Carbonyl sulfide exchange between terrestrial ecosystems and the atmosphere

Carbonyl sulfide (COS), a trace gas present in the atmosphere in tiny amounts, diffuses into leaves along a similar pathway as carbon dioxide (CO2) and water vapour (H2O) diffuse in and out, respectively. In contrast to CO2, however, no emission occurs for COS, which makes it a potential tracer for plant net photosynthesis, transpiration and stomatal conductance, processes which are difficult to quantify at the ecosystem scale. Very recent advances in spectroscopic analytical technologies now enable quantifying concurrent eddy covariance flux measurements of COS, CO2 and H2O at ecosystem scale and to test the potential of COS as a tracer for plant net photosynthesis and transpiration. If COS can be shown to represent a sensible tracer for canopy gas exchange of O2 and H2O, major progress is to be expected for our understanding and capability of quantifying these processes. The main aims of the project are to (i) quantify the COS, CO2 and H2O exchange over major biomes in the major climate zones and (ii) based on these data to scrutinise the assumptions involved in using COS as a tracer for canopy net photosynthesis and transpiration/stomatal conductance. To this end we propose eddy covariance flux measurements of COS, CO2 and H2O at several different ecosystems in different climates (evergreen/deciduous needle-/broadleaf forests and grasslands in Mediterraneanemperate/boreal climates) combined with concurrent measurements of key parameters of leaf and soil exchange.

Carbonyl sulfide (COS), a trace gas present in the atmosphere in tiny amounts, diffuses into leaves along a similar pathway as carbon dioxide (CO2) and water vapor (H2O) diffuse in and out, respectively. In contrast to CO2, however, no emission occurs for COS, which makes it a potential tracer for plant photosynthesis, transpiration and stomatal conductance, processes which are difficult to quantify at the ecosystem scale. The overarching goal of this project was to concurrently quantify the COS, CO2 and H2O at ecosystem scale using the eddy covariance method for investigating the potential of COS as a tracer for plant photosynthesis and transpiration. To that end a total of 9 measurement campaigns were conducted at different ecosystems in different climatic zones of Europe and Asia from semi-arid/Mediterranean forest/savanna ecosystems over temperate grasslands, croplands and deciduous forests to boreal coniferous forests. At each site the ecosystem-scale COS, CO2 and H2O exchange was quantified using the eddy covariance method and the soil COS and CO2 exchange with soil chambers. Results show that the soil contributes to the ecosystem-scale COS exchange in a significant fashion, especially in ecosystems characterized by sparse canopy architecture. In these, a large fraction of the sunlight is able to penetrate to the soil surface where, presumably through the interaction of organic matter and the short wavelengths of sunlight, it causes emissions of COS. At night and in ecosystems characterized by a closed canopy with little transmission of sunlight to the soil surface, soils usually act as sinks for COS. These data were used to develop models of the soil COS exchange, which then could be subtracted from the ecosystem-scale exchange to derive the canopy contribution. At ecosystem scale it could be shown that the COS and CO2 exchange by photosynthesis correlate with each other, but that the ratio of these two fluxes appears ecosystem-specific and not, as previously thought, universal. As this ratio is crucial for the applicability of COS for estimating canopy photosynthesis and transpiration, further research will be directed towards elucidating the processes and mechanisms underlying this variability.