Methyl chloride isotopic signatures from Irish forest soils and a comparison between abiotic and biogenic methyl halide soil fluxes

Forest soils demonstrate in a microcosm the difficulties that are faced in quantifying methyl halide budgets. Carbon isotopic analyses have been proposed as a potential tool to address these concerns and in this study we have measured significant enrichment of the methyl chloride ¹³C/¹²C isotopic ratio (from ?40.2 ± 0.8‰ to ?33.4 ± 7.4‰) after 9 min chamber emplacement on local Irish forest soils. This enrichment occurred independent of direction of methyl chloride fluxes. Measurements from soil cores in a flow‐through system (FTS) are comparable with chamber‐based isotopic measurements and indicate that methyl chloride produced abiotically from organic soil horizons has an isotopic ¹³C signature of ?53 ± 49‰, significantly less depleted than previously reported. Average net methyl chloride, methyl bromide and methyl iodide fluxes from soils (77.8 ± 2.1, 1.25 ± 3.63 and 0.35 ± 2.00 μg MeX m^?2 day^?1, respectively) are in line with previously reported values; however, a better understanding of spatial and temporal variability is needed for budget quantification. Methyl halide fluxes from FTS soil cores demonstrate that, on a per gram basis, most consumption occurs through biologically driven processes in the O horizon, with progressively smaller contributions in deeper horizons. Sporadic biogenic production was observed in shallow soil horizons only. Abiotic production was at most one‐tenth the net biological reaction rate in the O horizon and did not appear to be significantly different from zero in lower horizons. Modelled emissions based upon observed and reported rates for production, consumption and diffusion within the soil atmosphere system are unable to replicate all observed isotopic signatures from chamber fluxes.