Soils contain more than one activity consuming carbonyl sulfide

Exchange rates of carbonyl sulfide (OCS) were measured at 25°C in the laboratory in soil samples from a forest (PBE) and a former rape field (RA) over a range of OCS concentrations (250 – 120,000 pptv). The exchange of OCS changed from net production to net consumption above the OCS compensation concentration which was 785 pptv for PBE and 1470 pptv for RA. The OCS uptake rate constants were 0.17 and 0.14 l h−1 g−1 dry weight soil, respectively. However, at OCS concentrations higher than approximately 5000 pptv, uptake rate constants in RA became smaller suggesting saturation, and increased again at >50,000 pptv suggesting the existence of a second OCS consumption activity operating at elevated OCS concentrations. In PBE, the rate constant of OCS uptake increased at OCS concentrations higher than about 4000 pptv, also suggesting a second activity. Thus, our study indicated that OCS at close to ambient concentrations was consumed by a different activity than OCS at higher concentrations. Below the compensation point, the soil samples acted as a source rather than a sink for atmospheric OCS, stressing the necessity to measure OCS flux as function of OCS concentration to obtain reliable source or sink data for atmospheric budgets.