The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Developing an accurate and well-established method for measuring nitrification rate is important to understand the role of nitrification process in the nitrogen cycle across environments. Here, we present a rapid and more accurate method to determine the ammonia oxidation rate (R_a) and the nitrite oxidation rate (R_n) simultaneously using stable isotope tracing and acidified potassium iodide reduction (SIT-APIR). ¹⁵NH₄⁺ was added to the incubation as a tracer to measure the R_a by calculating produced ¹⁵NO₂^- and ¹⁵NO₃^-, while ¹⁵NO₂⁻ was used as another tracer for R_n determination by calculating produced ¹⁵NO₃^-. The produced ¹⁵NO₂^- and ¹⁵NO₃^- were chemically reduced to ³¹NO with potassium iodide (KI) and zinc-cadmium reducing agent couple with KI, respectively. High throughput measurements of ³¹NO were achieved by a membrane inlet mass spectrometer (MIMS). Based on the optimized reaction conditions, an improved method with lower detection limit (0.1 μmol L^-1), higher precision (relative error: 4.39% for ¹⁵NO₂^-, 0.65% for ¹⁵NO₃^-) and stable (relative standard deviation: 3.12% for ¹⁵NO₂^-, 8.97% for ¹⁵NO₃^-) was developed to quantify the concentrations of ¹⁵NO₂^- (0.1-200 μmol L^-1) and ¹⁵NO₃^- (0.1-80 μmol L^-1) with excellent calibration curves (R²≥0.994). In addition to water and sediment samples, this method can be widely applied to determine the R_a and R_n of dry soil. Overall, this SIT-APIR method provide a precise, fast, and promising approach for determining R_a and R_n in the environment with low cost.