Atmospheric Nitrogen Deposition along an Urban-Remote Elevation Gradient in the Colorado Front Range, USA

Abstract:

Chronic atmospheric nitrogen (N) deposition, even when only moderately elevated above background levels, can have adverse consequences for terrestrial and aquatic ecosystems. Although the National Atmospheric Deposition Program (NADP) has historically focused on measuring wet deposition in rural areas, there is growing recognition that agricultural and urban N emissions elevate N deposition to their immediate surroundings and to adjacent remote ecosystems. More work is needed, however, to understand the spatial extent, seasonal variability, and forms of N transported from urban to remote environments. In this three-year study (2017-2019), we investigated the delivery of urban and agricultural N to the Colorado Front Range along a transect from the urban plains (1600 m) to the remote subalpine forest (3159 m) using NADP wet-deposition collectors and ion-exchange resin bulk (wet and dry) deposition collectors (IERs).

Elevated annual N loads extended beyond the urban land-use boundary into the forested foothills. Over the three-year period, the average annual load of wet inorganic N deposition was significantly higher, with a greater proportion of ammonium, at the lower elevation urban (4.78 ± 0.71 kg N ha^-1, 72 ± 2% as ammonium) and foothills (4.61 ± 0.00 kg N ha^-1, 71 ± 1% as ammonium) sites compared to the highest elevation subalpine site (2.54 ± 0.64 kg N ha^-1, 58 ± 3% as ammonium). Seasonally, wet N deposition was highest May through July, when air masses from the urban and agricultural plains move up into the mountains. The NADP wet-only N deposition values fell within the IER error range (± two standard deviations) 50% of the time and were lower than the IER error range 42% of the time. Wet-only deposition values were lower than IER N deposition mostly during the spring and summer months, when wet deposition accounted for approximately 67% of the IER N deposition, likely because IERs capture dry deposition unaccounted for by wet deposition monitoring.

Our results highlight the importance of monitoring N deposition in urban areas and demonstrate the value of IERs, especially in remote regions. To make progress in reducing N deposition to areas like the Rocky Mountain West, it is critical to move beyond the current monitoring of wet deposition in predominantly rural areas.