Nitrogen removal through N cycling from sediments in a constructed coastal marsh as assessed by 15N–isotope dilution

Highlights: • We examined the effect of salinity on N removal capacity of a constructed salt marsh. • Higher salinity and lower organic C resulted in slower rates of microbial N processes. • The N removal capacity decreases with increasing salinity and decreasing organic C. • Denitrification was predicted well by sediment organic C rather by its salinity. • Heterotrophic denitrification dominated N removal (337 kg N day⁻¹) from this marsh. - Abstract: Constructed coastal marsh regulates land-born nitrogen (N) loadings through salinity-dependent microbial N transformation processes. A hypothesis that salinity predominantly controls N removal in marsh was tested through incubation in a closed system with added-¹⁵NH₄⁺ using sediments collected from five sub-marshes in Shihwa marsh, Korea. Time-course patterns of concentrations and ¹⁵N–atom% of soil-N pools were analyzed. Sediments having higher salinity and lower soil organic-C and acid-extractable organic-N exhibited slower rates of N mineralization and immobilization, nitrification, and denitrification. Rates of denitrification were not predicted well by sediment salinity but by its organic-C, indicating heterotrophic denitrification. Denitrification dominated N-loss from this marsh, and nitrogen removal capacity of this marsh was estimated at 337 kg N day⁻¹ (9.9% of the daily N-loadings) considering the current rooting depth of common reeds (1.0 m). We showed that sediment N removal decreases with increasing salinity and can increase with increasing organic-C for heterotrophic denitrification.