Saltmarshes in estuary regulate and control nutrients discharged into coastal area, however, limited studies have quantified porewater-derived nutrients exchange and identified the implication on coastal nitrogen cycling. Here, we hypothesized that porewater-derived nitrogen exchange may contribute to water environmental issues. Spatial investigation was processed in an estuary saltmarsh with vary plant species. Based on porewater exchange rate derived from the radon balance model, nitrogen fluxes (including total nitrogen, nitrate and ammonium) were estimated. While nitrate was absorbed by saltmarsh in 1.4 kmol d^-1, ammonium was export to surface water in 1.2 kmol d^-1. Since δ¹⁵N-NO₃ in porewater were generally higher than surface water, the nitrate might be uptake to organic nitrogen by plants. Despite total dissolved inorganic nitrogen was decreased, amount of ammonium discharge should be noticed. The property that is more easily used by organisms may lead to eutrophication in coastal area, and the potential poisonous can significantly threaten aquatic creatures. The main source of ammonium was considered as mineralize of organic nitrogen. Analyze for sediments cores had proved that Spatina Alterniflora (invasion C4 plant) can significantly decrease the nitrogen storage in sediments. During the vegetation invasion, potential nitrogen release might pose more threaten to coastal environments.