The atmosphere and the Earth’s surface are constantly exchanging gases. Some of these gases expose otherwise covert happenings in the land and air.  The ability to measure trace-trace compounds, gases a million times rarer than CO₂, has opened up a new toolbox for exploring processes that are important to climate and air quality.  The trace-trace gas carbonyl sulfide (COS) can help us understand how land carbon uptake changes over regional and daily time scales.  While supported by this fellowship, I led a workshop with over 40 participants that synthesized our understanding of COS and designated the most important questions left to be answered.  The resulting open-access article can be found at this link: https://doi.org/10.5194/bg-15-3625-2018. I also founded a professional organization of researchers with bi-annual meetings to foster collaboration between disciplines and over 4 continents (website: cosanova.org).  These efforts provided clarity to the community of scientists attempting to use COS for ecosystem research and promote a more efficient use of resources in furthering carbon cycle research.

In terrestrial ecosystems, CO₂ is taken up by photosynthesis and produced through plant and soil microbe respiration.  We can only measure the net effect of CO₂ production and consumption. COS is taken up in plant leaves at the same time as CO₂ and at a known ratio.  Measuring COS uptake over a region lets us know how much CO₂ uptake is occurring.  Continuous measurements reveal how photosynthesis changes dynamically with climate.  Comparing our new, standalone map of carbon uptake to those generated by our best models will challenge and advance our conceptions about terrestrial carbon balance.

A complication arises when COS interacts with soils.  Generally, the amount of COS consumed in soils is less than 5% of the amount destroyed by plants; however, when agricultural soils are hot and dry, the thermo- and photo-degradation of soil organic matter can yield emissions that are 25% of uptake.  Under these circumstances, using COS observations to infer carbon uptake would yield estimates that are too low.  Through this project, I acquired soils from several different ecosystems and demonstrated with laboratory studies that most all soils probably produce COS when hot and dry.  I was able to generate separate estimates of soil COS production and consumption and develop an empirical model based on soil temperature and moisture.  This study is published in an open access journal and can be found at this link: https://doi.org/10.5194/acp-16-3711-2016. The model framework was then applied in regional modeling studies (e.g. Hilton et al. 2017, link: https://doi.org/10.1038/nclimate3272) and has been implemented in the Simple Biosphere land surface model at Colorado State.

After this initial experiment, I refined our understanding by collaborating on several field and laboratory studies.  During a field campaign at the Wind River Experimental Forest I used an automatic soil chamber I re-designed to observe soil COS fluxes for several months in 2015.  This information aided multiple research articles (e.g. Rastogi, et al., 2018, link: https://doi.org/10.5194/bg-2018-85) and Rastogi’s PhD dissertation at Oregon State University.  The most interesting discovery from this activity was the strange dynamics of ground cover when the soil was frozen but the air temperate oscillated near freezing temperatures.  High COS uptake occurs when water condenses on mosses and lichen during the fluctuations from freezing to non-freezing atmospheric conditions.  I also collected field observations at an experimental corn field outside of University of Illinois, Urbana. These data suggest that high soil net COS emissions are not occurring when crops are growing because of high soil moisture, and are included in a manuscript in preparation.  Building on our new understanding of COS soil interaction, a group of collaborators and I were able to perform a pilot study to observe coastal redwood tree photosynthesis under different fog conditions (article link: https://doi.org/10.1002/2016JG003703).

To understand better the dynamics of soil COS uptake, I collaborated with another NSF fellow, Laura Meredith, to relate soil COS fluxes to the microbial communities present. COS destruction in plant leaves is thought to be controlled by the enzyme carbonic anhydrase, which is also present in soils.  Using the soil incubation chamber and method I developed, we incubated 20 different soils to determine their COS uptake or emission dynamics and Meredith assayed their microbial diversity. The results are published in an open-access journal (link: https://doi.org/10.3390/soilsystems2030037).  

This funding made possible the development of an empirical framework for describing COS soil interactions. Our broader scientific community concluded in our recent review article that soil COS fluxes are no longer an unknown and do not hinder the use of COS as a tracer for regional land carbon uptake.

Last Modified: 06/21/2018
Modified by: Mary E Whelan