Sea level rise-associated seawater intrusion exerts substantial impacts on the structure and function of coastal ecosystems. As nitrogen (N) availability is closely related to the structure and function of ecosystems, it is crucial to understand how seawater intrusion impacts biological N₂ fixation, a major pathway of new N input to natural ecosystems.

Here, a seawater addition experiment with two N₂-fixing plant species (Acacia auriculiformis and Casuarina equisetifolia) and ten non-N₂-fixing plant species was conducted to simulate sea level rise-associated seawater intrusion. The experiment included control (0 mm seawater yr^-1), low seawater addition (LSW, 200 mm seawater yr^-1), and high seawater addition (HSW, 400 mm seawater yr^-1). 

Symbiotic N₂ fixation (SNF) rate of A. auriculiformis was significantly higher under LSW (but not for HSW) than the control. For C. equisetifolia, SNF rates under LSW and HSW significantly increased by 8.0 and 19.5 times, respectively, compared with the control. In contrast, asymbiotic N₂ fixation rates under LSW and HSW significantly decreased by 18.5% and 30.4%, respectively, across all the plant species.

Our findings suggest that the responses of biological N₂ fixation to seawater intrusion are divergent either among different N₂-fixing plant species or between symbiotic and asymbiotic forms, which should be considered in terrestrial ecosystem models in order to improve the prediction of N dynamics under climate changes.