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.

Salt marshes are hotspots of nutrient processing en route to sensitive coastal environments. While our understanding of these systems has improved over the years, we still have limited knowledge of the spatiotemporal variability of critical biogeochemical processes within salt marshes. Sea-level rise will continue to force change on salt marsh functioning, highlighting the urgency of filling this knowledge gap. Our study was conducted in a central California estuary experiencing extensive marsh drowning and relative sea-level rise, making it a model system for such an investigation. Here we instrumented three marsh positions with different degrees of inundation (6.7%, 8.9%, and 11.2% of the time for the upper, middle, and lower marsh positions, respectively), providing locations with varied geochemical characteristics and hydrological interaction at the site. We continuously monitored redox potential (Eh) at depths of 0.1, 0.3, and 0.5 m, subsurface water levels (WL), and temperature at each marsh position to understand how drivers of subsurface biogeochemical processes fluctuate across tidal cycles, using wavelet analyses to explain the interactions between Eh and WL. We found that tidal forcing significantly affects biogeochemical processes by imparting controls on Eh variability, likely driving subsurface hydro-biogeochemistry of the salt marsh. Wavelet coherence showed that the Eh-WL relationship is non-linear, and their lead-lag relationship is variable. We found that precipitation events perturb Eh at depth over timescales of hours, even though WL show relatively minimal change during events. This work highlights the importance of high-frequency measurements, such as Eh, to help explain factors that govern subsurface geochemistry and hydrological processes in salt marshes.