Measuring field-scale isotopic CO2 fluxes with tunable diode laser absorption spectroscopy and micrometeorological techniques

Abstract

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 ${}^{12}\text{CO}{}_2$ and ${}^{13}\text{CO}{}_2$. 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 ${}^{12}\text{CO}{}_2$ and ${}^{13}\text{CO}{}_2$, respectively for $\text{u}{}_{\mathrm{\ast }}\text{>0.1}$ m s⁻¹. The average ${}^{12}\text{CO}{}_2$ and ${}^{13}\text{CO}{}_2$ flux for the period was 1.0 and 0.012 μmol m⁻² s⁻¹, respectively. The isotope ratio of respired carbon ($\text{δ}{}^{13}\text{C}{}_{\mathrm{<mtext>R</mtext>}}$) 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 $\text{δ}{}^{13}\text{C}{}_{\mathrm{<mtext>R</mtext>}}$ from the slope of a linear plot of ${}^{13}\text{CO}{}_2$ versus ${}^{12}\text{CO}{}_2$ flux. This method eliminates a number of potential limitations associated with the Keeling plot and provides a $\text{δ}{}^{13}\text{C}{}_{\mathrm{<mtext>R</mtext>}}$ 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 $\text{δ}{}^{13}\text{C}{}_{\mathrm{<mtext>R</mtext>}}$ 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 (R_E) and decrease in $\text{δ}{}^{13}\text{C}{}_{\mathrm{<mtext>R</mtext>}}$ 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.