CHARACTERISTICS OF TURNOVER OF CARBONYL SULFIDE IN 4 DIFFERENT SOILS

Release and uptake of carbonyl sulfide (OCS) were measured at 25 degre es C in samples of three forest soils (BL, BW, PBE) and one soil from a rape field (RA). The soil samples were flushed with a constant flow of either air toxic conditions) or nitrogen (anoxic conditions) contai ning defined concentrations of OCS. A cryogenic trapping technique wit h liquid argon (-186 degrees C) was used to collect gas samples for an alysis in a gas chromatograph equipped with a flame-photometric detect or. The dependence of net OCS fluxes between soil and atmosphere could be described by a simple model of simultaneous OCS production and OCS uptake. By using this model, production rates (P), uptake rate consta nts (k) and compensation concentrations (m(c)) of OCS could be determi ned as function of the soil type and the incubation conditions. Under oxic conditions, OCS production (P) and uptake were observed in all so ils tested. However, the compensation concentrations (< 166 ng l(-1) 1 ng OCS l(-1) = 0.41 ppbv) that were calculated from the model were hi gh relative to the ambient OCS concentration (ca. 0.5 ppbv). The produ ction rates (0.16-1.9 ng h(-1) g(-1) dw) that were actually measured w hen flushing the soil samples with air containing zero OCS were smalle r than those (17-114 ng h(-1) g(-1) dw) calculated from the model. Thi s observation was explained by two different concepts: one assuming th e existence of a threshold concentration (m(t)) below which OCS was no longer consumed in the soil; the other assuming the existence of two different OCS consumption processes, of which only the process active at elevated OCS concentrations was covered by the experiments. The lat ter concept allowed the estimation of OCS compensation concentrations that were partially low enough to allow the uptake of atmospheric OCS by soil. Both OCS production and uptake in PBE soil were dependent on soil temperature (optimum 20 degrees C) indicating a microbial process . However, both production and consumption of OCS were not consistentl y inhibited by sterilization of the soil, suggesting that they were no t exclusively due to microbiological processes. Under anoxic condition s, OCS was also produced, but was not consumed except in one soil (RA) . Production of OCS in the soils was stimulated after addition of thio cyanate, but not thiourea, thiosulfate, thioglycolate, tetrathionate, sulfate, elemental sulfur, cysteine and methionine.