## 1 Introduction
The transformation often does not result in a permanent loss of nitrogen mass unless the vegetation is harvested and removed from the wetland as a component of regular maintenance [15, 16]. Without routine removal, plant debris may decompose and organic nitrogen may mineralize [13, 17], contributing to nitrogen exports from the SCM. Adsorption is when NH4+ is bound to the negative charges on soil particles via cation exchange. However, desorption and release may also occur [18] and thus NH4+ removal via adsorption may only be temporary. The preferred method of nitrogen removal in an SCM is denitrification, the transformation of nitrate (NO3-) to N2 gas. Denitrification results in the loss of mass of nitrogen [17]. Denitrification requires anaerobic conditions, labile carbon, NO3-, and suitable environmental conditions (e.g., pH, temperature) for denitfying microorganisms [2, 8, 19]. In the absence of O2, denitfying microorganisms can oxidize organic compounds and use NO3- as a terminal electron acceptor, thus reducing NO3- to N2 gas [2, 19]. However, if oxygen is present in soil, then O2 will be utilized as the terminal electron acceptor during oxidation of organic compounds because it yields more energy [19]. Thus, denitrification may be inhibited if aerobic conditions are present. In some environments, denitrification rates may be limited based on the availability of carbon, which provides an energy (electron) source for microorganisms [8, 20]. SCMs such as wetlands, which incorporate dense vegetation and have zones of stagnant water or water-logged soils, have the highest denitrification rate potential due to the build-up of soil organic matter and anaerobic conditions [14]. The speciation of influent nitrogen may influence the performance of an SCM in reducing nitrogen transport. A review of nitrogen speciation and concentrations in over 3700 stormwater samples from different land uses showed that NO3- accounted for 30% of total nitrogen concentration, while total Kjeldahl nitrogen (organic + ammonium) comprised 70% [8]. Based on those numbers, the nitrogen removal efficiency via denitrification would not exceed 30% unless a higher percentage of the total nitrogen entering the SCM was transformed to NO3- or the transformation occurred at the entrance of the control measure, and NO3\({}^{-}\) was exposed to denitriying conditions prior to exiting. It is possible that organic nitrogen may mineralize to NH4\({}^{+}\), a process called ammonification [20], and under oxic conditions ammonium (NH4\({}^{+}\)) can rapidly undergo nitrification [2]. These steps would enable subsequent denitrification if alternating oxic and anoxic conditions are found within the SCM along with other suitable environmental conditions. Microorganisms that facilitate nitrification are active when the pH is between 4.5 and 10, aerobic conditions are present, and the temperature is greater than 5 \({}^{\circ}\)C [21]. Denitrifying microorganisms are active when the pH is above 5, anoxic conditions are present, and the temperature is above 2 \({}^{\circ}\)C [21]. Nitrogen transformations may not occur if the environmental conditions within an SCM fall outside the range of conditions needed for microbial activity. Therefore, the nitrogen treatment efficiency of a SCM may be influenced by the speciation of influent nitrogen and environmental conditions within the practice that facilitate nitrogen transformations including nitrification and denitrification. In North Carolina (NC), dry detention basins and constructed wetlands receive nitrogen reduction credits of 10% and 44% respectively, when installed to reduce stormwater runoff from new developments [22]. However, research has shown that the nitrogen treatment efficiency of dry basins and wetlands may be highly variable and more work is needed to better understand the factors which influence their performance [8].