Inverse dispersion modelling highlights the efficiency of slurry injection to reduce ammonia losses by agriculture in the Po Valley (Italy)

The performance of two inverse dispersion models to quantify ammonia (NH3) fluxes has been evaluated. Ammonia emissions from cattle slurry spreading have been estimated following two different application techniques in northern Italy: (i) surface spreading with incorporation after 24h and (ii) injection into the soil. Passive diffusion samplers were used to measure atmospheric ammonia concentrations at the treated fields and at background sites. A sonic anemometer was used to characterise the atmospehric turbulence and provided the necessary inputs for the inverse models. Two inverse dispersion models were then employed: FIDES, based on the Philip (1959) analytical solution of the advection-diffusion equation and WindTrax, a Lagrangian stochastic dispersion model. Both models were parameterised with friction velocity, Obukhov length and roughness as input. WindTrax was also alternatively used with wind components statistics from the sonic anemometer. The two models agreed well in estimating ammonia emissions under neutral conditions but diverged under stable and unstable conditions because of different parameterisation of the diffusivity profile. The maximum difference between the two models was less than 32%. The uncertainty associated with the concentration measurements with the passive samplers, was lower than 11%. The direct injection of slurry into the soil was the most effective method for reducing N loss from slurry application, resulting in an emission of 2% of the total ammoniacal nitrogen (TAN) applied when surface application resulted in losses of 5.4% of TAN.