Soil fluxes of carbonyl sulfide (COS), carbon monoxide, and carbon dioxide in a boreal forest in southern Finland.

Soil is a major contributor to the biosphere-atmosphere exchange of carbonyl sulfide (COS) and carbon monoxide (CO). COS is used to improve constraints on terrestrial photosynthesis based on the link between leaf uptake of COS and of CO₂, but this use requires the soil COS flux to be well quantified. For CO, soil is a main sink in natural environments that influences the tropospheric CO budget. We measured soil fluxes of COS, CO, and CO₂ hourly over the 2015 late growing season in a Scots pine forest in Hyytiälä, Finland. The soil acted as a net sink of COS and CO. Average uptake rates were around 3 pmol m^-2 s^-1 for COS, and 1 nmol m^-2 s^-1 for CO, respectively. Soil respiration showed seasonal dynamics controlled by soil temperature, peaking in late August and September with fluxes around 4 μmol m^-2 s^-1 and dropping to 1-2 μmol m^-2 s^-1 in October. In contrast, seasonal variations of COS and CO fluxes were weak and mainly driven by soil moisture changes through diffusion limitation. COS and CO fluxes did not appear to respond to temperature, although they both correlated well with soil respiration in specific temperature bins. We found that COS : CO₂ and CO : CO₂ flux ratios were modulated by temperature, possibly indicating shifts in active COS and CO-consuming microbial groups. Our results show that soil COS and CO uptake do not have strong variations over the late growing season in the boreal forest, and can be well described during the photosynthetically most active period. Well characterized and relatively invariant soil COS fluxes strengthen the case for using COS as a tracer for photosynthesis in this globally important biome. [ABSTRACT FROM AUTHOR]