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 13CO 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 12CO2 and 13CO2, respectively for u*>0.1 m s-1. The average 12CO2 and 13CO 2 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.
Bibliographical noteFunding Information:
Funding for this research was provided by the University of Minnesota, Grant-in-Aid-of-Research, Artistry and Scholarship Program (TJG). Field assistance was provided by W.A. Breiter and K. Vang, USDA-ARS Technicians, University of Minnesota. We gratefully acknowledge the logistical support provided by the Rosemount Research and Outreach Center, University of Minnesota and the USDA-ARS. We thank T.A. Black and K. Morgenstern for reviewing an earlier draft of the manuscript. We especially acknowledge the criticisms and helpful suggestions provided by D.R. Bowling and the anonymous reviewers.
- Ecosystem respiration
- Isotopic fluxes
- Keeling plot
- Net ecosystem exchange
- Stable isotopes
- Tunable diode laser absorption spectroscopy (TDLAS)