Stomatal control of leaf fluxes of carbonyl sulfide and CO<sub>2</sub> in a <i>Typha</i> freshwater marsh

Abstract

Abstract. Carbonyl sulfide (COS) is an emerging tracer to constrain land photosynthesis at canopy to global scales, because leaf COS and CO 2 uptake processes are linked through stomatal diffusion. The COS tracer approach requires knowledge of the concentration normalized ratio of COS uptake to photosynthesis, commonly known as the leaf relative uptake (LRU). LRU is known to vary with light, but the environmental controls over LRU variability in the field are poorly understood due to scant leaf scale observations. 5Here we present the first direct observations of the LRU versus light relationship in the field. We measured leaf COS and CO 2 fluxes at a freshwater marsh in summer 2013. Daytime leaf COS and CO 2 uptake showed similar peaks in the mid-morning and late afternoon, separated by a midday depression, highlighting the common stomatal control on COS and CO 2 diffusion.At night, in contrast to CO 2 , COS uptake continued, indicating partially open stomata. LRU ratios showed a clear relationship with photosynthetically active radiation (PAR), converging to 1.0 at high PAR, while increasing sharply at low PAR. Daytime 10 integrated LRU ranged from 1 to 1.5, with a mean of 1.2 across the campaign, significantly lower than the mean value reported from laboratory measurements (∼1.6). Our results indicate two major determinants of LRU-light and vapor pressure deficit (or evaporative demand). Light is the primary driver of LRU because CO 2 reactions are light limited but the COS reaction is not. In a secondary effect, high evaporative demand tends to reduce LRU values. During periods of high evaporative demand, leaves conserve water by partial stomatal closure. This reduces COS uptake more than CO 2 uptake because stomatal resistance 15 is a more dominant component in the COS diffusional pathway. High evaporative demand usually coincides with high PAR, leading to the lowest observed LRU in the afternoon. Our findings illustrate the stomatal coupling of COS and CO 2 uptake during the most photosynthetically active period in the field, and provide important characterization of LRU, a key parameter to support the use of COS as a photosynthetic tracer.

Seasonal Evolution of Canopy Stomatal Conductance for a Prairie and Maize Field in the Midwestern United States from Continuous Carbonyl Sulfide Fluxes

“…To a first order, the diurnal cycle in both g s and GPP are paced by incoming PAR, but a distinction between the two can be observed at the prairie during June and July when a midday suppression of g s emerged that was not present in GPP (Figure 2a). This feature has been observed elsewhere (Sun et al, 2018) and reflects the temperature or VPD limitation on g s relative to the light limitation on GPP (Figures 2e and S12). A difference between the shape of the diurnal cycle of g s and GPP was not present later in the season when cooler temperatures and lower light led to synchronous cycles (Figure 2b).…”

“…As OCS measurements have proliferated (Whelan et al, 2018), the data have often been utilized either as an independent tracer for GPP (Asaf et al, 2013) or assimilated with other data to constrain GPP (Spielmann et al, 2019). However, as Kooijmans et al (2019) and Sun et al (2018) have recently noted, there are distinctions in the response of OCS and CO 2 uptake to atmospheric forcings such as VPD and light. The different responses between the flux of the two gases to the canopy highlight how OCS is more specifically responsive to g s (Wehr et al, 2017;Wohlfahrt et al, 2012).…”