## 1 Introduction  
Urban runoff is one of the most common causes of surface water impairment [1]. Hard and impervious surfaces, including asphalt parking lots, roads, buildings, and houses cover soil and thus reduce infiltration capacity leading to more runoff during rain events [2]. As the global population increases, so too will the coverage of impervious surface [3]. Runoff can cause flooding to downstream properties and transport various contaminants that accumulate on surfaces between storms, including animal waste and fertilizers, which are enriched with nitrogen [4; 5; 6]. Excess nitrogen loading has resulted in overstimulation of algal growth, eutrophication and impairment of surface waters in many regions around the world [7]. Various stormwater control measures (SCMs) have been implemented in urban areas to retain and/or to treat runoff near the source [8; 9]. Dry detention basins were one of the first control measures widely used in many countries and most were designed with a main goal to reduce the peak rate of runoff [4; 10; 11]. Dry basins allow infiltration of runoff and slow the release of stormwater through an outlet structure. Dry detention basins are relatively easy to construct, require low maintenance, and may also provide for recreational activities during dry periods [10]. However, studies have shown that the nitrogen treatment efficiency of dry basins is poor, highly variable, and some basins may actually export nitrogen [8]. For example, in a literature review [8] that included influent and effluent data from 25 dry ponds, the mean and median nitrogen concentrations in dry pond effluent were more than 75% greater relative to influent concentrations. The dry ponds in those studies were sources of nitrogen. The authors [8] also reviewed data from 9 other dry ponds and reported median nitrogen load removals of 25%. The authors cited factors including detention time and density of vegetation as factors that influenced the treatment efficiency of the dry basins. Based on the data in the review [8], the expected median nitrogen removal rates of dry ponds may range from -75% to 25%. Constructed stormwater wetlands are another SCM used to reduce the peak rate of runoff and for nutrient treatment. Components of a stormwater wetland typically include a forebay to dissipate the energy of influent, shallow water channel that directs flow towards the outlet, shallow land that is vegetated with wetlands plants, and an outlet structure [12]. Wetlands are typically more expensive to design and install, and require more land area, but may provide better nutrient removal relative to dry basins [12]. In a review [8] of the total nitrogen (TN) treatment performance of 19 stormwater wetlands, mean and median effluent concentrations of TN were 43% and 48% lower respectively, relative to influent concentrations. Wetlands in those studies were sinks for nitrogen. Wetlands typically have a higher diversity and density of vegetation than dry basins. The wetland vegetation facilitate nitrogen removal via plant uptake and increase of soil organic carbon, which may fuel denitrification in anaerobic environments [8, 11, 12]. However, in highly aerated wetlands, mineralization of soil organic nitrogen and/or nitrification of ammonium (NH4+) may cause net exports of nitrogen [8]. While most stormwater wetlands have shown great capacity to reduce nitrogen transport, others were sources of nitrogen. More research is needed to quantify the nitrogen treatment efficiency of common SCM, including dry basins and wetlands, especially in nutrient-sensitive watersheds. The effectiveness of SCMs in reducing nitrogen transport depends on the biochemical processes that are active within the sites. There are several potential removal mechanisms for nitrogen in SCMs including immobilization, adsorption, and denitrification [8, 13, 14]. Immobilization is the uptake of NH4+ and/or nitrate (NO3-) by plants and/or microorganisms resulting in a transformation of inorganic to organic nitrogen.