Influence of landscape patterns on nitrate and particulate organic nitrogen inputs to urban stormwater runoff

This study investigated the effect of the landscape pattern of permeable/impermeable patches on NO3--N and particulate organic nitrogen (PON) concentrations during stormwater runoff transport and their source contributions. Six landscape pattern indices, namely, mean proximity index (MPI), largest patch index (LPI), mean shape index (MSI), landscape shape index (LSI), connect index (CONNECT), and splitting index (SPLIT), were selected to reflect the fragmentation, complexity, and connectivity of permeable patches in urban catchments. The results show that lower fragmentation, higher complexity, and greater connectivity can reduce NO3--N concentrations in road runoff and drainage flow (i.e., the flow in the stormwater drainage network), as well as PON concentrations in road runoff. Further, the above landscape pattern is effective for mitigating the contributions of NO3--N and PON from road runoff. Low impact development (LID) can be incorporated with the landscape pattern of permeable/impermeable patches to mitigate nitrogen pollution in urban stormwater at the catchment scale by optimizing the spatial arrangement.

Nitrogen transport and sources in urban stormwater with different rainfall characteristics

Nitrogen pollution in urban stormwater has led to serious quality issues in urban water. Nitrogen pollution mitigation requires fully understanding the transport process and major nitrogen sources in urban stormwater. In this study, the concentrations and flux of various nitrogen forms during urban stormwater transport were analyzed. It was found that the concentration and flux of NO₃^--N and NH₃-N decreased in the order of roof runoff, road runoff, and drainage runoff, while the concentration and flux of dissolved organic nitrogen (DON) and particulate nitrogen (PN) were found to be the highest in road runoff. Source quantification found that roof runoff (34%) and rainwater (34%) were the main contributors to nitrate pollution during light and moderate rains (<25 mm), while road runoff was identified as the major source (49%) of nitrate pollution during heavy rains (>25 mm) due to the large road runoff volume. Regarding particulate organic nitrogen (PON), the road runoff in commercial areas contributed most (23%) to PON pollution during light and moderate rains, while the runoff generated from pervious surfaces and drainage sediments were the primary two sources (22%) of PON during heavy rains. Moreover, the influence of rainfall characteristics on nitrate and PON source contributions was identified. The results show that antecedent dry periods were an important factor influencing nitrogen source contributions during light and moderate rains, while rainfall amount and intensity were critical factors impacting the nitrogen source contributions during heavy rains. Taking various transport processes, source contributions, and rainfall characteristics into consideration, several recommendations were given for the mitigation of nitrogen pollution in urban stormwater. This study can provide a useful perspective to understand the transport and sources of nitrogen, thus developing constructive strategies to control urban nonpoint source pollution management.

Nitrogen isotope constraint on the zonation of multiple transformations between dissolved and particulate organic nitrogen in the Changjiang plume

Information on the sources and transformations of particulate organic N (PN) and dissolved organic N (DON) at the coastal interface remains insufficient due to technological difficulties and complicated features of intensive physical mixing and rapid biological activities. Here, we investigated the spatial distribution of concentrations and isotopic compositions of PN and DON in the Changjiang plume during the summer flood period. In average, DON and PN accounted for 25.6 ± 12.1% and 8.1 ± 9.1% (n = 55), respectively, of the total N pool, with the remaining N primarily in the form of nitrate (NO₃^-). Mean δ¹⁵N values were the lowest for DON (-0.1 ± 2.7‰, n = 58) and slightly higher for PN (2.0 ± 1.6‰, n = 101), and the highest for NO₃^- (6.5 ± 2.2‰, n = 67), suggesting multiple transformations had occurred to differentiate isotopic characteristics among the three N pools. By applying a conservative mixing model, we found DON deficits (-3.5 ± 3.7 μmol L^-1, n = 43) and negative shift in δ¹⁵N_DON (-3.6 ± 2.2‰, n = 43) in the Changjiang plume, revealing nonconservative DON behaviors. In the offshore surface plume where Chlorophyll a was high, the most likely cause is the DON uptake by phytoplankton with a strong inverse isotope effect (around -40‰). This DON assimilation by phytoplankton contributed to ~16 ± 12% of the PN production, with the remaining supported by NO₃^- assimilation, producing an overall isotope effect of 4-9‰. However, in waters near the river mouth and at the bottom of the offshore plume where total suspended matter concentrations were high (>5 mg L^-1), the DON deficit was most likely induced by the selective adsorption of ¹⁵N enriched moieties of DON onto particulate surfaces (with an isotope effect of -20‰ to -5‰). Unlike dissolved organic carbon to behave conservatively in most estuaries, our results show that active transformations had occurred between the DON and PN pools in the Changjiang plume.

Organic nitrogen in residential stormwater runoff: Implications for stormwater management in urban watersheds

Stormwater runoff containing organic nitrogen (N) is a source of potentially bioavailable N in water bodies. Characterization and concentrations of dissolved organic N (DON) and particulate organic N (PON) in urban stormwater runoff are rarely reported and considered in stormwater management. Our objectives were to (1) characterize the organic (DON, PON) and inorganic (NO₃^- and NH₄⁺) N pools in residential stormwater runoff and (2) determine the rainfall driven landscape sources of runoff PON using an isotopic mixing model with ¹³C and ¹⁵N during a wet season (June-September). We instrumented a 13 ha (0.13 km²) residential catchment located in Florida, United States with an ISCO autosampler and collected stormwater runoff samples (n = 52) over 11 individual stormwater runoff events. Mean concentration of total N in runoff during the wet season was 1.61 mg L^-1, of which 37% was DON and 25% was PON. A strong seasonal first flush of PON, giving rise to a large PON:TN ratio, was observed as the wet season progressed from June (PON:TN = 0.39;) to September (PON:TN = 0.12), whereas DON did not display any seasonal variability (mean: 0.66 mg L^-1). The isotope mixing model estimated that 76% of PON in the runoff originated from oak detritus (leaves: 50%, acorns: 26%) and the remaining 24% from lawn grass clippings. The dominance of organic N fractions in the urban stormwater runoff suggests that landscape controls on PON and DON are needed to reduce N loading in the urban stormwater runoff. The seasonal first flush of PON indicates that monitoring strategies should focus on how nutrient concentrations in runoff may respond to seasonal drivers such as leaf litterfall and that there may be optimal times for N management, such as after a prolonged dry season in which materials accumulate and pose the risk for later mobilization.

Determining Inorganic and Organic Nitrogen

Nitrogen (N) is one of the key nutrients for algal growth and is an integral part of many cellular components, for example in proteins. Being able to determine the inorganic and organic pools of N is consequently critical for algal cultivation. In this chapter we present the methods we use for determining dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON). The methods presented here for DIN rely on colorimetric methods and those of DON and PON on filtration and high temperature catalytic oxidation.