Organic Nitrogen in Urban Stormwater: Where It Comes from and Why It Matters.

The current focus of many nutrient management stategies for urban areas is on controlling the input of fertilizer nitrogen (N) and the subsequent export of inorganic N (mainly nitrate). The premise behind this is the fact that inorganic N is highly soluble in water and so it is easily carried by stormwater runoff to receiving water bodies, where it is a biologically available form of N to the organisms that can cause eutrophication and algal blooms. But there is growing evidence that portions of the organic N pool in urban stormwater runoff are also highly bioavailable. We are assessing the organic N in stormwater from the urban area around Tampa, Florida. We instrumented the inflow pipe of an urban stormwater pond with an ISCO autosampler and flow meter and are collecting runoff under varying storm conditions from a 13-hectare drainage basin. Average total N concentration for 73 stormwater samples taken during 18 storm events to date was 2.7 mg/L, 77% of which was organic N. Isotopic data shows that oak leaf litter is the main source of stormwater organic N and that different intensity storms produce different patterns in N loading. For example, Tropical Storm Andrea, which produced 93.5 mm of rain in just a few hours was effective at driving both organic N and nitrate-N from watershed soils; whereas the bulk of other season events (90% of all storms; <10mm rain) were effective at carrying only organic N. The dominance of organic N in urban stormwater demands that we examine it as a potential vector of labile N transport to receiving water bodies.