Atmospheric Nitrogen Depositions in a Highly Human-Impacted Area

Nutrients that fall on the ground from the atmosphere represent a minor component of the total nitrogen (N) 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 N 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. Rainwater collection was executed in the surroundings of Milan, in northern Italy, starting from February 2017 to February 2019. The presence of N compounds and their temporal variations in rainwater are consistent with pollution coming from local anthropogenic emission sources of nitrogen oxides and ammonia, mainly related to the use of the heating systems in the cold seasons and the spreading of fertilizers and manure on agricultural fields. Consequently, the total amount of N wet depositions range between 14 and about 30 kg/ha·yr in the study area. As leaching of N compounds from soils generally increases at deposition rates higher than about 10 kg(N)/ha·yr, this work suggests that the N atmospheric input to soils could not be neglected when evaluating the impacts of N 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 minimizing the risk to deteriorate surficial ecosystems and groundwater.