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Abstract: Stable isotopes of carbon dioxide (CO2) can be used as natural tracers to better understand carbon cycle processes and exchange pathways between the biosphere and atmosphere. In this study, we used a tunable diode laser (TDL) technique for continuous fast measurement of the stable isotopomers C18O16O and C16O2 and their ratio, δ a. The TDL system was configured to measure mixing ratios of [C16O2] and [C18O16O] at wavenumber frequencies of 2308.225 and 2308.416cm−1, respectively. Two-minute precision (1standard deviation) was 0.0004, 0.09μmolmol−1, and 0.26‰ for [C18O16O], [C16O2] and δ a, respectively. Comparison of TDL and mass spectrometry flask measurements showed relatively good agreement (r 2 =0.94) with a standard deviation of 0.55‰ for the residual values. Estimates of the isotope signature of ecosystem flux components over a soybean (Glycine max) field were examined. These data represent one of the first continuous flux measurements of C18O16O. The isotope signature of net ecosystem CO2 exchange at night ranged from −15 to −7‰ and during the daytime from −40 to −20‰. A simple estimate of canopy-scale photosynthetic discrimination showed significant diurnal variation and averaged 10.5‰ (±8.8‰). The large difference between the isotope signature of respiration and midday canopy photosynthesis represented significant isotopic disequilibrium. Coupled with recent advances in measuring water vapor isotopomers with the TDL technique, a new opportunity is emerging to better understand the dynamics, complex interactions, and discrimination mechanisms controlling land-atmosphere C18O16O exchange. [Copyright &y& Elsevier]

Abstract: Ecosystem-atmosphere fluxes of 12CO2 and 13CO2 are needed to better understand the impacts of climate and land use change on ecosystem respiration (F R), net ecosystem CO2 exchange (F N), and canopy-scale photosynthetic discrimination (Δ). We combined micrometeorological and stable isotope techniques to quantify isotopic fluxes of 12CO2 and 13CO2 over a corn–soybean rotation ecosystem in the Upper Midwest United States. Results are reported for a 192-day period during the corn (C4) phase of the 2003 growing season. The isotopomer flux ratio, d13CO2/d12CO2, was measured continuously using a tunable diode laser (TDL) and gradient technique to quantify the isotope ratios of F R (δ R) and F N (δ N). Prior to leaf emergence δ R was approximately −26‰. It increased rapidly following leaf emergence and reached an average value of −12.5‰ at full canopy. δ R decreased to pre-emergence values following senescence. δ N also showed strong seasonal variation and during the main growing period averaged −11.6‰. δ R and δ N values were used in a modified flux partitioning approach to estimate canopy-scale Δ and the isotope ratio of photosynthetically assimilated CO2 (δ P) independent of calculating canopy conductance or assuming leaf-scale discrimination factors. The results showed substantial day-to-day variation in Δ with an average value of 4.0‰. This flux-based estimate of Δ was approximately 6‰ lower than the Keeling mixing model estimate and in better agreement with leaf-level observations. These data were used to help constrain and partition F R into its autotrophic (F Ra) and heterotrophic (F Rh) components based on the numerical optimization of a mass balance model. On average F Ra accounted for 44% of growing season F R and reached a maximum of 59% during peak growth. The isotope ratio of F Rh (δ Rh), was −26‰ prior to leaf emergence, and became increasingly 13C enriched as the canopy developed indicating that recent photosynthate became the dominant substrate for microbial activity. Sensitivity analyses substantiated that F Rh had a major influence on the seasonal pattern of δ R, δ N and the isotopic disequilibrium of the ecosystem. These data and parameter estimates are critical for validating and constraining the parameterization of land surface schemes and inverse models that aim to estimate regional carbon sinks and sources and interpreting changes in the atmospheric signal of δ 13CO2. [Copyright &y& Elsevier]

The combination of micrometeorological and stable isotope techniques offers a relatively new approach for elucidating ecosystem-scale processes. Here we combined a micrometeorological gradient technique with tunable diode laser absorption spectroscopy (TDLAS) using the Trace Gas Analyzer (TGA100, Campbell Scientific, Inc., Utah, USA) to measure field-scale isotopic CO2 mixing ratios and fluxes of 12CO2 and 13CO2. The experiment was conducted in a recently harvested soybean (Glycine max) field that had been in corn (Zea mays) production the previous 4 years. Measurements were made over a period of 26 days from October 25 to November 19, 2002. Weather conditions were unusually cold and dry during the experiment. Isotopic gradients were small and averaged -0.153 and -0.0018 μmol mol-1 m-1 for 12CO2 and 13CO2, respectively for u*>0.1 m s-1. The average 12CO2 and 13CO2 flux for the period was 1.0 and 0.012 μmol m-2 s-1, respectively. The isotope ratio of respired carbon (δ13CR) obtained from the linear intercept of a Keeling plot was -27.93‰ (±0.32‰) for the experimental period. The Keeling plot technique was compared to a new flux ratio methodology that estimates δ13CR from the slope of a linear plot of 13CO2 versus 12CO2 flux. This method eliminates a number of potential limitations associated with the Keeling plot and provides a δ13CR value that can be directly related to the flux footprint. In this initial comparison, our analysis showed that the flux ratio method produced a similar δ13CR value (-28.67‰), but with greater uncertainty (±2.1‰). Better results are expected during growing season conditions when fluxes are substantially larger and the signal to noise ratio is improved. The isotope ratio of respired carbon was consistent with C3 agricultural systems indicating that soybean decomposition was the dominant substrate for respiration. The observed increase in ecosystem respiration (RE) and decrease in δ13CR following tillage indicated that the incorporation of fresh soybean residue provided the major source for decomposition and further illustrates that the combination of micrometeorological and stable isotope techniques can be used to better interpret changes in carbon cycle processes. Long-term and continuous measurements of isotopic CO2 exchange using tunable diode laser absorption spectroscopy and micrometeorological techniques offers a new opportunity to study carbon cycle processes at the field-scale. [Copyright &y& Elsevier]

Analyzes the eddy-covariance carbon dioxide flux measurements from two contrasting deciduous forest ecosystems in the boreal and temperate regions of central Canada. Difference between the mean annual net ecosystem productivity (NEP); Indication of the contrasting impact of warm spring on NEP; Need for a consistent data post-processing methodology in ecosystem intercomparisons.