Insights to anomalous positive carbonyl sulfide fluxes in a boreal forest by using eddy covariance flux measurements.

Vegetation is the major sink of carbonyl sulfide (COS), and for this reason, COS is a valuable proxy for assessing gross primary productivity (GPP). In this study, a comprehensive analysis of three years (2020-2022) of eddy-covariance measurements over a boreal forest in Hyytiälä (Finland) has unravelled intriguing patterns in COS fluxes in response to environmental conditions. The measurements were done following a partial harvest of the forest stand, where 40% of the basal area was removed from the footprint area. An anticipated reduction in uptake was observed in the summer of both 2020 and 2022. However, during the summer of 2021, anomalous positive COS fluxes (emission of COS) were consistently observed. The primary mechanism responsible for the removal of COS in leaves is hydrolysis, facilitated by the enzyme carbonic anhydrase (CA), leading to the production of hydrogen sulfide (H2S) and carbon dioxide (CO2). In the case of the Hyytiälä forest, CA-driven hydrolysis of the canopy acted as the dominant sink for COS. A detailed examination of environmental conditions prevailing during this period revealed a confluence of factors contributing to the unusual COS fluxes. Elevated temperatures, higher vapour pressure deficit, and decreased soil water content during the 2021 summer were identified as potential reasons for the anomalous COS flux responses. These conditions could collectively exert a suppressing effect on both stomatal and non-stomatal uptake of COS by the vegetation and soil. The empirical soil model used in the study also points towards increased abiotic production of COS from the soil due to increased soil temperature. However, the environmental conditions alone cannot explain the positive emissions during the daytime. Examining night-time fluxes shows that the canopy still uptakes COS even at a reduced rate. The analyses point towards the photodegradation production of COS from litter from the forest floor, which is overlooked in the empirical soil models and canopy uptake models. The thinning of the forest stand has led to a more open subcanopy, allowing increased sunlight penetration to the forest floor. The pine needles and residuals from the thinning are speculated to be the source of the photodegradation production of COS. Understanding the possible sources of these anomalous COS fluxes is crucial for refining our interpretation of COS as a proxy for GPP and, consequently, enhancing our ability to model and predict ecosystem productivity in a changing climate. In summary, the discovery of anomalous positive COS fluxes in the Hyytiälä boreal forest during the summer of 2021 represents a unique and significant observation, prompting further research into the complex interplay of environmental variables influencing COS fluxes within the boreal forest ecosystem.