Articles | Volume 14, issue 18
Biogeosciences, 14, 4161–4193, 2017
Biogeosciences, 14, 4161–4193, 2017

Research article 21 Sep 2017

Research article | 21 Sep 2017

Process-based modelling of NH3 exchange with grazed grasslands

Andrea Móring1,2,3, Massimo Vieno2, Ruth M. Doherty3, Celia Milford4,5, Eiko Nemitz2, Marsailidh M. Twigg2, László Horváth6, and Mark A. Sutton2 Andrea Móring et al.
  • 1University of Edinburgh, High School Yards, Edinburgh, EH8 9XP, UK
  • 2Centre for Ecology and Hydrology, Bush Estate, Penicuik, EH26 0QB, UK
  • 3University of Edinburgh, The King's Buildings, Alexander Crum Brown Road, Edinburgh, EH9 3FF, UK
  • 4Associate Unit CSIC University of Huelva ”Atmospheric Pollution”, CIQSO, University of Huelva, Huelva, E21071, Spain
  • 5Izaña Atmospheric Research Centre, AEMET, Joint Research Unit to CSIC “Studies on Atmospheric Pollution”, Santa Cruz de Tenerife, Spain
  • 6Hungarian Meteorological Service, Gilice tér 39, Budapest, 1181, Hungary

Abstract. In this study the GAG model, a process-based ammonia (NH3) emission model for urine patches, was extended and applied for the field scale. The new model (GAG_field) was tested over two modelling periods, for which micrometeorological NH3 flux data were available. Acknowledging uncertainties in the measurements, the model was able to simulate the main features of the observed fluxes. The temporal evolution of the simulated NH3 exchange flux was found to be dominated by NH3 emission from the urine patches, offset by simultaneous NH3 deposition to areas of the field not affected by urine. The simulations show how NH3 fluxes over a grazed field in a given day can be affected by urine patches deposited several days earlier, linked to the interaction of volatilization processes with soil pH dynamics. Sensitivity analysis showed that GAG_field was more sensitive to soil buffering capacity (β), field capacity (θfc) and permanent wilting point (θpwp) than the patch-scale model. The reason for these different sensitivities is dual. Firstly, the difference originates from the different scales. Secondly, the difference can be explained by the different initial soil pH and physical properties, which determine the maximum volume of urine that can be stored in the NH3 source layer. It was found that in the case of urine patches with a higher initial soil pH and higher initial soil water content, the sensitivity of NH3 exchange to β was stronger. Also, in the case of a higher initial soil water content, NH3 exchange was more sensitive to the changes in θfc and θpwp. The sensitivity analysis showed that the nitrogen content of urine (cN) is associated with high uncertainty in the simulated fluxes. However, model experiments based on cN values randomized from an estimated statistical distribution indicated that this uncertainty is considerably smaller in practice.

Finally, GAG_field was tested with a constant soil pH of 7.5. The variation of NH3 fluxes simulated in this way showed a good agreement with those from the simulations with the original approach, accounting for a dynamically changing soil pH. These results suggest a way for model simplification when GAG_field is applied later at regional scale.

Short summary
This study describes and evaluates a new ammonia (NH3) exchange model for grazed fields (GAG_field). GAG_field is able to simulate the main features of the observed NH3 fluxes. A sensitivity analysis for the non-meteorological model parameters showed that the sensitivity of the NH3 fluxes to a parameter varies among urine patches. Moreover, the fluxes modelled with a dynamic soil pH are similar if a constant pH 7.5 is used, suggesting a useful simplification for regional-scale model application.
Final-revised paper