Drivers of ammonia volatilization in Mediterranean climate cropping systems

Ammonia (NH₃) volatilization is the major source of nitrogen (N) loss resulting from the application of synthetic and organic N fertilizers to croplands. It is well known that in Mediterranean cropping systems, there is a relationship between the intrinsic characteristics of the climate and nitrous oxide (N₂O) emissions, but whether the same relation exists for NH₃ emissions remains uncertain. Here, we estimated the impact of edaphoclimatic conditions (including meteorological conditions after N fertilization), crop management factors, and the measurement technique on both the cumulative emissions and the NH₃ emission factor (EF) in Mediterranean climate zones, drawing on a database of 234 field treatments. We used a machine learning method, random forest (RF), to predict volatilization and ranked variables based on their importance in the prediction. Random forest had a good predictive power for the NH₃ EF and cumulative emissions, with an R² of 0.69 and 0.76, respectively. Nitrogen fertilization rate (N rate) was the top-ranked predictor variable, increasing NH₃ emissions substantially when N rate was higher than 170 kg N ha^-1. Soil pH was the most important edaphoclimatic variable, showing greater emissions (36.7 kg NH₃ ha^-1, EF = 19.3%) when pH was above 8.2. Crop type, fertilizer type, and N application method also affected NH₃ emission patterns, while water management, mean precipitation, and soil texture were ranked low by the model. Our results show that intrinsic Mediterranean characteristics had only an indirect effect on NH₃ emissions. For instance, relatively low N fertilization rates result in small NH₃ emissions in rainfed areas, which occupy a very significant surface of Mediterranean agricultural land. Overall, N fertilization management is a key driver in reducing NH₃ emissions, but additional field factors should be studied in future research to establish more robust abatement strategies.