Divergent responses of symbiotic and asymbiotic N2 fixation to seawater additions

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 wit...