Terrestrial freshwater influences salt marsh nitrogen processing through plant productivity

Abstract Tidal marshes serve as control points for nitrogen (N) removal at the terrestrial-marine interface. However, marshes are inherently heterogeneous, making it challenging to predict the timing, magnitude, and distribution of N processing. We investigated the role of terrestrial freshwater inputs in driving N-cycling in a tidal marsh dominated by a single plant species ( Salicornia pacifica ) (Elkhorn Slough National Estuarine Research Reserve). At this site, elevation-driven differences in tidal inundation and freshwater inputs have influenced the timing and distribution of S. pacifica productivity. We measured nitrogen fluxes using intact sediment cores collected from the lower, middle, and upper marsh positions during the wet (winter-spring) and dry (summer-fall) seasons to capture changes in N transformations, particularly N-removal. We found that changes in N transformations were associated with freshwater-driven belowground plant production across the marsh platform. Specifically, net N-removal rates were > 2.5X higher in upper and middle marsh positions, which had increased belowground plant productivity due to fresh groundwater upwelling. We also found a strong seasonal effect on N fluxes. The marsh served as a N source in the wet season and a N sink in the dry season. Overall, terrestrial freshwater inputs can help predict patterns in N-transformations within heterogeneous marsh systems and will be useful as climate change continues to affect the composition and persistence of tidal marshes.