Dynamics of ammonia exchange with cut grassland: synthesis of results and conclusions of the GRAMINAE Integrated Experiment
- 1Centre for Ecology and Hydrology (Edinburgh Research Station) (CEH), Bush Estate, Penicuik, Midlothian, EH26 0QB, UK
- 2Energy research Centre of the Netherlands (ECN), Postbus 1, 1755 ZG Petten, The Netherlands
- 3Institut National de la Recherche Agronomique (INRA), UMR Environnement et Grandes Cultures, Thiverval-Grignon, 78850, France
- 4Plant and Soil Science Laboratory, University of Copenhagen (UoC), Faculty of Life Sciences, Thorvaldsensvej 40, 1871 Frederiksberg C, Copenhagen, Denmark
- 5School for Earth, Atmospheric and Environmental Sciences, University of Manchester (UoM), P.O. Box 88, Manchester, M60 1QD, UK
- 6Hungarian Meteorological Service (HMS), P.O. Box 39 1675, Budapest, Hungary
- 7Department of Meteorology, Eötvös Loránd University (ELU), Pázmány Péter sétány 1/A, P.O. Box 32, 1518 Budapest, Hungary
- 8Institut für Agrarökologie, Bundesforschungsanstalt für Landwirtschaft (FAL-D) (now the von Thunen Institute), Bundesallee 50, 38116 Braunschweig, Germany
- 9Agroscope Reckenholz-Tänikon Research Station ART, (formerly FAL-CH) Reckenholzstrasse 191, 8046 Zürich, Switzerland
- 10Soils, Agronomy and Spatialization Unit, UMR-SAS, INRA, Rennes, France
- 11Institute for Crop Science and Resource Conservation, INRES-PE, University of Bonn, Karlrobert-Kreiten-Str. 13, 53115 Bonn, Germany
- *now at: Institute of Earth Sciences "Jaume Almera", CSIC, Lluis Solé i Sabarís, 08028, Barcelona, Spain
- **now at: Section for Economy & Technology, Halmstad University, 30118 Sweden
- ***formerly at: University of Bern (UoB), Bern, Switzerland
Abstract. Improved data on biosphere-atmosphere exchange are fundamental to understanding the production and fate of ammonia (NH3) in the atmosphere. The GRAMINAE Integrated Experiment combined novel measurement and modelling approaches to provide the most comprehensive analysis of the interactions to date. Major inter-comparisons of micrometeorological parameters and NH3 flux measurements using the aerodynamic gradient method and relaxed eddy accumulation (REA) were conducted. These showed close agreement, though the REA systems proved insufficiently precise to investigate vertical flux divergence. Grassland management had a large effect on fluxes: emissions increased after grass cutting (−50 to 700 ng m−2 s−1 NH3) and after N-fertilization (0 to 3800 ng m−2 s−1) compared with before the cut (−60 to 40 ng m−2 s−1).
Effects of advection and air chemistry were investigated using horizontal NH3 profiles, acid gas and particle flux measurements. Inverse modelling of NH3 emission from an experimental farm agreed closely with inventory estimates, while advection errors were used to correct measured grassland fluxes. Advection effects were caused both by the farm and by emissions from the field, with an inverse dispersion-deposition model providing a reliable new approach to estimate net NH3 fluxes. Effects of aerosol chemistry on net NH3 fluxes were small, while the measurements allowed NH3-induced particle growth rates to be calculated and aerosol fluxes to be corrected.
Bioassays estimated the emission potential Γ = [NH4+]/[H+] for different plant pools, with the apoplast having the smallest values (30–1000). The main within-canopy sources of NH3 emission appeared to be leaf litter and the soil surface, with Γ up to 3 million and 300 000, respectively. Cuvette and within-canopy analyses confirmed the role of leaf litter NH3 emission, which, prior to cutting, was mostly recaptured within the canopy.
Measured ammonia fluxes were compared with three models: an ecosystem model (PaSim), a soil vegetation atmosphere transfer model (SURFATM-NH3) and a dynamic leaf chemistry model (DCC model). The different models each reproduced the main temporal dynamics in the flux, highlighting the importance of canopy temperature dynamics (Surfatm-NH3), interactions with ecosystem nitrogen cycling (PaSim) and the role of leaf surface chemistry (DCC model). Overall, net above-canopy fluxes were mostly determined by stomatal and cuticular uptake (before the cut), leaf litter emissions (after the cut) and fertilizer and litter emissions (after fertilization). The dynamics of ammonia emission from leaf litter are identified as a priority for future research.