A laboratory-scale greenhouse for spectroscopic monitoring of plants and associated gas-phase isotopic fractionation

The ability to monitor isotopic fractionation in terrestrial ecosystems is a challenge due to the presence of interacting variables. A laboratory-scale apparatus for controlled experiments could serve as a useful platform to deconvolute the variables that affect isotopic fractionation. Such a device could offer a powerful means to understand fractionation of carbon stocks in terrestrial ecosystems and to probe the effects of photosynthesis or interactions between the soil and plants on carbon fractionation. To this end, an enclosed and artificially-lit benchtop soil and plant chamber was constructed and equipped to monitor atmospheric isotopic composition. Fourier-transform infrared (FTIR) spectroscopy was employed for isotopic sensing since it enables in-situ measurements. The validity of FTIR for isotopic ratio determination was confirmed by comparing FTIR and isotope ratio mass spectrometry data for a series of CO2 gas samples with known quantities of C-13 and C-12. The greenhouse chamber was also equipped with an optically-based trace gas analyzer capable of continuously tracking CO2, CH4, and H2O concentrations and a residual gas analyzer mass spectrometer. Reflectance spectroscopy was also incorporated by way of sealed fiber optic feed-throughs coupled to a spectro-radiometer, for quantifying changes in leaf spectra induced by various environmental stressors. The resulting greenhouse chamber can be a useful tool for determining the effects of atmospheric trace gases on plant morphology and physiology as a function of concentration and isotopic composition. Microecosystems can be examined under controlled laboratory conditions and a wide variety of plant species can be accommodated. The bench-scale greenhouse should prove useful in assessing the impact of environmental variables and in guiding the design of field experiments.