Quantifying dry NH(3) deposition to an ombrotrophic bog from an automated NH(3) field release system

Providing an accurate estimate of the dry component of N deposition to low N background, semi-natural habitats, such as bogs and upland moors dominated by Calluna vulgaris is difficult, but essential to relate nitrogen deposition to effects in these communities. To quantify the effects of NH(3) inputs to moorland vegetation growing on a bog at a field scale, a field release NH(3) fumigation system was established at Whim Moss (Scottish Borders) in 2002. Gaseous NH(3) from a line source was released along of a 60 m transect, when meteorological conditions (wind speed > 2.5 m s(-1) and wind direction in the sector 180-215') were met, thereby providing a profile of decreasing NH(3) concentration with distance from the source. In a complementary study, using a NH(3) flux chamber system, the relationships between NH(3) concentrations and cuticular resistances were quantified for a range of NH(3) concentrations and micrometeorological conditions for moorland vegetation. Cuticular resistances increased with NH(3) concentration from 11 s m(-1) at 3.0 kg m(-3) to 30 s m(-1) at 30 kg m(-3). The NH(3) concentration data and the concentration-dependent canopy resistance are used to calculate NH(3) deposition taking into account leaf surface wetness. The implications of using an NH(3) concentration-dependent cuticular resistance and the importance for refining critical loads are discussed.