Fluxes of NH3 and CO2 over upland moorland in the vicinity of agricultural land

Abstract

Intensive field measurements of NH3 and CO2 exchange were made over a wet heathland in the vicinity (<500 m) of sheep pastures in the Cairngorm mountains of Scotland for a two‐week period in the summer. Fluxes of NH3 were determined using the aerodynamic gradient method with a 3‐height continuous denuder system; fluxes of CO2 were determined using eddy correlation, while sensible 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 bidirectionally, in response to a combination of biological (foliar, soil) and physico‐chemical controls (solubility). NH3 was deposited rapidly to leaf surfaces, although during warm, dry daytime conditions periods of emission occurred, explained by the existence of a compensation point concentration for NH3. By contrast, CO2 followed a characteristic pattern of absorption during the day associated with net photosynthesis and emission at night. Both gases showed net uptake from the atmosphere, at 30 μmol 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. The analysis highlights how advection modifies the classical one‐dimensional inferential resistance approach. It is concluded that ecosystems in the vicinity of agricultural land receive more dry deposition than would be estimated using NH3 concentration monitoring and standard inferential models. In the present study, this effect represented an overall increase in total NH3 deposition of 32%.