Process-based modelling of NH3 exchange over a grazed field

Andrea Móring1,2, Massimo Vieno2, Ruth M. Doherty3, Celia Milford4,5, Eiko Nemitz2, Marsailidh M. Twigg2, Mark A. Sutton2

1 University of Edinburgh, High School Yards, Edinburgh, United Kingdom, EH8 9XP,
2 Centre for Ecology & Hydrology, Bush Estate, Penicuik, United Kingdom, EH26 0QB
3 University of Edinburgh, The King’s Buildings, Alexander Crum Brown Road, Edinburgh, United Kingdom, EH9 3FF
Associate Unit CSIC University of Huelva ”Atmospheric Pollution”, CIQSO, University of Huelva, Huelva, Spain, E21071

5 Izaña Atmospheric Research Center, AEMET, Joint Research Unit to CSIC “Studies on Atmospheric Pollution”, Santa Cruz de Tenerife, Spain


In this study a process-based ammonia (NH3) exchange model for a grazed field has been described and evaluated. The presented model is based on a patch-scale NH3 exchange model, GAG (Generation of Ammonia from Grazing), which has been here extended to the field scale. GAG accounts for the total ammoniacal nitrogen and water content of the soil as well as the soil pH under a single urine patch. The new, field scale model combined multiple runs of the patch-scale model including both urine-affected and unaffected areas. The field-scale model was tested over two modelling periods, using NH3 flux measurements taken at an intensively managed grassland, Easter Bush, UK. The model represented well the observed fluxes. It was found that the temporal evolution of the NH3 exchange flux was dominated by the NH3 emission from the urine patches. The results also showed that the evolution of NH3 emission from urine patches deposited in different time steps could be substantially different: in some cases the first high NH3 emission peak occurred a day or two days after the deposition of the given urine patch. Furthermore, according to our findings, NH3 fluxes over the field in a given day could be considerably affected by the NH3 emission from urine patches deposited several days earlier. The approach is designed to provide a balance between simplicity and process representation to allow it to be ultimately applied in regional scale atmospheric emission, transport and deposition modelling.