These data were collected during July 2020 from a sandy subterranean estuary (STE) located in Gloucester Point, Virginia, USA. A pole-corer was used to collect triplicate sediment cores roughly 50cm in length from the mid tide line of the beach during low tide. Cores were sectioned into ten centimeter increments. Porewater was collected from surface water and piezometers placed at 10cm increments to the same depth of 50cm.  Core sections were used in isotope tracer incubations with 15N-labeled substrates to measure potential nitrification rates. Briefly, 10 g of the composited sediment from each 10 cm section were used in sediment slurry incubation experiments with 40mL of porewater from the same depth interval. Samples were incubated for 0, 6, and 12 hours in in 100mL HDPE bottles and amended with 5 atom% 15 µM 15NH4+ (Cambridge Isotope) at the measured in situ temperature. After the incubation period, samples were spun down, porewater was decanted and filtered with a 0.45 µm syringe filter (Whatman GE) and were frozen (-20 °C) until analysis.  Porewater nutrient concentrations were analyzed for NOx (nitrate + nitrite), nitrite, and ammonium with a Lachat autoanalyzer. The isotopic enrichment of nitrate  and nitrite products were measured  using a gas bench isotope ratio mass spectrometer (IRMS, Delta V Plus, Thermo Fisher Scientific, Waltham, MA) using the bacterial reduction method. A culture of Pseudomonas aureofaciens reduced NOx in collected samples to N2O. The isotopic composition of the N2O was measured with isotope-ratio mass spectrometry (IRMS) (Sigman et al., 2001).