Using nitrate stable isotopes to untangle the drivers of wet nitrogen deposition in New South Wales

Abstract

Anthropogenic nitrogen deposition, which stems from fossil fuel combustion, agricultural soils, and biomass burning, is assumed to be negligible in Australia due to its low population density and rainfall. Perhaps because of this assumption, neither the amount nor the source of nitrogen deposited via rainfall has been systematically quantified for the country. Here we present two years of daily measurements of rain in Sydney and Lismore. We measured both inorganic nitrogen (NO3- and NH4+) concentrations and the isotopic composition of NO3- (δ15N-NO3- and δ18O-NO3-) to identify event, seasonal, and spatial differences in the origin of wet nitrogen deposition. The two sites have contrasting precipitation regimens (Lismore receives ~1,600 mm of rain, predominantly in the summer, while Sydney receives ~900 mm of rain sporadically over the year) and land-use (Lismore population: 40,000; Sydney population: 5 million). Despite lower rainfall in Sydney, N deposition was higher (~4 kg N ha-1 y-1, v 2 kg N ha-1 y-1 in Lismore). Although NO3- isotopes varied widely between events, consistent differences between the two sites emerged: Sydney δ15N-NO3- ranged from -10 to +10 ‰ and δ18O-NO3- from 50 to 70 ‰, while Lismore δ15N-NO3- ranged from -15 to +5 ‰ and δ18O-NO3- from 30 to 70 ‰. These findings suggest that Australian ecosystems are affected by local atmospheric nitrogen pollution, and highlights the need to re-examine the spatial and temporal drivers of nitrogen across the continent.