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Abstract: Monthly nitrogen isotopes of ammonium and nitrate in wet deposition in the city of Guangzhou, and the causes of their variability, are reported in this paper. Nitrate δ15N showed nearly constant values around zero in the dry season (October to April), but oscillating values from negative to positive in the rainy season (May to September). By contrast, ammonium δ15N displayed lower values during the rainy season than in the dry season. The rural area north of the city was considered as the prominent source of ammonium and nitrate in spring and early summer (May and June), as suggested by their concurrent negative isotopic trends and higher NH4 +/NO3 − ratios. From July to September, different dominating sources from the city, i.e., fossil fuel combustion for nitrate, and sewage and waste emission for ammonium, caused disparate δ15N trends of the two species, showing positive nitrate δ15N, but still negative ammonium δ15N. During the cool dry season, the high values of ammonium δ15N and concurrently low NH4 +/NO3 − ratios suggested the decrease in NH3 volatilization and relatively important thermogenic origin of ammonium, but the intermediate nitrate δ15N values around zero may be a result of a balanced emission of NOx from the city and the rural areas. The isotopic effects of chemical conversion of NOx to nitrate and washout of nitrate were ruled out as significant causes of nitrate δ15N variability, but ammonium washout, during which 15N is assumed to be preferentially removed, may partly contribute to the ammonium δ15N variability. [Copyright &y& Elsevier]

Abstract: Understanding the role of the oceans in the Earth's changing climate requires comprehension of the relevant metabolic pathways which produce climatically important trace gases. The global ocean represents one of the largest natural sources of nitrous oxide (N2O) that is produced by selected archaea and/or bacteria during nitrogen (N) metabolism. In this study, the role of nitrite (NO2 −) in the production of N2O in the upper water column of the oligotrophic North Pacific Subtropical Gyre was investigated, focusing primarily on the lower euphotic zone where NO2 − concentrations at the primary NO2 − maximum reached 195nmolL−1. Free-drifting sediment trap arrays were deployed to measure N cycle processes in sinking particulate material and the addition of selected N substrates to unpreserved sediment traps provided an experimental framework to test hypotheses regarding N2O production pathways and controls. Sinking particles collected using NO2 −-amended, unpreserved sediment traps exhibited significant production of N2O at depths between 100 and 200m. Subsequent stable isotope tracer measurements conducted on sediment trap material amended with 15NO2 − yielded elevated δ15N values of N2O, supporting N2O production via a NO2 − metabolism pathway. Experiments on seawater collected from 150m showed N2O production via NO2 − metabolism also occurs in the water-column and indicated that the concentration of NO2 − relative to NH4 + availability may be an important control. These findings provide evidence for the production of N2O via nitrifer-denitrification in the lower euphotic zone of the open ocean, whereby NO2 − is reduced to N2O by ammonia-oxidizing microorganisms. [Copyright &y& Elsevier]