Biosphere-atmosphere interactions of ammonia with grasslands: experimental strategy and results from a new European initiative

A new study to address the biosphere-atmosphere exchange of ammonia (NH₃) with grasslands is applying a European transect to interpret NH₃ fluxes in relation to atmospheric conditions, grassland management and soil chemistry. Micrometeorological measurements using the aerodynamic gradient method (AGM) with continuous NH₃ detectors are supported by bioassays of the NH₃ 'stomatal compensation point' (Χs). Relaxed eddy accumulation (REA) is also applied to enable flux measurements at one height; this is relevant to help address flux divergence due to gas-particle inter-conversion or presence of local sources in a landscape. Continuous measurements that contrast intensively managed grasslands with semi-natural grasslands allow a scaling up from 15 min values to seasonal means. The measurements demonstrate the bi-directional nature of NH₃ fluxes, with typically daytime emission and small nocturnal deposition. They confirm the existence of enhanced NH₃ emissions (e.g. 30 g N ha^-1 d^-1) following cutting of intensively managed swards. Further increased emissions follow fertilization with NH₄NO₃ (typically 70 g N ha^-1 d^-1). Measurements using REA support these patterns, but require a greater analytical precision than with the AGM. The results are being used to develop models of NH₃ exchange. 'Canopy compensation point' resistance models reproduce bi-directional diurnal patterns, but currently lack a mechanistic basis to predict changes in relation to grassland phenology. An advance proposal here is the coupling of Χs to dynamic models of grassland C-N cycling, and a relationship with modelled plant substrate-N is shown. Applications of the work include incorporation of the resistance models in NH₃ dispersion modelling and assessment of global change scenarios.