Nutrients that fall on the ground from the atmosphere represent a minor component of the total nitrogen input to soils, especially when compared to agricultural, civil and industrial inputs (i.e., sewage treatment plants or sewage systems, fertilizer and manure applications). However, integrating all nitrogen forms, processes and scales can represent a breakthrough challenge for the understanding and the management of the nitrogen cycle.
A monitoring experiment was set up to collect wet atmospheric depositions in a human impacted area with multiple land uses, representing different emission sources (i.e., extended urban areas with residential buildings and industrial activities, high traffic roads and agricultural activities). Wet deposition is measured at 17 sites, homogeneously distributed in the western sector of Lombardy Region (northern Italy), in the surroundings of Milan. Rainwater collection was executed almost at each single rainfall event at all the sites, starting from February 2017 to February 2019. In summary, 16 precipitation events were monitored and 155 rainwater samples were collected, involving, on average, 10 sites each time. After collection, samples were analysed for pH, electric conductivity, ammonium, nitrate, nitrite, major cations (calcium, magnesium, sodium, potassium), and major anions (sulphate, chloride, fluoride).
In June 2018, an instrumented field site has been settled to reconstruct the path of nitrogen from the atmospheric emissions to rainwater precipitations to soil and groundwater, monitoring the infiltration and drainage processes in the vadose zone. The field site is located within a well field of the public manager of the water service of the Province of Milan, 20 km east from the City of Milan.
Results show a direct relationship between high levels of air pollutants (e.g., nitrogen dioxide, sulphur dioxide, ammonia) and relatively high contaminant concentrations (e.g., nitrate, sulphate, ammonium) in rainwater samples. Consequently, given an average annual precipitation of about 800 mm for the period 2016-2018, a wet deposition of inorganic nitrogen equal to 9 kg(nitrate-N)/ha·yr and 15.5 kg(ammonium-N)/ha·yr was estimated. Considering both the variability of the spatial or temporal distributions of precipitations and the variability of concentrations of nitrogen compounds in rainwaters, the total amount of nitrogen depositions can range between 20 and 30 kg/ha·yr in the study area.
As leaching of nitrogen compounds from soils generally increases at nitrogen deposition rates higher than about 10 kg/ha·yr, this work suggests that the nitrogen atmospheric input to soils could not be neglected when evaluating the impacts of nitrogen sources to terrestrial and aquatic ecosystems, as well as to groundwater resources. This highlights the need of wisely integrating air, soil and water policies for the planning and the management of equitable and sustainable cities.