Canopy uptake dominates nighttime carbonyl sulfide fluxes in a boreal forest

Abstract

Abstract. Nighttime vegetative uptake of carbonyl sulfide (COS) can exist due to the incomplete closure of stomata and the light independence of the enzyme carbonic anhydrase, which complicates the use of COS as a tracer for gross primary productivity (GPP). In this study we derived nighttime COS fluxes in a boreal forest (the SMEAR II station in Hyytiälä, Finland; 61 • 51 N, 24 • 17 E; 181 m a.s.l.) from June to November 2015 using two different methods: eddycovariance (EC) measurements (F COS-EC ) and the radontracer method (F COS-Rn ). The total nighttime COS fluxes averaged over the whole measurement period were −6.8 ± 2.2 and −7.9 ± 3.8 pmol m −2 s −1 for F COS-Rn and F COS-EC , respectively, which is 33-38 % of the average daytime fluxes and 21 % of the total daily COS uptake. The correlation of 222 Rn (of which the source is the soil) with COS (average R 2 = 0.58) was lower than with CO 2 (0.70), suggesting that the main sink of COS is not located at the ground. These observations are supported by soil chamber measurements that show that soil contributes to only 34-40 % of the total nighttime COS uptake. We found a decrease in COS uptake with decreasing nighttime stomatal conductance and increasing vapor-pressure deficit and air temperature, driven by stomatal closure in response to a warm and dry period in August. We also discuss the effect that canopy layer mixing can have on the radon-tracer method and the sensitivity of (F COS-EC ) to atmospheric turbulence. Our results suggest that the nighttime uptake of COS is mainly driven by the tree foliage and is significant in a boreal forest, such that it needs to be taken into account when using COS as a tracer for GPP.

Gross Primary Productivity of Four European Ecosystems Constrained by Joint CO2 and COS Flux Measurements

“…net CO 2 release), the light‐independent canopy uptake of COS continued at lower rates during night time (Figures and S5–S12) due to incomplete stomatal closure. This finding is in agreement with other studies (Kooijmans et al, , ; Novick et al, ), although we did not observe an earlier peak in COS uptake as compared to NEE (Figure ) or GPP as Kooijmans et al () did. Note that the nighttime residual uptake of COS, when GPP is zero, does not void the general approach of using COS as a proxy for GPP, as this is accounted for by the light‐dependent parameterization of LRU, which approaches infinity at low light (Eq.…”

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

“…Although the majority of the analyses presented here has focused on daytime values when the ecosystems were productive, we also investigated the nighttime dynamics of g because of the role of this process in nighttime water loss and as an indicator of other controls on g such as ozone damage (Caird et al, ). Consistent with previous studies, we see strong evidence for nighttime stomatal uptake of OCS throughout the summer at the prairie (Berkelhammer et al, ; Kooijmans et al, ). The magnitude of nighttime g ranged from between 1% and 20% of the previous day's midday value with an average of 6% (Figure a).…”

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

“…At this site, nocturnal uptake contributed 23% of the total daily leaf COS uptake. This fraction is comparable to those reported from a wheat field (29 ± 5%, Maseyk et al, 2014), an alpine temperate forest (25-30%, Berkelhammer et al, 2014), a boreal pine forest (17%, Kooijmans et al, 2017), and a New England mixed forest (< 20% after subtracting soil uptake, Commane et al, 2015;Wehr et al, 2017). Collectively, these studies indicate that nocturnal uptake is typically 17-30% of the total canopy COS budget, a fraction that is too large to ignore in ecosystem and 15 regional COS budget studies.…”

“…To understand the relationship between daily integrated COS and CO 2 fluxes for regional flux inversion (e.g., Hilton et al, 2015), nighttime COS uptake needs to be constrained . Nighttime COS uptake has been found in a wheat field , a boreal pine forest (Kooijmans et al, 2017), and temperate forests (Berkelhammer et al, 2014;Commane et al, 2015;Wehr et al, 2017). However, most field studies based their findings upon indirect evidence of nighttime ecosystem COS uptake, with only one study reporting some direct leaf observations of nighttime uptake 30 (Berkelhammer et al, 2014).…”

“…This translates to 10±2 mmol m −2 s −1 for the stomatal conductance to water (g s ), after accounting 20 for the different diffusivities of water and COS in the air with a ratio of 2.0 (Seibt et al, 2010). The nocturnal g s,COS is at the lower end of values reported for other ecosystems, ranging from 1.6 mmol m −2 s −1 for a New England mixed forest (Wehr et al, 2017) to 5-20 mmol m −2 s −1 for a Scots pine forest (Kooijmans et al, 2017), 11.5 mmol m −2 s −1 for a wheat field , and 13-20 and 22-66 mmol m −2 s −1 for pine and poplar trees, respectively, in an alpine temperate forest (Berkelhammer et al, 2014).…”