Abstract
Abstract. Gross primary productivity (GPP), the CO2 uptake by means of photosynthesis, cannot be measured directly on the ecosystem scale but has to be inferred from proxies or models. One newly emerged proxy is the trace gas carbonyl sulfide (COS). COS diffuses into plant leaves in a fashion very similar to CO2 but is generally not emitted by plants. Laboratory studies on leaf level gas exchange have shown promising correlations between the leaf relative uptake (LRU) of COS to CO2 under controlled conditions. However, in situ measurements including daily to seasonal environmental changes are required to test the applicability of COS as a tracer for GPP at larger temporal scales. To this end, we conducted concurrent ecosystem-scale CO2 and COS flux measurements above an agriculturally managed temperate mountain grassland. We also determined the magnitude and variability of the soil COS exchange, which can affect the LRU on an ecosystem level. The cutting and removal of the grass at the site had a major influence on the soil flux as well as the total exchange of COS. The grassland acted as a major sink for CO2 and COS during periods of high leaf area. The sink strength decreased after the cuts, and the grassland turned into a net source for CO2 and COS on an ecosystem level. The soil acted as a small sink for COS when the canopy was undisturbed but also turned into a source after the cuts, which we linked to higher incident radiation hitting the soil surface. However, the soil contribution was not large enough to explain the COS emission on an ecosystem level, hinting at an unknown COS source possibly related to dead plant matter degradation. Over the course of the season, we observed a concurrent decrease in CO2 and COS uptake on an ecosystem level. With the exception of the short periods after the cuts, the LRU under high-light conditions was rather stable and indicated a high correlation between the COS flux and GPP across the growing season.
Highlights
Carbonyl sulfide (COS) is the most abundant sulfurcontaining gas in the atmosphere, with tropospheric mole fractions of ∼ 500 ppt
The vapor pressure deficit (VPD) reached values of above 2 kPa during 25 d and plant-available water dropped below 50 % on 111 d during the campaign (Fig. 1), we did not observe any relationship with COS
The Normalized Difference Vegetation Index (NDVI) further decreased in the subsequent days as a consequence of dying plant parts remaining at the field site (Fig. 2a–c)
Summary
Carbonyl sulfide (COS) is the most abundant sulfurcontaining gas in the atmosphere, with tropospheric mole fractions of ∼ 500 ppt. The relative decrease in ambient mole fraction during summer of the Northern Hemisphere is 6 times stronger for COS than for CO2 (Montzka et al, 2007) as COS is generally not emitted by plants like CO2, which is released in respiration processes. The uptake of COS by plants is mostly mediated by the enzyme carbonic anhydrase (CA) and photolytic enzymes like Ribulose-1,5-bisphosphate-carboxylase/oxygenase (Rubisco; Lorimer and Pierce, 1989). This in turn means that COS and CO2 share a similar pathway into leaves through the boundary layer, the stomata and the cytosol, up to their reaction sites. That makes COS an interesting tracer for estimating the stomatal conductance and the gross uptake of CO2, referred to as gross primary production (GPP), on an ecosystem level (Asaf et al, 2013; Kooijmans et al, 2017, 2019)
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