Salt marshes remove terrestrially derived nutrients en route to coasts. While these systems play a critical role in improving water quality, we still have a limited understanding of the spatiotemporal variability of biogeochemically reactive solutes and processes within salt marshes, particularly nitrogen species. To investigate this knowledge gap, we implemented a high-frequency sampling system to monitor sub-hourly nitrate (NO₃) concentrations in salt marsh porewater at Elkhorn Slough in central California, USA. We instrumented three marsh positions along an elevation gradient subjected to different extents of tidal inundation, which we hypothesized would lead to varied biogeochemical characteristics and hydrological interactions. At each marsh position, we continuously monitored NO₃ concentrations at depths of 10, 30, and 50 cm with subsurface water levels measured from 70 cm wells over seven deployments of ~10 days each. We quantified tidal event hysteresis between NO₃ and water level to understand how NO₃ concentrations and sources fluctuate across tidal cycles. There was significant differences in the NO₃-subsurface water level hysteresis patterns across seasonal wet/dry periods common to Mediterranean climates. In dry periods, the NO₃-subsurface water level relationship indicated that the source was likely estuarine surface water that flooded the transect during high tides. In wet periods, the NO₃-subsurface water level relationship suggested the salt marsh was a source of NO₃. These findings suggest that tidal and seasonal hydrologic fluxes control NO₃ porewater dynamics and influence ecological processes in coastal environments.