotopic composition (disequilibrium effect). We suggest that the frequently observed progressive δ13C enrichment of SOM may be related to a g...

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Carbon isotope ratios in belowground carbon cycle processes

[1] Photosynthesis and respiration impart distinct isotopic signatures to the atmosphere that are used to constrain global carbon source/sink estimates and partition ecosystem fluxes. Increasingly, the ‘‘Keeling plot’’ method is being used to determine the carbon isotope composition of ecosystem respiration (d 13 CR) in order to better understand the processes controlling ecosystem isotope discrimination. In this paper we synthesize emergent patterns in d 13 CR by analyzing 146 Keeling plots constructed at 33 sites across North and South America. In order to interpret results from disparate studies, we discuss the assumptions underlying the Keeling plot method and recommend standardized methods for determining d 13 CR. These include the use of regression calculations that account for error in the x variable, and constraining estimates of d 13 CR to nighttime periods. We then recalculate d 13 CR uniformly for all sites. We found a high degree of temporal and spatial variability in C3 ecosystems, with individual observations ranging from � 19.0 to � 32.6%. Mean C3 ecosystem discrimination was 18.3%. Precipitation was a major driver of both temporal and spatial variability of d 13 CR, suggesting (1) a large influence of recently fixed carbon on ecosystem respiration and (2) a significant effect of previous climatic effects on d 13 CR. These results illustrate the importance of water availability as a key control on atmospheric 13 CO2 and highlight the potential of d 13 CR as a useful tool for integrating environmental effects on dynamic canopy and ecosystem processes. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 1615 Global Change: Biogeochemical processes (4805); 1694 Global Change: Instruments and techniques; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; KEYWORDS: carbon cycle, carbon isotopes, ecosystem respiration, carbon dioxide, terrestrial ecosystems

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The application and interpretation of Keeling plots in terrestrial carbon cycle research

Estimates of the extent of the discrimination against13CO2 during photosynthesis (ΔA) on a global basis were made using gridded data sets of temperature, precipitation, elevation, humidity and vegetation type. Stomatal responses to leaf-to-air vapour mole fraction difference (D, leaf-to-air vapour pressure difference divided by atmospheric pressure) were first determined by a literature review and by assuming that stomatal behaviour results in the optimisation of plant water use in relation to carbon gain. Using monthly time steps, modelled stomatal responses toD were used to calculate the ratio of stomatal cavity to ambient CO2 mole fractions and then, in association with leaf internal conductances, to calculate ΔA. Weighted according to gross primary productivity (GPP, annual net CO2 asimilation per unit ground area), estimated ΔA for C3 biomes ranged from 12.9‰ for xerophytic woods and shrub to 19.6‰ for cool/cold deciduous forest, with an average value from C3 plants of 17.8‰. This is slightly less than the commonly used values of 18–20‰. For C4 plants the average modelled discrimination was 3.6‰, again slightly less than would be calculated from C4 plant dry matter carbon isotopic composition (yielding around 5‰). From our model we estimate that, on a global basis, 21% of GPP is by C4 plants and for the terrestrial biosphere as a whole we calculate an average isotope discrimination during photosynthesis of 14.8‰. There are large variations in ΔA across the globe, the largest of which are associated with the precence or absence of C4 plants. Due to longitudinal variations in ΔA, there are problems in using latitudinally averaged terrestrial carbon isotope discriminations to calculate the ratio of net oceanic to net terrestrial carbon fluxes.

13C discrimination during CO2 assimilation by the terrestrial biosphere

Stable carbon isotopes are used extensively to examine physiological, ecological, and biogeochemical processes related to ecosystem, regional, and global carbon cycles and provide information at a variety of temporal and spatial scales. Much is known about the processes that regulate the carbon isotopic composition (delta(13)C) of leaf, plant, and ecosystem carbon pools and of photosynthetic and respiratory carbon dioxide (CO(2)) fluxes. In this review, systematic patterns and mechanisms underlying variation in delta(13)C of plant and ecosystem carbon pools and fluxes are described. We examine the hypothesis that the delta(13)C of leaf biomass can be used as a reference point for other carbon pools and fluxes, which differ from the leaf in delta(13)C in a systematic fashion. Plant organs are typically enriched in (13)C relative to leaves, and most ecosystem pools and respiratory fluxes are enriched relative to sun leaves of dominant plants, with the notable exception of root respiration. Analysis of the chemical and isotopic composition of leaves and leaf respiration suggests that growth respiration has the potential to contribute substantially to the observed offset between the delta(13)C values of ecosystem respiration and the bulk leaf. We discuss the implications of systematic variations in delta(13)C of ecosystem pools and CO(2) fluxes for studies of carbon cycling within ecosystems, as well as for studies that use the delta(13)C of atmospheric CO(2) to diagnose changes in the terrestrial biosphere over annual to millennial time scales.

Carbon isotopes in terrestrial ecosystem pools and CO2 fluxes.

