Dynamics of ammonia exchange with cut grassland: synthesis of results and conclusions of the GRAMINAE Integrated Experiment

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 (&minus;50 to 700 ng m^&minus;2 s^&minus;1 NH₃) and after N-fertilization (0 to 3800 ng m^&minus;2 s^&minus;1) compared with before the cut (&minus;60 to 40 ng m^&minus;2 s^&minus;1). <br><br> 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. <br><br> 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. <br><br> 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.