Drivers of ammonia volatilization in Mediterranean climate cropping systems.

Ammonia (NH) 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 (NO) emissions, but whether the same relation exists for NH 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 NH 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 NH EF and cumulative emissions, with an R of 0.69 and 0.76, respectively. Nitrogen fertilization rate (N rate) was the top-ranked predictor variable, increasing NH emissions substantially when N rate was higher than 170 kg N ha. Soil pH was the most important edaphoclimatic variable, showing greater emissions (36.7 kg NH ha, EF = 19.3%) when pH was above 8.2. Crop type, fertilizer type, and N application method also affected NH 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 NH emissions. For instance, relatively low N fertilization rates result in small NH emissions in rainfed areas, which occupy a very significant surface of Mediterranean agricultural land. Overall, N fertilization management is a key driver in reducing NH emissions, but additional field factors should be studied in future research to establish more robust abatement strategies.