Stable isotopes are a powerful research tool in environmental sciences and their use in ecosystem re- search is increasing. In this review we introduce and dis- cuss the relevant details underlying the use of carbon and oxygen isotopic compositions in ecosystem gas ex- change research. The current use and potential develop- ments of stable isotope measurements together with con- centration and flux measurements of CO2 and water va- por are emphasized. For these applications it is critical to know the isotopic identity of specific ecosystem compo- nents such as the isotopic composition of CO 2 , organic matter, liquid water, and water vapor, as well as the associated isotopic fractionations, in the soil-plant- atmosphere system. Combining stable isotopes and con- centration measurements is very effective through the use of "Keeling plots." This approach allows the identifi- cation of the isotopic composition and the contribution of ecosystem, or ecosystem components, to the exchange fluxes with the atmosphere. It also allows the estimation of net ecosystem discrimination and soil disequilibrium effects. Recent modifications of the Keeling plot ap- proach permit examination of CO2 recycling in ecosys- tems. Combining stable isotopes with dynamic flux mea- surements requires precision in isotopic sampling and analysis, which is currently at the limit of detection. Combined with the micrometeorological gradient ap- proach (applicable mostly in grasslands and crop fields), stable isotope measurements allow separation of net CO 2 exchange into photosynthetic and soil respiration compo- nents, and the evapotranspiration flux into soil evapora- tion and leaf transpiration. Similar applications in con- junction with eddy correlation techniques (applicable to forests, in addition to grasslands and crop fields) are more demanding, but can potentially be applied in com- bination with the Keeling plot relationship. The advance and potential in using stable isotope measurements should make their use a standard component in the limit- ed arsenal of ecosystem-scale research tools.

The use of stable isotopes to study ecosystem gas exchange.

Our objective was to measure the carbon isotope ratio of CO 2 released by respiration (δ r ) within forest canopies at different times during the growing season and to use this information to estimate forest ecosystem carbon isotope discrimination. We made measurements in the three major forest types (black spruce, jack pine, and aspen) at the southern and northern ends of the boreal forest in central Canada. This research was part of a larger study, the Boreal EcosystemAtmosphere Study (BOREAS). The δ r values, calculated from measurements of change in the concentration and carbon isotope ratio of atmospheric CO 2 in air samples collected at night, ranged from -28.1‰ to -25.9‰ with an average (± s.d.) of -26.8‰ ± 0.5‰. There was good correlation between calculated δ r values and measurements of (1) the carbon isotope ratio of CO 2 released directly from the soil and (2) the δ 13 C values of foliage collected from the dominant tree species at each site. Carbon isotope discrimination during photosynthetic gas exchange (Δ A ) by each forest ecosystem was estimated as the difference between the carbon isotope ratio of atmospheric CO 2 at the top of the canopy (δ a ) and the isotopic composition of respired CO 2 : Δ A = δ a - δ r . All three of the major forest types had similar values of Δ A , with an average (± s.d.) of 19.1‰ ± 0.5‰. However, a seasonal change in forest discrimination was observed for aspen forests in both the northern and southern study areas, with an increase in Δ A occurring between the middle and end of the growing season. In contrast, the evergreen conifer canopies exhibited relatively constant discrimination values throughout the active growing season.

Carbon isotope discrimination during photosynthesis and the isotope ratio of respired CO2 in boreal forest ecosystems

Our objective was to evaluate the relative importance of gradients in light intensity and the isotopic composition of atmospheric CO2 for variation in leaf carbon isotope ratios within a Pinus resinosa forest. In addition, we measured photosynthetic gas exchange and leaf carbon isotope ratios on four understory species (Dryopteris carthusiana, Epipactus helleborine, Hieracium floribundum, Rhamnus frangula), in order to estimate the consequence of the variation in the understory light microclimate for carbon gain in these plants. During midday, CO2 concentration was relatively constant at vertical positions ranging from 15 m to 3 m above ground. Only at positions below 3 m was CO2 concentration significantly elevated above that measured at 15 m. Based on the strong linear relationship between changes in CO2 concentration and δ13C values for air samples collected during a diurnal cycle, we calculated the expected vertical profile for the carbon isotope ratio of atmospheric CO2 within the forest. These calculations indicated that leaves at 3 m height and above were exposed to CO2 of approximately the same isotopic composition during daylight periods. There was no significant difference between the daily mean δ13C values at 15 m (-7.77‰) and 3 m (-7.89‰), but atmospheric CO2 was significantly depleted in 13C closer to the ground surface, with daily average δ13C values of -8.85‰ at 5 cm above ground. The light intensity gradient in the forest was substantial, with average photosynthetically active radiation (PAR) on the forest floor approximately 6% of that received at the top of the canopy. In contrast, there were only minor changes in air temperature, and so it is likely that the leaf-air vapour pressure difference was relatively constant from the top of the canopy to the forest floor. For red pine and elm tree samples, there was a significant correlation between leaf δ13C value and the height at which the leaf sample was collected. Leaf tissue sampled near the forest floor, on average, had lower δ13C values than samples collected near the top of the canopy. We suggest that the average light intensity gradient through the canopy was the major factor influencing vertical changes in tree leaf δ13C values. In addition, there was a wide range of variation (greater than 4‰) among the four understory plant species for average leaf δ13C values. Measurements of leaf gas exchange, under natural light conditions and with supplemental light, were used to estimate the influence of the light microclimate on the observed variation in leaf carbon isotope ratios in the understory plants. Our data suggest that one species, Epipactus helleborine, gained a substantial fraction of carbon during sunflecks.

Stephanie C. Berry

Papers

Carbon isotope discrimination during photosynthesis and the isotope ratio of respired CO2 in boreal forest ecosystems

Leaf δ13C in Pinus resinosa trees and understory plants: variation associated with light and CO2 gradients.