Coupled Stable Isotope Tracing and Sulfamic Acid Reduction (SIT–SAR) Method to Determine the Ammonia and Nitrite Oxidation Rates in Water and Sediments

Herein, a method was developed to measure the ammonia oxidation rate (Ra) and the nitrite oxidation rate (Rn) of water and sediment samples using a coupled stable isotope tracing and sulfamic acid reduction (SIT-SAR) method. 15NH4+ was used as a tracer to determine the ammonia oxidation rates (Ra) by calculating the concentrations of produced 15NO2- and 15NO3- during incubation, while 15NO2- was used as a tracer to determine the nitrite oxidation rates (Rn) by calculating the increase of 15NO3- during incubation. 15NO2- was chemically reduced to 29N2 with 15 mmol·L-1 sulfamic acid (SA). 15NO3- was first reduced to 15NO2- with a zinc-cadmium reducing agent, and then 15NO2- was subsequently reduced to 29N2 with SA. The produced 29N2 was measured by a membrane inlet mass spectrometer (MIMS). Under optimized experimental conditions, this method provides a sensitive (detection limit: 0.5 μmol·L-1) and precise (relative standard deviation: 4.80% for 15NO2-, 3.82% for 15NO3-) approach to quantify the concentrations of 15NO2- (0.5-150 μmol·L-1) and 15NO3- (0.5-120 μmol·L-1) in water and sediment samples over a wide range of salinities (0-30‰) with excellent calibration curves (R2 ≥ 0.999). This method was a successful application to estuarine water and sediments along the salinity gradient. Overall, the SIT-SAR method provided a rapid, accurate, and cost-effective means to determine Ra and Rn simultaneously.