Seasonal fluxes of carbonyl sulfide in a midlatitude forest

Abstract

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 CO 2 . 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 | carbon cycle | sulfur cycle | stomatal conductance C arbonyl 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 H 2 S and CO 2 . CA is the same enzyme that hydrolyzes carbon dioxide (CO 2 ) 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 CO 2 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 w...

Canopy uptake dominates nighttime carbonyl sulfide fluxes in a boreal forest

“…This means that vegetative uptake of COS can continue during the night if stomata are not completely closed . Caird et al (2007) showed that nighttime stomatal conductance exists in a wide variety of plant species and several studies report nighttime depletion of COS mole fractions (White et al, 2010;Belviso et al, 2013;Commane et al, 2013Commane et al, , 2015Berkelhammer et al, 2014;Maseyk et al, 2014;Wehr et al, 2017). The measurements presented in White et al (2010), Maseyk et al (2014), Berkelhammer et al (2014) and Wehr et al (2017) indicated that nighttime ecosystem COS fluxes were indeed dominated by the vegetation, and not by the soil.…”

“…Besides restricting COS as a GPP tracer to light conditions, the leaf relative uptake ratio (LRU), which is the normalized ratio between COS and CO 2 fluxes, can be expected to increase when GPP becomes zero around sunrise and sunset while at the same time COS is continuously being taken up by vegetation. So far, only Stimler et al (2011) have showed the light dependence of LRU from leaf-scale measurements and Maseyk et al (2014) and Commane et al (2015) observed a light dependence in the ratio of ecosystem fluxes of COS and CO 2 . Other studies have focused on LRU values under high-light conditions (e.g., Sandoval-Soto et al, 2005;Berkelhammer et al, 2014).…”

Reviews and Syntheses: Carbonyl Sulfide as a Multi-scale Tracer for Carbon and Water Cycles

“…The available observations are limited in time and do not cover tropical ecosystems, which contribute almost 60% of global GPP (Beer et al, 2010). The only study reporting year-round OCS flux measurements is from a mixed temperate forest, which was a sink 15 for OCS with a net flux of -4.7 pmol m -2 s -1 during the observation period (Commane et al, 2015).…”

Soil fluxes of carbonyl sulfide (COS), carbon monoxide, and carbon dioxide in a boreal forest in southern Finland

“…Since ocean COS emissions are geographically separated from the terrestrial COS sinks (leaf and soil), and anthropogenic emissions are usually concentrated as point sources, the spatial separation of dominant COS sources and sinks enables us to constrain land COS fluxes, and hence photosynthetic carbon uptake, from atmospheric COS observations (Campbell et al, 2008;Berry et al, 2013;Hilton et al, 2015). However, for the use of COS as a photosynthetic tracer, soil COS flux, which is unrelated to photosynthesis, needs to be understood and separated from the ecosystem COS flux that is the sum of leaf and soil fluxes Commane et al, 2015;Wehr et al, 2017).…”