The central theme of my research is ecosystem carbon cycle, with emphases on improving observations and the process-based understanding of photosynthesis using novel tracer methods. A robust quantification of the terrestrial photosynthesis has long been a holy grail for ecosystem carbon cycle research, because CO2 measurements alone cannot reliably separate photosynthesis and respiration — two opposing fluxes of comparable magnitudes. To resolve this problem, my research has been dedicated to developing novel tracer techniques for photosynthesis observations — carbonyl sulfide and solar induced chlorophyll fluorescence. These tracers offer valuable constraints on different stages in photosynthesis and are not affected by respiration. Accurate estimates of photosynthesis, anticipated by these new methods, will improve the understanding of its climate sensitivities and reduce the uncertainty in projected future trends of photosynthesis.

Carbonyl sulfide (COS) as a tracer for quantifying photosynthesis. COS is taken up by leaves through stomata due to reactions with carbonic anhydrase. The concurrency of COS and CO2 uptake processes and the lack of a COS source in leaves allow photosynthesis to be constrained with COS fluxes. To improve the COS method, I have been pursuing field and modeling research to understand environmental and physiological controls on the relationship between leaf COS and CO2 fluxes, represented by the leaf COS : CO2 relative uptake ratio (LRU). Using dynamic leaf chambers for COS and CO2 flux measurements, my research showed that light and vapor pressure deficit (VPD) are the main drivers of LRU in field conditions. I have also developed a theoretical framework that explains LRU variability. With a better understanding of the COS : CO2 flux relationship, the COS method will deliver more accurate estimates of photosynthesis.

Process-based modeling of solar induced chlorophyll fluorescence (SIF). During the light reaction stage of photosynthesis, a fraction of the absorbed photosynthetically active radiation is re-emitted as SIF. Measuring SIF signals from photosynthesizing leaves can therefore offer constraints on photosynthesis efficiency, which cannot be obtained from gas exchange measurements alone. In this line of research, I will simulate SIF signals in a tropical rainforest canopy to (i) aid the interpretation of field data and (ii) improve model representation of canopy radiation and gas exchange processes. Planned future research will combine COS tracer with SIF to simultaneously utilize the constraints from COS and SIF on photosynthesis.