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

A new study to address the biosphere-atmosphere exchange of ammonia (NH3) w ith grasslands is applying a European transect to interpret NH3 fluxes in r elation to atmospheric conditions, grassland management and soil chemistry. Micrometeorological measurements using the aerodynamic gradient method (AG M) with continuous NH3 detectors are supported by bioassays of the NH3 'sto matal compensation point' (chi (s)). Relaxed eddy accumulation (REA) is als o applied to enable flux measurements at one height; this is relevant to he lp address flux divergence due to gas-particle inter-conversion or the pres ence of local sources in a landscape. Continuous measurements that contrast intensively managed grasslands with s emi-natural grasslands allow a scaling up from 15 min values to seasonal me ans. The measurements demonstrate the bi-directional nature of NH3 fluxes, with typically daytime emission and small nocturnal deposition. They confir m the existence of enhanced NH3 emissions (e.g. 30 g N ha(-1) d(-1)) follow ing cutting of intensively managed swards. Further increased emissions foll ow fertilization with NH4NO3 (typically 70 g N ha(-1) d(-1)). Measurements using REA support these patterns, but require a greater analytical precisio n than with the AGM. The results are being used to develop models of NH3 exchange. 'Canopy compe nsation point' resistance models reproduce bi-directional diurnal patterns, but currently lack a mechanistic basis to predict changes in relation to g rassland phenology. An advance proposal here is the coupling of chi (s) to dynamic models of grassland C-N cycling, and a relationship with modelled p lant substrate-N is shown. Applications of the work include incorporation o f the resistance models in NH3 dispersion modelling and assessment of globa l change scenarios.