Stomatal control of leaf fluxes of carbonyl sulfide and CO₂ in a Typha freshwater marsh

Wu Sun , Kadmiel Maseyk (1) , Céline Lett (2) , Ulli Seibt (1)

1 Institute of Ecology & Environmental Sciences of Paris
2 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]

Carbonyl sulfide (COS) is an emerging tracer to constrain land photosynthesis at canopy to global scales, because leaf COS and CO₂ 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 increase under low light, but the environmental controls over LRU variability in the field are poorly understood due to scant leaf scale observations.

Here we present the first direct observations of LRU responses to environmental variables in the field. We measured leaf COS and CO₂ fluxes at a freshwater marsh in summer 2013. Daytime leaf COS and CO₂ uptake showed similar peaks in the mid-morning and late afternoon separated by a prolonged midday depression, highlighting the common stomatal control on diffusion. At night, in contrast to CO₂, 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 integrated LRU (calculated from daytime mean COS and CO₂ uptake) ranged from 1 to 1.5, with a mean of 1.2 across the campaign, significantly lower than the previously reported laboratory mean value (∼ 1.6). Our results indicate two major determinants of LRU - light and vapor deficit. Light is the primary driver of LRU because CO₂ assimilation capacity increases with light, while COS consumption capacity does not. Superimposed upon the light response is a secondary effect that high vapor deficit further reduces LRU, causing LRU minima to occur in the afternoon, not at noon. The partial stomatal closure induced by high vapor deficit suppresses COS uptake more strongly than CO₂ uptake because stomatal resistance is a more dominant component in the total resistance of COS. Using stomatal conductance estimates, we show that LRU variability can be explained in terms of different patterns of stomatal vs. internal limitations on COS and CO₂ uptake. Our findings illustrate the stomata-driven coupling of COS and CO₂ uptake during the most photosynthetically active period in the field and provide an in situ characterization of LRU - a key parameter required for the use of COS as a photosynthetic tracer.

Domaines: Planète et Univers [physics]

Wu Sun, Kadmiel Maseyk, Céline Lett, Ulli Seibt. Stomatal control of leaf fluxes of carbonyl sulfide and CO₂ in a Typha freshwater marsh. Biogeosciences, 2018, 15, pp.3277-3291. ⟨10.5194/bg-15-3277-2018⟩. ⟨insu-03660293⟩