[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 C R ) in order to better understand the processes controlling ecosystem isotope discrimination. In this paper we synthesize emergent patterns in d 13 C R 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 C R . These include the use of regression calculations that account for error in the x variable, and constraining estimates of d 13 C R to nighttime periods. We then recalculate d 13 C R uniformly for all sites. We found a high degree of temporal and spatial variability in C 3 ecosystems, with individual observations ranging from À19.0 to À32.6%. Mean C 3 ecosystem discrimination was 18.3%. Precipitation was a major driver of both temporal and spatial variability of d 13 C R , suggesting (1) a large influence of recently fixed carbon on ecosystem respiration and (2) a significant effect of previous climatic effects on d 13 C R . These results illustrate the importance of water availability as a key control on atmospheric 13 CO 2 and highlight the potential of d 13 C R as a useful tool for integrating environmental effects on dynamic canopy and ecosystem processes.

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In this paper we describe how a model of stable isotope fractionation processes, originally developed by H. Craig and L.I. Gordon ([1965] in E Tongiorgi, ed, Proceedings of a Conference on Stable Isotopes in Oceanographic Studies and Paleotemperature, Spoleto, Italy, pp 9-130) for evaporation of water from the ocean, can be applied to leaf transpiration. The original model was modified to account for turbulent conditions in the leaf boundary layer. Experiments were conducted to test the factors influencing the stable isotopic composition of leaf water under controlled environment conditions. At steady state, the observed leaf water isotopic composition was enriched above that of stem water with the extent of the enrichment dependent on the leaf-air vapor pressure difference (VPD) and the isotopic composition of atmospheric water vapor (AWV). The higher the VPD, the larger was the observed heavy isotope content of leaf water. At a constant VPD, leaf water was relatively depleted in heavy isotopes when exposed to AWV with a low heavy isotope composition, and leaf water was relatively enriched in heavy isotopes when exposed to AWV with a large heavy isotope composition. However, the observed heavy isotope composition of leaf water was always less than that predicted by the model. The extent of the discrepancy between the modeled and observed leaf water isotopic composition was a strong linear function of the leaf transpiration rate.The stable isotopic composition of plant leafwater is altered during transpiration. Water vapor molecules containing the lighter isotopes of oxygen and hydrogen escape from the leaf more readily than do heavy isotope molecules, so that during transpiration, leaf water becomes enriched in heavy isotope molecules (19,24,28 photosynthetic gas exchange and plant water-use efficiency (10,11,20,24,28).Previous attempts to use the Craig and Gordon model in studies of leaf water isotopic enrichment have been complicated by two factors. First, there has been uncertainty about the value to use for the kinetic fractionation factor. Values for the relative rates of diffusion of water vapor molecules containing light and heavy isotopes of oxygen and hydrogen have been measured (16). These measured values for the kinetic fractionation factor are appropriate for molecular diffusion only, however, and need to be modified for turbulent conditions in a boundary layer. In previous studies there has been uncertain and inconsistent modification of the kinetic fractionation factor to account for turbulence in the water vapor diffusion pathway (3,7,11,14,20,22,23,25). Second, a major assumption in the derivation of the evaporative enrichment model is that isotopic steady state is reached. When leaf water is at isotopic steady state, the isotopic composition of transpiration water is the same as the source or stem water isotopic composition (24). The assumption of isotopic steady state has not been verified in most studies attempting to test the ability of the Craig and Gordon model to predict leaf wat...

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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 gradual shift in the relative contributions of microbial vs. plant components in the residual SOM and not to differential SOM degradation or to microbial fractionation during decomposition. Clarifying this mechanism is critical for applying δ13C analyses to quantification of SOM turnover rates. Across latitudinal gradients, large differences should occur in the δ13C values of CO2 effluxing from soils, but as of yet a global database is lacking with which to test this prediction. Such a global database would be a useful input for global carbon cycle models that rely on δ values to constrain source and sink relations.