High site elevation enhanced nitrogen fixation and the stability of diazotrophic community in planted Sonneratia apetala mangrove sediments

Site elevation was decisive in determining effective mangrove restoration, and nitrogen availability was a decisive factor for mangrove growth and development. The biological nitrogen fixation conducted by diazo-trophs was responsible for 40-60 % of the nitrogen demand of mangrove ecosystems, and the nitrogen dynamics of mangrove forests were primarily correlated with stand age. Understanding how site elevation affects sediment diazotrophs and their variation across stand age was critical for site selection for mangrove restoration and evaluating the functional rehabilitation and primary productivity of mangrove wetlands. Diazotrophic com-munity and nitrogen fixation rate (NFR) in mangrove sediments with depth were compared among 12-year and 18-year-old S. apetala mangrove forests planted under high (H18) and low site elevations (L18 and L12), respectively. The results indicated the sediment nutrient pools (e.g., TOC, TN, and TS) and NFR in mangrove sediments followed an increasing trend with increased site elevation over time (H18 > L18 > L12), and NFR weakly increased with sediment depth. Moreover, the diazotrophic community structure was both elevation-and age-dependent. The relative abundance of the dominant diazotrophic group Desulfuromonadales (belonged to sulfate-reducing bacteria, SRB) decreased with increasing site elevation, while another SRB group Syntropho-bacterales showed an opposite trend. Moreover, network analyses revealed more stable and less complicated networks in sediments with higher site elevation, which followed a similar trend with NFR by H18 > L18 > L12. This study provided new perspectives on the microbe-based assessment of mangrove functional restoration from the viewpoint of both diazotrophic communities and NFR. From this perspective, high site elevation had a higher nutrient pool, NFR, and diazotrophic community stability, which should be recommended for mangrove resto-ration in the future, and long-term rehabilitation would benefit the ecological functional (nitrogen fixation) restoration.