Carbonyl sulfide (OCS) exchange between soils and the atmosphere affected by soil moisture and compensation points

Abstract. Carbonyl sulfide (OCS) is a chemically quite stable gas in the troposphere (lifetime ~ 2–6 years) and consequently some of it is transported up to the stratosphere where it contributes to the stratospheric sulfate layer. Due to the similarities in uptake mechanism between OCS and CO₂, the use of OCS as a proxy for CO₂ in ecosystem gross primary production (GPP) has been proposed. For this application a good understanding of uptake (U_OCS) and production (P_OCS) processes of OCS in an ecosystem is required. A new OCS quantum cascade laser coupled with an automated soil chamber system enabled us to measure the soil-atmosphere OCS exchange of four different soil samples with high precision. The adjustment of the chamber air to different OCS mixing ratios (50, 500, and 1000 ppt) allowed us to separate production and consumption processes and to estimate compensation points (CPs) for the OCS exchange. At an atmospheric mixing ratio of 1000 ppt, the maximum U_OCS was of the order of 22 to 110 pmol g⁻¹ h⁻¹ for needle forest soil samples and of the order of 3 to 5 pmol g⁻¹ h⁻¹ for an agricultural mineral soil, both measured at moderate soil moisture. Uptake processes (U_OCS) were dominant at all soil moistures for the forest soils, while P_OCS exceeded U_OCS at higher soil moistures for the agricultural soil, resulting in net emission. Hence, our results indicate that in (spruce) forests U_OCS might be the dominant process, while in agricultural soils P_OCS at higher soil moisture and U_OCS under moderate soil moisture seem to dominate the OCS exchange. The OCS compensation points (CPs) were highly dependent on soil water content and extended over a wide range of 130 ppt to 1600 ppt for the forest soils and 450 ppt to 5500 ppt for the agricultural soil. The strong dependency between soil water content and the compensation point value must be taken into account for all further analyses. The lowest CPs were found at about 20 % water filled pore space (WFPS_lab), implying the maximum of U_OCS under these soil moisture conditions and excluding OCS emission under such conditions. We discuss our results in view of other studies about compensation points and the potential contribution of microbial groups.