Soils contain more than one activity consuming carbonyl sulfide

Exchange rates of carbonyl sulfide (OCS) were measured at 25 degrees C in t he 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 o f OCS changed from net production to net consumption above the OCS compensa tion concentration which was 785 pptv for PBE and 1470 pptv for RA. The OCS uptake rate constants were 0.17 and 0.141 h(-1)g(-1) dry weight soil, resp ectively. However, at OCS concentrations higher than approximately 5000 ppt v, uptake rate constants in RA became smaller suggesting saturation, and in creased again at > 50,000 pptv suggesting the existence of a second OCS con sumption activity operating at elevated OCS concentrations. In PBE, the rat e constant of OCS uptake increased at OCS concentrations higher than about 4000 pptv, also suggesting a second activity. Thus, our study indicated tha t OCS at close to ambient concentrations was consumed by a different activi ty than OCS at higher concentrations. Below the compensation point, the soi l samples acted as a source rather than a sink for atmospheric OCS, stressi ng the necessity to measure OCS Bur as function of OCS concentration to obt ain reliable source or sink data for atmospheric budgets. (C) 2000 Elsevier Science Ltd. All rights reserved.