Continuous In-situ Measurements of Carbonyl Sulfide to Constrain Ecosystem Carbon and Water Exchange

Thursday, 18 December 2014
Bharat Rastogi1, Youngil Kim2, Max B Berkelhammer3, David C Noone4, Chun-Ta Lai5, David Y Hollinger6, Kenneth Bible7, J. Brian Leen8, Manish Gupta8 and Christopher J Still2, (1)UCSB, Corvallis, OR, United States, (2)Oregon State University, Corvallis, OR, United States, (3)University of Illinois at Chicago, Chicago, IL, United States, (4)Dept Atmospheric & Oceanic Sci, Boulder, CO, United States, (5)San Diego State University, San Diego, CA, United States, (6)University of New Hampshire Main Campus, Durham, NH, United States, (7)University of Washington, Seattle, School of Environmental and Forest Sciences College of the Environment, Seattle, WA, United States, (8)Los Gatos Research, Mountain View, CA, United States
Understanding the processes that control the terrestrial exchange of carbon and water are critical for examining the role of forested ecosystems in changing climates. A small but increasing number of studies have identified Carbonyl Sulfide (OCS) as a potential tracer for photosynthesis. OCS is hydrolyzed by an irreversible reaction in leaf mesophyll cells that is catalyzed by the enzyme, carbonic anhydrase. Leaf-level field and greenhouse studies indicate that OCS uptake is controlled by stomatal activity and that the ratio of OCS and CO2 uptake is reasonably constant. Existing studies on ecosystem OCS exchange have been based on laboratory measurements or short field campaigns and therefore little information on OCS exchange in a natural ecosystem over longer timescales is available.

The objective of this study is to further assess the stability of OCS as a tracer for canopy photosynthesis in an active forested ecosystem and also to assess its utility for constraining transpiration, since both fluxes are mediated by canopy stomatal conductance. An off-axis integrated cavity output spectroscopy analyzer (Los Gatos Research Inc.) was deployed at the Wind River Experimental Forest in Washington (45.8205°N, 121.9519°W). Canopy air was sampled from three heights to measure vertical gradients of OCS within the canopy, and OCS exchange between the forest and the atmosphere. Here we take advantage of simultaneous measurements of the stable isotopologues of H2O and CO2 at corresponding heights as well as NEE (Net Ecosystem Exchange) from eddy covariance measurements to compare GPP (Gross Primary Production) and transpiration estimates from a variety of independent techniques. Our findings seek to allow assessment of the environmental and ecophysicological controls on evapotranspiration rates, which are projected to change in coming decades, and are otherwise poorly constrained.