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

Abstract

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 parts per thousand to -33.4 +/- 7.4 parts per thousand) 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 parts per thousand, 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 mu g 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

Microbial metabolism directly affects trace gases in (sub) polar snowpacks

“…The impact of archaea on methyl halide cycling is not yet established and they may play a role in either methyl halide production or consumption within soils and snowpacks. In temperate forest soils, with an estimated 0.1 billion microbial cells per cubic centimetre [47], methyl bromide is reported to be consumed at a rate of 5 mg m 22 d 21 [41]. If we assume that the density of microbial cells in the snowpack is approximately 50 000 cc 21 [48], then we would expect the microbial consumption rate for methyl bromide in the snowpack to be roughly equal to 2.5 ng m 22 d 21 , assuming all else to be equal.…”

“…Methyl bromide and methyl chloride are consumed by bacteria in soils [13,41]. Fungal production may play a role in net fluxes from terrestrial surfaces [8,9].…”

“…Trace gas concentrations were analysed on an HP 5972 GC/MSD fitted with a PoraPlot Q column (25 m, 0.32 ID, 5 mm thickness; Restek, Bellefonte, PA), similar to methods used in previous studies [39,41]. This instrument has been equipped with low-concentration halocarbon and DMS gas standards and calibration tests indicate detection limits of approximately 0.2 pptv for methyl iodide, less than 1.0 pptv for methyl bromide, less than 10 pptv for DMS and less than 50 pptv for methyl chloride.…”

Hydrogen and carbon isotope fractionation during degradation of chloromethane by methylotrophic bacteria

“…() for a more detailed treatment of stable isotope fractionation). Stable carbon isotope signatures (ratio of 13 C/ 12 C or δ 13 C values) of chloromethane sources were determined for forest soils (Redeker and Kalin ), plants (Harper et al. ; Keppler et al.…”

Chloromethane Degradation in Soils: A Combined Microbial and Two‐Dimensional Stable Isotope Approach

“…The combination of compound-specific isotope analysis and CH 3 Cl flux measurement has the potential to better constrain the atmospheric CH 3 Cl budget (Keppler et al, 2005;Saito and Yokouchi, 2008). Several studies have determined the stable carbon isotope signatures (d 13 C values) for CH 3 Cl in the atmosphere (Thompson et al, 2002); of its sources in forest soils (Redeker and Kalin, 2012), plants Keppler et al, 2004), fungi , and from biomass burning (Czapiewski et al, 2002); and of its sinks, including methylotrophic CH 3 Cl-degrading bacteria (Miller et al, 2001;Nadalig et al, 2013Nadalig et al, , 2014. Bacterial degradation of CH 3 Cl has been demonstrated in soils (Miller et al, 2004), but modeling studies (Keppler et al, 2005) have suggested that the microbial soil sink for the global atmospheric CH 3 Cl budget might be larger than originally estimated (Montzka and Fraser, 2003).…”