The 1990 Clean Air Act Amendments led to declining rates of atmospheric nitrogen deposition in the northeastern U.S., but urban areas remain as hot spots with rates significantly above nearby rural sites. Small amounts of nitrogen deposition can serve as fertilizer, stimulating plant growth. However, high rates of nitrogen deposition can lead to a series of negative consequences, including reductions in plant species diversity, acidification of soils and waterways, and harmful effects on human health. Compared to rates of inorganic nitrogen deposition, less is known about rates of organic nitrogen, phosphorous, and carbon deposition in urban ecosystems. These fluxes are important to understand as more than half of the world’s population lives in cities and this number is expected to rise over the coming decades.

We measured nitrogen, carbon, and phosphorus in bulk deposition and throughfall at multiple sites throughout the greater Boston area, including two urban NADP National Trends Network (NTN) sites. We find that rates of atmospheric nitrogen deposition are on average twice as high as nearby rural areas, but vary more than threefold within the greater Boston area. Organic nitrogen makes up approximately one third of bulk and throughfall nitrogen inputs and ammonium makes up about 70% of total inorganic nitrogen deposition. Rates of ammonium and total inorganic N deposition are strongly correlated with on-road emissions of nitrogen oxides and distance to roads, suggesting a significant source of ammonia emissions from urban vehicles. However, we find enhanced rates of throughfall nitrogen in late spring, suggesting either a local source from dry deposition or biogenic processes within the canopy.

Comparing throughfall to bulk deposition, we find that the urban tree canopy enhances growing season inputs of nitrogen, phosphorus, and carbon to urban ecosystems, likely due to a combination of the capture of dry deposition on canopy surfaces, canopy processes, and production of biological matter. With Boston’s tree canopy covering approximately 26% of the city, inputs from throughfall roughly double the amount of nitrogen and triple the amount of phosphorus inputs to the ground surface compared to bulk deposition across the City of Boston.

The results of our work demonstrate that atmospheric deposition of nitrogen, phosphorus, and carbon can be both high and variable within an urban area and highlight the need for more measurements of both dry deposition and biogenic fluxes in tree canopies across urban areas.