Methyl bromide emissions to the atmosphere from temperate woodland ecosystems

Abstract

The environmental importance of methyl bromide (CH3Br) arises from its contribution to stratospheric ozone loss processes and, as a consequence, its emissions from anthropogenic sources are subject to the Montreal Protocol. A better understanding of the natural budget of CH3Br is required for assessing the benefit of anthropogenic emission reductions and for understanding any potential effects of environmental change on global CH3Br concentrations. Measurements of CH3Br flux in temperate woodland ecosystems, in particular, are very sparse, yet these cover a large fraction of terrestrial land surface. Results presented here from 18 months of field measurements of CH3Br fluxes in four static flux chambers in a woodland in Scotland and from enclosures of rotting wood and deciduous and coniferous leaf litter suggest net emissions from temperate woodlands. Net CH3Br fluxes in the woodland varied between the chambers, fluctuating between net uptake and net emissions (-73 to 279 ng m-2 h-1 across 161 individual measurements), and with no strong seasonality, but with time-averaged net emission overall (27 +/- 57 (1SD)) ng m-2 h-1). This work demonstrates that scale-up needs to be based on sufficient individual measurements to provide a reasonably-constrained estimate of the long-term mean. Mean (+/- 1SD) net CH3Br emissions from deciduous and coniferous leaf litter were 43 (± 33) ng kg 1 (dry weight) h-1 and 80 (± 37) ng kg-1 (dry weight) h-1, respectively, and ~1-2 ng kg-1 (fresh weight) h-1 from rotting woody litter. Despite the intrinsic variability, the data obtained here consistently point to the conclusion that the temperate forest soil/litter ecosystem is a net source of CH3Br to the atmosphere.