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' (s). 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 the 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 s 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.