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

Chloromethane (CH 3 Cl, methyl chloride) is the most abundant volatile halocarbon in the atmosphere and involved in stratospheric ozone depletion. The global CH 3 Cl budget, and especially the CH 3 Cl sink from microbial degradation in soil, still involves large uncertainties. These may potentially be resolved by a combination of stable isotope analysis and bacterial diversity studies. We determined the stable isotope fractionation of CH 3 Cl hydrogen and carbon and investigated bacterial diversity during CH 3 Cl degradation in three soils with different properties (forest, grassland, and agricultural soils) and at different temperatures and headspace mixing ratios of CH 3 Cl. The extent of chloromethane degradation decreased in the order forest > grassland > agricultural soil. Rates ranged from 0.7 to 2.5 μg g −1 dry wt. d −1 for forest soil, from 0.1 to 0.9 μg g −1 dry wt. d −1 for grassland soil, and from 0.1 to 0.4 μg g −1 dry wt. d −1 for agricultural soil and increased with increasing temperature and CH 3 Cl supplementation. The measured mean stable hydrogen enrichment factor of CH 3 Cl of −50 ± 13‰ was unaffected by temperature, mixing ratio, or soil type. In contrast, the stable carbon enrichment factor depended on CH 3 Cl degradation rates and ranged from −38 to −11‰. Bacterial community composition correlated with soil properties was independent from CH 3 Cl degradation or isotope enrichment. Nevertheless, increased abundance after CH 3 Cl incubation was observed in 21 bacterial operational taxonomical units (OTUs at the 97% 16S RNA sequence identity level). This suggests that some of these bacterial taxa, although not previously associated with CH 3 Cl degradation, may play a role in the microbial CH 3 Cl sink in soil. Core Ideas Chloromethane biodegradation is affected by temperature and soil properties. The stable carbon enrichment factor of CH 3 Cl strongly depends on CH 3 Cl consumption. The overall bacterial community is unaffected by chloromethane degradation. Unexpected bacterial taxa displayed increased abundance upon CH 3 Cl incubation.