Intense denitrification and sewage effluent result in enriched 15N in N2O from urban polluted rivers

Abstract

Urban polluted rivers are important sources of N2O because they receive heavy nitrogen loads containing N2O and its precursors. Understanding the N2O budget is vital to the formulation of N2O emissions reduction policies. The isotopic ratios of N2O from different sources can provide insights into global N2O budgets. Isotopic ratios of N2O from soil and marine has been well studied, yet few studies have explored dynamics of dissolved N2O and its isotopic ratios in urban polluted rivers. In this study, we quantified N2O concentrations, environmental variables and the production pathways of N2O, measured the nitrogen isotope ratios of nitrate and ammonia to revealed the nitrogen pollution sources, assessed the isotopic ratios of N2O emitted from a series of polluted rivers and compare them with those of soils, oceans, and freshwaters, and revealed the influence of N2O production pathways and nitrogen pollution sources on the δ15N–N2O and N2O concentrations of urban polluted rivers in Shenzhen City, China. The concentrations of dissolved N2O ranged from 0.3 to 2482.9 nmol/L, with most values exceeding those equilibrium with the atmosphere, indicating that these rivers in urban subtropical areas are significant N2O sources to atmosphere. The isotopic ratios of dissolved N2O from polluted rivers varied widely, with δ15N–N2O ranging from −11.9‰ to 34.8‰, δ18O–N2O from 37.9‰ to 68.2‰, and SP from −10.5‰ to 36.2‰. The δ15N of emitted N2O from water surface to atmosphere ranged from −11.9‰ to +34.8‰, with a mean value of +9.2‰, which was higher than that from natural rivers, soil, and the troposphere. δ15Nsp and δ18O values of dissolved N2O were adopted to quantify the relative contributions of nitrification (f), denitrification (1-f), and the magnitude of N2O reduction (Fr) by denitrification. The results showed that denitrification was dominant pathway of N2O production, although the relative contribution of denitrification and nitrification had a great temporal and spatial variability. There is no distinct linear relationship between the concentrations of dissolved N2O, δ15N–N2O and environmental variables (DO, DOC, NO3−, and NH4+). The non-linear relationship among δ15N–N2O and δ15N–NO3−, f, Fr, and environmental variables were explored by regression tree analysis. The results suggested that f and Fr, not DO, DOC, NO3− and NH4+, were the primary correlates of dissolved N2O, for which the N2O concentrations were higher when N2O was primarily produced via nitrification (f > 77.5%) or denitrification (f < 8%), and were lower when Fr > 69.5%. It is the first attempt to reveal the connection between production and consumption processes with N2O concentrations directly. In addition, the elevated values of δ15N–NO3− (4.3 to 29.8‰) and δ15N–NH4+ (3.2‰ to 9.3‰) in these urban rivers consequence that the nitrogen contamination is predominantly sourced from urban sewage. Regression tree analysis also indicated that intense denitrification and sewage effluent result in enriched δ15N in N2O from polluted rivers, for which the δ15N values were higher when N2O was primarily produced via denitrification (f < 12%) or δ15N–NO3− were higher than 15.6‰. These findings provide a foundation upon which to devise strategies to reducing N2O production and emission in urban rivers.