Impact of increased water and salt on nitrogen dynamics in soil pore water of the Minjiang River estuary wetland.

Estuarine wetlands, functioning as dynamic interfaces among terrestrial, atmospheric, and marine systems, are critical components of global nitrogen cycle. Here, we examined the effects of increased salinity and inundation induced by seawater intrusion on total nitrogen (TN), inorganic nitrogen fractions in soil pore water, as well as the physicochemical properties of soil and pore water in the brackish <i>Cyperus malaccensis</i> wetland in Shanyutan of the Minjiang River Estuary, by conducting an experiment with three treatments, control (bare soil,CK), tidal water (flooding depth 5 cm, salinity 2-11, TW), and saline water (flooding depth 5 cm, salinity 15, SW). The result showed that: 1) TW treatment reduced the contents of NO₃^--N and NO₂^--N in soil pore water by 30.0% and 25.0%, respectively. The SW treatment reduced the contents of nitrate nitrogen (NO₃^--N), nitrite nitrogen (NO₂^--N), and TN in soil pore water by 65.0%, 50.0%, and 16.1%, respectively, while slightly increased the ammo-nium nitrogen (NH₄⁺-N) content by 8.7%. 2) TN was positively correlated with soil temperature but negatively correlated with soil moisture. NH₄⁺-N was significantly positively correlated with soil temperature and negatively correlated with pore water pH and soil moisture, whereas NO₃^--N exhibited a significant negative correlation with soil temperature. 3) The structural equation modeling (SEM) analysis indicated that salinity had a significant negative and direct effect on NO₃^--N and NO₂^--N, and a significant positive and direct effect on NH₄⁺-N. Increased water-salinity coupling induced by seawater intrusion would more strongly affect nitrogen transformations in soil pore water by suppressing nitrification and promoting denitrification, thereby accelerating nitrogen loss in the estuarine wetland.