Long-Term Cumulative Exposure Exacerbates the Effects of Atmospheric Ammonia on an Ombrotrophic Bog: Implications for Critical Levels

A line source of ammonia, simulating NH₃ emissions from an intensive livestock unit, was established in 2002 on an ombrotrophic bog, Whim bog in the Scottish Borders. The site is at 55° 46'N, 3° 16'W and (based on 4.5 years) has a mean monthly temperature of 8.8°C and mean annual rainfall 971 mm, supporting a Calluna Eriophorum vegetation, NVC M19. Release of NH₃ is controlled by meteorological conditions i.e. wind direction and speed. NH₃ concentrations were measured at >10 distances along the transect of release using passive ALPHA samplers, fixed at two or more heights above the vegetation. Effects were recorded for the ericoids, mosses, Sphagnum and Cladonia lichens with respect to cover changes, visible damage, nutrients and interactions with abiotic stress. These data, collected over >4 years, have been used to evaluate the Critical Level (CLE) for NH₃ and compared with values derived from short—term (weeks) exposures at high concentrations in controlled conditions. Results suggest the annual Critical Level of 8 μg m−³ is too high to protect sensitive species experiencing long-term exposures to NH₃. In particular, they show exposure periods longer than 1 year lead to detrimental effects at lower mean NH₃ concentrations through relationships between CLE and exposure period that are still linear after 4.5 years exposure. These results highlight the need to set a CLE representing the mean concentration of NH₃ in air for long term protection of sensitive habitats (e.g. over 20–30 years), that will be substantially smaller than the current annual CLE.