Nitrogen availability and denitrification in urban agriculture and regreened vacant lots

Many cities demolish abandoned homes and create regreened vacant lots (RVLs), and an increasingly popular, high-intensity use of RVLs is as urban agriculture (UA) sites. UA may potentially result in higher nitrogen (N) runoff to aquatic ecosystems, but this potential has not been quantified. We examined the role that varying land reuse intensity plays in determining potential for N export via runoff or leaching, focusing on soil N availability and N removal capacity via denitrification. We contrasted three levels of land use intensity for vacant parcels: intact vacant properties, turfgrass RVLs, and regreened UA lots in Buffalo, NY. We examined soil N and C availability, denitrification potential, and isotopic evidence of denitrification. Land use intensity only affected soil properties in surficial soil horizons. Total N was 2.5x higher in UA soils (mean = 0.51%) than non-UA (mean = 0.21%). Soil nitrate was 2.6x higher in winter (mean = 12.4 µg NO₃⁻-N g⁻¹) than summer (mean = 4.7 µg NO₃⁻-N g⁻¹) and was generally higher in UA soils. Despite higher soil N availability at UA sites, there were no differences in denitrification potential between UA and non-UA sites (mean = 620 ng N g soil⁻¹ h⁻¹). Isotopic evidence further confirms that denitrification was not a major sink of N. As UA had high N availability compared to non-UA sites and low rates of denitrification, UA has potential for runoff-driven N export, particularly of organic N, though lower nitrate concentrations than values typical for conventional agricultural soils make nitrate leaching less of a concern by comparison.