Hotspots of nitrogen deposition in the world's urban areas: a global data synthesis

Human activities have altered the global nitrogen (N) cycle, elevating rates of atmospheric N deposition up to tenfold above pre-industrial levels, with consequences for ecosystem function and human health. To date, most N deposition studies have been carried out in rural areas; however, there has been a recent surge of N deposition studies conducted in urban ecosystems due to the increased recognition that humans are greatly altering the N cycle in these environments. We synthesized data from 174 publications over a period of 40 years that examined rates of N deposition in urban and nearby rural areas worldwide. Results of this meta-analysis help to quantify urban N deposition, demonstrate that total N deposition in cities is predominately composed of chemically reduced - as opposed to oxidized - forms of N like ammonia, and identify regional hotspots of urban N deposition, particularly in China. These findings highlight the need to examine and address the N cycle in cities as the world continues to urbanize.