Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/26/2011
Publication Date: 2/15/2012
Publication URL: http://handle.nal.usda.gov/10113/57476
Citation: Fassbinder, J.J., Griffis, T.J., Baker, J.M. 2012. Evaluation of carbon isotope flux partitioning theory under simplified and controlled environmental conditions. Agricultural and Forest Meteorology. 153:154-164. Interpretive Summary: A better understanding of the carbon cycle is essential to the prediction of impacts of global climate change. There is now a wealth of data on net carbon exchange in different ecosystems, but often those data don't distinguish between the component fluxes of photosynthesis and respiration. Recent developments with tunable diode laser absorption spectroscopy (TDLAS) offer the potential to separate photosynthesis and respiration by separately measuring the isotopic fluxes of CO2, but they have not been critically tested. Our goal was to test their use under controlled conditions in a greenhouse. We combined TDLAS with automated chambers, to evaluate isotope partitioning theory in both corn and soybean systems. Since the system also allowed separate estimation of evaporation and transpiration, it permitted more accurate determination of stomatl conductance, a key parameter in partitioning theory. Overall, the system showed that reliable partitioning of net ecosystem exchange into photosynthesis and respiration depends on accurate specification of conductance, and also showed the conditions under which isotope partitioning can be expected to perform better than traditional regression methods.
Technical Abstract: Separation of the photosynthetic (Fp) and respiratory (Fr) fluxes of net CO2 exchange (Fn)remains a necessary step toward understanding the biological and physical controls on carbon cycling between the soil, biomass, and atmosphere. Despite recent advancements in stable carbon isotope partitioning methodology, several potential limitations cause uncertainty in the partitioned results. Here, we combined an automated chamber system with a tunable diode laser (TDL) to evaluate isotopic partitioning theory under controlled environmental conditions. Experiments were conducted in a climate controlled greenhouse utilizing both soybean (C3 pathway) and corn (C4 pathway) treatments during the winter of 2009. Under these conditions, the isotopic CO2 exchange (dn) between the soil, biomass, and atmosphere was obtained with improved signal to noise ratio. Further, the chamber system was used to estimate soil evaporation and plant transpiration, allowing for a more precise measurement of the bulk stomatal conductance to CO2 (gs). These experiments provided an opportunity for an in-depth analysis of the limitations of stable carbon isotope partitioning theory. In particular, this study found that the incorporation of short-term variability in the isotopic composition of respiration (dr) caused estimated Fp values to nearly double in the corn system but only slightly increase in the soybean system. Further, variability in both gs and the CO2 bundle sheath leakage factor (F) had a significant influence on estimated Fp values. Specifically, this study examined 1) if reliable partitioning results can be obtained if the signal to noise ratios of gs and the isoflux are improved, 2) how both short-term and long-term variability in dr affect partitioning results, and 3) the environmental and physiological conditions where the stable isotope partitioning method is more effective than traditional temperature regression partitioning methods.