Spartina alterniflora invasion and mangrove restoration alter diversity and composition of sediment diazotrophic community

Ecological restoration using native mangrove species (i.e., Kandelia obovata) is a practical approach for con-trolling Spartina alterniflora invasion in the coastal wetlands. Diazotrophs play a critical role in enhancing pro-ductivity of both S. alterniflora and mangrove plant, via providing new nitrogen to the coastal wetlands ecosystems. However, response of the diazotrophic community to S. alterniflora invasion and subsequent mangrove restoration remains unclear. The present study monitored sediment physicochemical properties and diazotrophic communities across a chronosequence of restored mangrove wetland (bare mudflat, invasive S. alterniflora stands, 3-year and 10-year K. obovata restoration areas, and mature K. obovata forests over 30 years). The results showed an orderly succession in the sediment properties and diazotrophic community composition after S. alterniflora invasion and along mangrove restoration chronosequences. Almost all sediment nutrient contents (e.g., TC, TN, and C/N) were significantly (P < 0.05) increased by S. alterniflora invasion, then they decreased sharply in the newly restored mangrove and increased gradually with restoration ages, with the highest values in the mature mangrove. The diazotrophs demonstrated distinct community structure in different seasons. Diazotrophic alpha diversity increased with S. alterniflora invasion and mangrove restoration age in winter. Sulfate-reducing bacteria (SRB) was the dominant diazotrophic group, especially Desulfuromonadales. Moreover, network analyses revealed that SRB groups were the major keystone taxa. The relative abundance of Desulfuromonadales decreased, while the abundance of Chromatiales increased gradually with the S. alterniflora invasion and mangrove restoration age. Redundancy analysis revealed that TC, TN, and TS were the significant (P < 0.05) environmental factors in altering sediment diazotrophic communities in both seasons. These findings expand the current understanding of succession patterns of diazotrophic communities in mangrove restoration and provide new perspectives on the sustainable management of mangrove ecosystems.