Identifying sources and sinks: An investigation of Irish forest soil fluxes of methyl chloride using isotopic analysis

Methyl chloride provides 20% of the inorganic chlorine in the stratosphere and is therefore responsible for approximately 10-15% of the annual ozone loss. Methyl chloride is primarily naturally produced, with minor human induced changes in its global budget. Several key areas within the budget remain relatively unexplored. Atmospheric fluxes of methyl chloride to and from soil ecosystems are poorly understood due to the complexity of the microbial, fungal and abiotic interactions. Isotopic studies have shown their utility in separating source and sink terms for the same compound. Here we present results from analyses of Irish forest soils as an indication of the functionality of this method. Flow-through chambers were used to separate microbial, fungal and abiotic signatures. Surprisingly, we find that the isotopic signature of abiotic production at ambient temperatures is nearly identical to the ambient atmospheric methyl chloride signature (-39±Â±1.5 per mil). Living soils are generally significantly enriched (-28±Â±8 per mil). This study does not provide clear evidence of correlation between soil temperature and isotopic signature and isotopic signatures do not correlate with flux except on a chamber by chamber basis, indicating that there are multiple processes occurring simultaneously. Our results indicate that within some chambers headspace air is exchanged nearly completely within 5 minutes with the soil pore space.