In-situ Measurements of Isotope Exchange using Tunable Diode Laser Spectroscopy in a Tall-Grass Prairie Ecosystem

Understanding the carbon cycle is quintessential to developing more accurate global climate models, to improve future climate predictions, and to understand ecosystem responses to environmental variables and management strategies. There is an increasing interest in exploring new tools to investigate the mechanisms and processes that impact the carbon exchange in the soil-plant-atmosphere continuum (Brüggeman et al, 2011). Grassland ecosystems cover a large area of Earth's land surface and play an important role in the global carbon cycle. The use of high temporal resolution measurements of stable CO2 isotopologues (12CO2, 13CO2, 12C18O16O, and 13C18O16O) has become more popular in years and has the potential to improve our understanding of processes driving the carbon exchange at the ecosystem level. The objective of this study was to evaluate the use of a tunable diode laser spectrometer and a multiport sampling system to measure 12CO2 and 13CO2 exchange above a tall-grass prairie ecosystem. The system was deployed from June to November at the Konza Prairie Biological Station south of Manhattan, Kansas (39°6'0.30"N Latitude and 96°33'44.50"W Longitude). The Konza Prairie site is dominated by C4 grasses with small communities of C3 forbs, sedges, and woody species. Management of the Konza site entails prescribed burn treatments at various time scales to study the changes in plant communities such as woody plant encroachment in prairie ecosystems. The measurement system consisted of a tunable diode laser gas analyzer (TGA200, Campbell Sci., Logan, UT) that was connected to a multiport manifold sampling system. This system, controlled using a data logger (CR3000, Campbell Sci., Logan, UT), was used to draw air through eight gas inlets at different heights (0.18m, 0.31m, 0.45m, 0.56m, 0.73m, 1.29m, 2.0m, and 3.0m) and from calibration tanks. The isotope concentration data for two inlets just above the tall-grass canopy were used to calculate isotope composition of the net CO2 ecosystem exchange, using the isotope flux ratio method. In addition to the isotope measurements, the net CO2 ecosystem exchange, the soil CO2 flux, plant phenology, and other environmental variables were monitored at the site. Our preliminary results show that the system is indeed suitable for monitoring CO2 isotopologues above tall-grass canopies. The use of mixing volumes reduced the noise of the CO2 isotopologue concentration data, by minimizing errors due to partial sampling time of the air intakes. The d13C of the CO2 in the ecosystem air showed a clear diel trend indicating the effect of the canopy discrimination on the isotope exchange in the grassland ecosystem. Values of dF were more uncertain towards the end of the growing season when the CO2 gradients became less intense due to plant senescence.