Influence of environmental factors on biosphere-atmosphere exchange of carbonyl sulfide (OCS) with special focus on elevated CO2-levels and soils
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Abstract
Carbonyl sulfide (OCS) is the most abundant sulfur containing trace gas in the troposphere. It is linked to the carbon and sulfur cycle and, acts as a greenhouse gas in the troposphere and is related to the stratospheric aerosol layer. It is also considered a possible tracer for CO2 in Gross primary productivity (GPP), because there is a close link between the uptake of OCS and exchange of CO2 between plants and atmosphere. Both soils and plants play an important role in the budget of OCS. Therefore, any estimation of GPP based on the OCS exchange must take into account the contribution of the exchange with the corresponding soil. Many environmental factors influence soil-atmosphere or plant-atmosphere OCS exchange. In this work the influence of the ambient CO2 and OCS mixing ratio, soil moisture and antimicrobial agents on this exchange has been examined.
The OCS exchange of four arable soils was examined at elevated CO2 mixing ratios (soil pore concentrations) at varying soil moisture. These arable soils showed a tendency towards OCS emission at low and high soil moisture and towards OCS uptake at medium soil moisture. With increasing CO2 mixing ratio, three soils showed an increase of OCS emission at high and low soil moisture and a reduction of OCS uptake at medium soil moisture. One organically fertilized soil showed an increase in OCS uptake at medium soil moisture with increasing CO2 mixing ratio instead. Treatment with chloroform vapor yielded ambivalent results but demonstrated the involvement of biotic processes in soil-atmosphere OCS exchange for those soils. Application of Nystatin and Streptomycin solutions to one arable soil and one rainforest soil demonstrated that fungi possibly dominated OCS uptake in those soils.
In another set of experiments OCS exchange of three forest soils and one arable soil at high and low OCS mixing ratio at varying soil moistures was measured. Based on these measurements OCS production (POCS) and consumption (UOCS) at 1000 ppt OCS mixing ratio were determined. Their compensation points were found to vary with soil moisture but generally indicated that needle forest soils mainly act as sinks. Compensation point variations calculated based on these process studies were in good agreement with single values as found in the literature. Furthermore, the OCS exchange measured with one forest soil sample (soil from the SMEAR II station in Hyytiälä, Finland) agreed very well with field data from the same site as published recently by other researchers.
OCS uptake of plants at elevated CO2 mixing ratios was examined in a third set of experiments. OCS uptake declined with increasing CO2 mixing ratios. Calculations of stomatal conductance based on the water vapor released by the examined plants demonstrate that the reduction of OCS uptake at higher OCS concentration is mediated by stomatal aperture. This control over OCS uptake by stomatal conductance is in good agreement with literature. An experiment with a toxin causing a stomatal opening further confirmed that mechanism.