High Atmospheric Nitrate Inputs and Nitrogen Turnover in Semi-arid Urban Catchments

The influx of atmospheric nitrogen to soils and surfaces in arid environments is of growing concern due to increased N emissions and N usage associated with urbanization. Atmospheric nitrogen inputs to the critical zone can occur as wet (rain or snow) or dry (dust or aerosols) deposition, and can lead to eutrophication, soil acidification, and groundwater contamination through leaching of excess nitrate. The objective of this research was to use the δ¹⁵N, δ¹⁸O, and Δ¹⁷O values of atmospheric nitrate (NO₃ ⁻) (precipitation and aerosols) and NO₃ ⁻ in runoff to assess the importance of N deposition and turnover in semi-arid urban watersheds. Data show that the fractions of atmospheric NO₃ ⁻ exported from all the urban catchments, throughout the study period, were substantially higher than in nearly all other ecosystems studied with mean atmospheric contributions of 38% (min 0% and max 82%). These results suggest that catchment and stream channel imperviousness enhance atmospheric NO₃ ⁻ export due to inefficient N cycling and retention. In contrast, catchment and stream channel perviousness allow for enhanced N processing and therefore reduced atmospheric NO₃ ⁻ export. Overall high fractions of atmospheric NO₃ ⁻ were primarily attributed to slow N turn over in arid/semi-arid ecosystems. A relatively high fraction of nitrification NO₃ ⁻ (~30%) was found in runoff from a nearly completely impervious watershed (91%). This was attributed to nitrification of atmospheric NH₄ ⁺ in dry-deposited dust, suggesting that N nitrifiers have adapted to urban micro niches. Gross nitrification rates based on NO₃ ⁻ Δ¹⁷O values ranged from a low 3.04 ± 2 kg NO₃-N km⁻² day⁻¹ in highly impervious catchments to a high of 10.15 ± 1 kg NO₃-N km⁻² day⁻¹ in the low density urban catchment. These low gross nitrification rates were attributed to low soil C:N ratios that control gross autotrophic nitrification by regulating gross NH₄ ⁺ production rates.