Ecosystem models differ widely in their predictions of how forest carbon dynamics will interact with a changing climate, one of the largest sources of uncertainty in predictions of future climate. We are investigating the mechanisms controlling carbon sequestration at the Harvard Forest by combining newly developed instrumentation for automated, in situ CO2 isotopologue measurements (including isotopologue eddy flux measurements) with novel experimental manipulations of belowground processes, as well as with traditional methods like soil chambers and plot trenching. The resulting data will be integrated in—and used to refine—the Ecosystem Demography 2 (ED2) model.
Here we focus on one key aspect of that work: partitioning the net ecosystem-atmosphere CO2 exchange (NEE) into its gross photosynthetic and respiratory components. Partitioning NEE is a longstanding problem in carbon cycle science. The most common approaches to this problem treat respiration as an empirical function of temperature (and sometimes moisture) according to nighttime data. Methods in this vein, though long-used, have been difficult to validate, and are challenged by mounting evidence of a soil respiratory response to photosynthesizing vegetation.
Isotopic flux partitioning (IFP) is an alternative method that uses the distinct influences of photosynthesis and respiration on the stable isotopic signature of the CO2 flux. Here we present the first ever IFP estimates of ecosystem photosynthesis and respiration based on direct eddy covariance measurements of the fluxes of 13CO2 and 12CO2. This set of flux measurements (which also include 18O12C16O) spans the 2011 growing season and constitutes the first long-term continuous record of CO2 isotopologue flux in any ecosystem. The measurements were obtained using a quantum cascade laser spectrometer with a precision on par with the gold standard laboratory technique, isotope ratio mass spectrometry.
The 3379 40-minute flux measurements between May and October contain much information about ecosystem behavior. We find that on the average, ecosystem respiration shows at most minor diel variation, consistent with short-term, temperature-based extrapolation from night to day. We also observe a gradual decrease from June to September in the isotopic discrimination due to photosynthesis, while diel variation in that discrimination shows that stomata close as photosynthesis becomes increasingly light-limited in this ecosystem (as opposed to photosynthesis becoming increasingly diffusion-limited as stomata close due to water stress). The 18O12C16O fluxes evince evaporative isotopic enrichment of leaf water during the day and equilibration of the leaf and soil water isotopic compositions during the night.