Florent LELENDA KEBALO

There is an increasing use of digestates as organic fertilisers in agriculture but they are sensitive to ammonia volatilisation. The latter has consequences on the environment, agriculture and human health: its control requires better knowledge regarding the underlying factors as well as their integration into modelling. The ability of the Volt’Air model to predict ammonia volatilisation after field application of digestates was assessed by using experimental data mainly collected in the field but also under controlled conditions, as part of two recent projects, MetaMetha and EVAPRO. Study showed a correlation between ammonia volatilisation rate and digestate characteristics, dry matter content and pH, as well as meteorological variables such as rain, temperature and wind speed over two days. The results revealed that ammonia volatilisation is overestimated by Volt’ Air after digestate spreading in both the field and under controlled conditions. Some explanations are suggested: the datasets do not allow to validate that the infiltration is adequately modeled, so this point requires a more in-depth study; on the other hand, it is clear that the effect on volatilisation decrease due to the presence of plant cover is not taken into account. Furthermore, the results highlighted the propensity of Volt’ Air to properly simulate the start of ammonia volatilisation subsequent to liquid digestate application. However the volatilisation extends beyond what is measured. It is likely that different processes consuming ammoniacal nitrogen, such as nitrification, equilibria (that decrease ammonia availability) and adsorption are either not or not well modeled. The good adequacy at the start of the volatilisation period, as well as the Volt’Air model’s ability to integrate new effects and / or modules is a promising sign of its potential to make a better prediction after been improved.