Water column ammonium concentration and salinity influence nitrogen uptake and growth of Spartina alterniflora

- Ammonium concentration and salinity affect Spartina alterniflora nitrogen uptake.
- Salinity effects on nitrogen uptake and biomass outweighed those of ammonium.
- Low-level ammonium addition has the potential to mitigate salinity-induced stress.
- Incorporating both salinity and nitrogen in water management plans is recommended.

Salt marsh macrophytes, such as Spartina alterniflora, play a critical role in uptake and transformation of inorganic nitrogen before it reaches coastal waters, but it may be possible to exceed S. alterniflora nitrogen uptake capacity, particularly when salinity is elevated. While it is well known how inorganic nitrogen availability or salinity influences S. alterniflora nitrogen uptake individually, investigating the combined effects of both factors is essential because changes in inorganic nitrogen supply often occur simultaneously with altered freshwater flow. Nitrogen uptake and growth responses of Spartina alterniflora to inorganic nitrogen (0, 10, or 100 μM ammonium, NH4+) and salinity (20, 30, or 40) treatments were measured in greenhouse microcosms with tidal simulation. Water column NH4+ uptake decreased as salinity increased with the addition of 10 μM NH4+ after 48 h. In contrast, with 100 μM NH4+ addition uptake rates were twice as high in the lowest (20) and highest (40) salinity compared to the mid-level (30) salinity treatment. After 6 months, above and belowground S. alterniflora plant tissue NH4+ uptake (δ15N) decreased by 50% with increasing salinity across all NH4+ addition treatments. Furthermore, at the highest salinity, above and belowground biomass, stem density and culm height were greater in the 10 μM NH4+ addition compared to the 0 and 100 μM NH4+ addition, indicating potential for low-level NH4+ additions to mitigate salinity-induced stress. Overall the effects of salinity on S. alterniflora nitrogen uptake and biomass generally outweighed those of water column N concentration, suggesting the interaction of salinity and nutrient loading should be considered when developing predictive models for the fate of coastal ecosystems under changing environmental conditions.