Seasonal fluxes of carbonyl sulfide in a midlatitude forest

Carbonyl sulfide (OCS), the most abundant sulfur gas in the atmosphere, has a summer minimum associated with uptake by vegetation and soils, closely correlated with CO₂. We report the first direct measurements to our knowledge of the ecosystem flux of OCS throughout an annual cycle, at a mixed temperate forest. The forest took up OCS during most of the growing season with an overall uptake of 1.36 ± 0.01 mol OCS per ha (43.5 ± 0.5 g S per ha, 95% confidence intervals) for the year. Daytime fluxes accounted for 72% of total uptake. Both soils and incompletely closed stomata in the canopy contributed to nighttime fluxes. Unexpected net OCS emission occurred during the warmest weeks in summer. Many requirements necessary to use fluxes of OCS as a simple estimate of photosynthesis were not met because OCS fluxes did not have a constant relationship with photosynthesis throughout an entire day or over the entire year. However, OCS fluxes provide a direct measure of ecosystem-scale stomatal conductance and mesophyll function, without relying on measures of soil evaporation or leaf temperature, and reveal previously unseen heterogeneity of forest canopy processes. Observations of OCS flux provide powerful, independent means to test and refine land surface and carbon cycle models at the ecosystem scale.Carbonyl sulfide (OCS) is the most abundant sulfur gas in the atmosphere (1), and biogeochemical cycling of OCS affects both the stratosphere and the troposphere. The tropospheric OCS mixing ratio is between 300 and 550 parts per trillion (ppt) (1) (10⁻¹² mol OCS per mol dry air), decreasing sharply with altitude in the stratosphere (2). In times of low volcanic activity, the sulfur budget and aerosol loading of the stratosphere are largely controlled by transport and photooxidation of OCS from the troposphere (3). The processes regulating emission and uptake of OCS are thus important factors in determining how changes in climate and land cover may affect the stratospheric sulfate layer.Oceans are the dominant source of atmospheric OCS (4), with smaller emissions from anthropogenic and terrestrial sources, such as wetlands and anoxic soils (e.g., refs. 5 and 6) and oxic soils during times of heat or drought stress (e.g., refs. 7 and 8). The terrestrial biosphere is the largest sink for OCS (1, 4, 9, 10) with uptake by both oxic soils (e.g., ref. 11) and vegetation (e.g., ref. 9). Once OCS molecules pass through the stomata of leaves, the uptake rate of OCS is controlled by reaction with carbonic anhydrase (CA) within the mesophyll, to produce H₂S and CO₂. CA is the same enzyme that hydrolyzes carbon dioxide (CO₂) in the first chemical step of photosynthesis (12).Studies considering the large-scale atmospheric variability of OCS have linked OCS fluxes and the photosynthetic uptake of CO₂ for regional and global scales (1, 4, 13). Leaf-scale studies have confirmed the OCS link to photosynthesis (14, 15). Initial OCS ecosystem flux estimations were made using flask sampling followed by analysis via gas chromatography–mass spectrometry (GC-MS) (13, 16), but these studies did not have sufficient resolution to examine daily or hourly controls on the OCS flux. Laser spectrometers have been developed (17, 18) to enable direct, in situ measurement of OCS fluxes by eddy covariance, and measurements of OCS ecosystem fluxes have been reported, for periods of up to a few weeks, above arid forests (19) and an agricultural field (8, 20).Net carbon exchange in terrestrial ecosystems [net ecosystem exchange (NEE)] can be measured by eddy flux methods. NEE may be regarded as the sum of two gross fluxes: gross ecosystem productivity (GEP) and ecosystem respiration (R_eco). GEP is the light-dependent part of NEE, estimated by subtracting daytime ecosystem respiration (R_eco), computed by extrapolation of the temperature dependence of nighttime NEE (NEE – R_eco = GEP) (e.g., refs. 21–24). At night, NEE includes all autotrophic and heterotrophic respiration processes. During the day, GEP approximates the carboxylation rate minus photorespiration at the ecosystem scale (25). Extrapolation of nighttime R_eco introduces major uncertainty in the interpretation of GEP, which could be reduced, and the ecological significance of GEP increased, by developing independent methods of measuring rates of photosynthetic processes. As shown below, fluxes of OCS give more direct information on one of the major controls on GEP, stomatal conductance, rather than GEP itself, providing a powerful means for testing and improving ecosystem models and for scaling up leaf-level processes to the whole ecosystem.Here we describe the factors controlling the hourly, daily, seasonal, and total fluxes of OCS in a forest ecosystem, using a year (2011) of high-frequency, direct measurements at Harvard Forest, MA. We report the seasonal cycle, the response to environmental conditions, and the total deposition flux of OCS throughout the year 2011. We compare these fluxes to corresponding measurements of CO₂ flux and to simulations using the Simple Biosphere model (SiB3).