Abstract
Abstract. The role that soil, foliage, and atmospheric dynamics have on surface carbonyl sulfide (OCS) exchange in a Mediterranean forest ecosystem in southern France (the Oak Observatory at the Observatoire de Haute Provence, O3HP) was investigated in June of 2012 and 2013 with essentially a top-down approach. Atmospheric data suggest that the site is appropriate for estimating gross primary production (GPP) directly from eddy covariance measurements of OCS fluxes, but it is less adequate for scaling net ecosystem exchange (NEE) to GPP from observations of vertical gradients of OCS relative to CO2 during the daytime. Firstly, OCS and carbon dioxide (CO2) diurnal variations and vertical gradients show no net exchange of OCS at night when the carbon fluxes are dominated by ecosystem respiration. This contrasts with other oak woodland ecosystems of a Mediterranean climate, where nocturnal uptake of OCS by soil and/or vegetation has been observed. Since temperature, water, and organic carbon content of soil at the O3HP should favor the uptake of OCS, the lack of nocturnal net uptake would indicate that its gross consumption in soil is compensated for by emission processes that remain to be characterized. Secondly, the uptake of OCS during the photosynthetic period was characterized in two different ways. We measured ozone (O3) deposition velocities and estimated the partitioning of O3 deposition between stomatal and non-stomatal pathways before the start of a joint survey of OCS and O3 surface concentrations. We observed an increasing trend in the relative importance of the stomatal pathway during the morning hours and synchronous steep drops of mixing ratios of OCS (amplitude in the range of 60–100 ppt) and O3 (amplitude in the range of 15–30 ppb) after sunrise and before the break up of the nocturnal boundary layer. The uptake of OCS by plants was also characterized from vertical profiles. However, the time window for calculation of the ecosystem relative uptake (ERU) of OCS, which is a useful tool for partitioning measured NEE, was limited in June 2012 to a few hours after midday. This was due to the disruption of the vertical distribution of OCS by entrainment of OCS rich tropospheric air in the morning and because the vertical gradient of CO2 reverses when it is still light. Moreover, polluted air masses (up to 700 ppt of OCS) produced dramatic variation in atmospheric OCS ∕ CO2 ratios during the daytime in June 2013, further reducing the time window for ERU calculation.
Highlights
Terrestrial ecosystems modulate the water balance over land and fix carbon dioxide (CO2) from the atmosphere in the form of carbon-rich materials
Experimental and modeling studies have shown that changes in atmospheric CO2 concentration and changes in climate, induced by increasing anthropogenic emissions of greenhouse gases, impact the fixation of atmospheric CO2 by plants and the release of CO2 by terrestrial ecosystems as modulated by temperature and water availability and the effects of fertilization (e.g., Arora and Boer, 2014)
Diel changes in the OCS mixing ratio and in its vertical distribution show that net soil exchange of OCS is negligible compared to the uptake of the gas through the stomata, a feature that is not shared by other oak woodland ecosystems characterized by a Mediterranean climate
Summary
Terrestrial ecosystems modulate the water balance over land and fix carbon dioxide (CO2) from the atmosphere in the form of carbon-rich materials. Experimental and modeling studies have shown that changes in atmospheric CO2 concentration and changes in climate, induced by increasing anthropogenic emissions of greenhouse gases, impact the fixation of atmospheric CO2 by plants (gross primary production, GPP) and the release of CO2 by terrestrial ecosystems (respiration, Reco) as modulated by temperature and water availability and the effects of fertilization (e.g., Arora and Boer, 2014). Recent studies demonstrated limitations to the use of OCS as a GPP proxy at canopy and ecosystem scales because (1) consumption and/or production of OCS occur in soil and litter (Van Diest and Kesselmeier, 2008; Sun et al, 2015; Ogée et al, 2016; Whelan et al, 2016 and references therein), (2) in agricultural fields and midlatitude forests OCS can be taken up by plants at night (Maseyk et al, 2014; White et al, 2010; Commane et al, 2015), and (3) the leaf relative uptake of OCS and of CO2 (LRU), which is of central importance in the calculation of GPP from eddy covariance measurements of OCS exchange (LOCS) following Eq (1), exhibit daily and seasonal variations of variable amplitudes (Berkelhammer et al, 2014; Maseyk et al, 2014; Commane et al, 2015)
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