Fluxes of NH3 and CO2 over upland moorland in the vicinity of agriculturalland

Intensive field measurements of NH3 and CO2 exchange were made over a wet h eathland in the vicinity (< 500 m) of sheep pastures in the Caimgorm mounta ins of Scotland for a two-week period in the summer. Fluxes of NH3 were det ermined using the aerodynamic gradient method with a 3-height continuous de nuder system; fluxes of CO2 were determined using eddy correlation, while s ensible and latent heat fluxes were determined by both methods. Few studies have measured NH3 and CO2 fluxes simultaneously, making these measurements relevant to compare exchange dynamics. Both NH3 and CO2 exchanged bidirect ionally, in response to a combination of biological (foliar, soil) and phys ico-chemical controls (solubility). NH3 was deposited rapidly to leaf surfa ces, although during warm, dry daytime conditions periods of emission occur red, explained by the existence of a compensation point concentration for N H3. By contrast, (CO2 followed a characteristic pattern of absorption durin g the day associated with net photosynthesis and emission at night. Both ga ses showed net uptake from the atmosphere, at 30 mu mol NH3 m(-2) d(-1) and 74 mmol CO2 m(-2) d(-1). In southeast winds, NH3 emissions from the sheep pasture caused a significant advection error to the measured fluxes (> 10%) . Corrections were applied using a local-scale dispersion-exchange model. T he analysis highlights how advection modifies the classical one-dimensional inferential resistance approach. It is concluded that ecosystems in the vi cinity of agricultural land receive more dry deposition than would be estim ated using NH3 concentration monitoring and standard inferential models. In the present, study, this effect represented an overall increase in total N H3 deposition of 32%.