The capillary fringe is a subsurface terrestrial-aquatic interface that can be a significant hotspot for biogeochemical cycling of terrestrially derived organic matter and nutrients. However, pathways of nitrogen(N)cycling within this environment are poorly understood, and observations of temporal fluctuations in nitrate (NO3−) concentrations lack the necessary resolution to partition between biotic or abiotic mechanisms. At discrete sampling points, we measured NO3−, nitrite (NO2−), ammonium (NH4+), gaseous nitrous oxide (N2O), and nitrogen (N2), and the corresponding isotopic composition of NO3− within floodplain sediments at Rifle, Colorado. Coincident with an annually reoccurring spring/summer excursion in groundwater elevation driven by snowmelt, we observed a rapid decline in NO3− followed by transient peaks in NO2−, at three depths (2, 2.5, and 3 m) below the ground surface. Isotopic measurements (δ15N and δ18O of NO3−) suggest an immediate onset of biological N loss at 2 m. At 2.5 and 3 m, NO3− concentrations declined initially with no observable isotopic response, indicating dilution of NO3− as the NO3 −-deficient groundwater rose, followed by denitrification after prolonged saturation. A simple Rayleigh model further supports this depth-dependent variability in the significance of actively fractionating mechanisms (i.e., NO3− reduction) relative to non-fractionating mechanisms (mixing and dilution). NO3− reduction was calculatedtoberesponsiblefor64%oftheNO3 − decline at 2m, 28% at 2.5 and 47% at 3m, respectively. Finally, by accounting for previous molecular and geochemical analysis at this site and comparing the trajectories between 1δ18O: 1δ15N, we conclude that biological NO3− consumption at the two deeper and frequently saturated depths (2.5 and 3 m) can be attributed to heterotrophic denitrification. However, the 1δ18O: 1δ15N trajectory at the shallower, irregularly saturated site at 2 m shows a more complicated relationship best explained by the cyclic production of NO3− via aerobic oxidation and consumption via NO3 − reduction.