AbstractSalt 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. We implemented a high‐frequency sampling system to monitor sub‐hourly nitrate () concentrations in salt marsh porewater at Elkhorn Slough in central California, USA. We instrumented three marsh positions along an elevation gradient subjected to different amounts of tidal inundation, which we predicted would lead to varied biogeochemical characteristics and hydrological interactions. At each marsh position, we continuously monitored porewater concentrations at depths of 10, 30, and 50 cm and porewater levels measured at 70 cm depth over seven deployments of ~10 days each that spanned seasonal wet/dry periods common to Mediterranean climates. We quantified tidal event hysteresis between and water level to understand how concentrations and sources fluctuate across tidal cycles. In dry periods, the ‐porewater level relationship indicated that the source was likely estuarine surface water that flooded the transect during high tides and the salt marsh was a sink. In wet periods, the ‐porewater level relationship suggested the salt marsh was a source of . These findings suggest that tidal and seasonal hydrologic fluxes together control porewater dynamics and export and influence ecological processes in coastal environments.