Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2003
Publication Date: 6/1/2004
Citation: Griffis, T.J., Baker, J.M., Sargent, S.D., Tanner, B.D., Zhang, J.M. 2004. Measuring field-scale isotopic co2 fluxes with tunable diode laser absorption spectroscopy and micrometeorological techniques. Agricultural and Forest Meteorology. 124:15-29. Interpretive Summary: Atmospheric levels of carbon dioxide (CO2) have been rising since the advent of the industrial revolution, and there is increasing evidence that this may cause changes in the earth's climate. Consequently there is strong interest in studying ways to minimize rising CO2 by storing carbon in soils and biomass. One powerful technique involves the measurement of the relative abundance of the stable isotopes of carbon in the CO2 respired by a system. This measurement allows inference about the source of the CO2 because different types of plants have different characteristic ratios of 12CO2 to 13CO2. Up to this point such measurements could only be made in the laboratory with a mass spectrometer. Tunable diode laser absorption spectroscopy is a new technology that offers considerable promise, but it has not yet been applied to long-term, field-scale measurement. We installed such a system in a field at Rosemount, MN and used it to monitor respiration from a recently harvested soybean field that had previously been cropped in corn for 4 years. The system worked well under difficult weather conditions, and the isotopic ratio of the respired C suggested that nearly all of it was apparently coming from the fresh soybean residue, with little contribution from the older corn residue. The system has an important advantage over older mass spectrometer-based methods in that it provides continuous data, rather than discrete samples at a single point in time. It will be valuable in determining the sources of respired carbon in agricultural and forest ecosystems.
Technical Abstract: The combination of micrometeorological and stable isotope techniques offers a relatively new approach for improving the description of ecosystem scale processes. Here we combined a micrometeorological gradient technique with tunable diode laser absorption spectroscopy (TDLAS) using the Trace Gas Analyzer (TGA 100, 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 field that had been in corn production the previous four years. Measurements were made over a period of 26 days from October 25 to November 19, 2002. Climate conditions were unusually cold and dry during the experiment. Isotopic gradients were small and averaged -0.153 and -0.0018 umol 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 umol m-2 -1,repectively. The isotopic ratio of respired carbon (delta 13CR) obtained from the linear intercept of a Keeling plot was -26.41 per mil for the experimental period. The Keeling plot intercept was substantiated by using a more direct approach based on a linear plot of 13CO2 versus 12CO2 fluxes. Our analysis provides further support that the Keeling plot is a robust technique. The isotopic ratio of respired carbon was consistent with C3 agricultural systems indicating that soybean decomposition was the dominant substrate for respiration. Significant temporal variability was observed in delta 13CR, which may have important implications for flux partitioning studies. If the flux ratio of respiration is conservative on relatively short timescales (< 1 day) then the direct partitioning of daytime net ecosystem CO2 exchange into photosynthesis and respiration will be possible from the daytime flux measurements of 12CO2 and 13CO2 exchange. 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.