Changing Characteristics and Influencing Factors of the Bacterial and Fungal Communities in the Litter of Casuarina Equisetifolia Forests of Different Ages in Hainan Island

Abstract Casuarina equisetifolia, an important pioneer tree with resistance to typhoon and stress tolerance, is mainly cultivated in the coastal areas of tropical and subtropical zones. Casuarina equisetifolia forests have high litter accumulation rates and slow litter decomposition rates. The accumulation of litter may affect the secondary regeneration of Casuarina equisetifolia, and the decomposition of litter is largely controlled by microorganisms such as bacteria and fungi. While the characteristics of the microbial diversity in the litter of Casuarina equisetifolia forests have not yet been resolved. In order to explore the microbial distribution characteristics and community dynamic changes in the litter of a Casuarina equisetifolia forest, we used Illumina Miseq high-throughput sequencing technology to analyze the endophytic and exophytic bacterial and fungal communities of Casuarina equisetifolia forests of three ages (young forest, half-mature forest, and mature forest). The results showed that the young and half-mature forests had the highest bacterial diversity, and the half-mature and mature forests had the highest fungal diversity. The community structure and endophytic and exophytic communities of different forest ages were also different. Exophytic bacteria of the phylum Proteobacteria had a gradually decreasing abundance with stand age, while endophytic bacteria of the phylum Actinomycetes had a gradually increasing abundance with stand age; in the half-mature forest, the abundance of nitrogen-fixing microorganisms, such as the genera Rhizobium, Bradyrhizobium, and Actinoplanes, were increased relative to all of the other forest age types. The fungal community structure was different than the bacterial structure. Specifically, the fungal communities of both the young and mature forests were dominated by the class Dothideomycetes of the phylum Ascomycota, while phylum Basidiomycota was dominant in the half-mature forest. The correlation with the physicochemical properties of the litter showed that exophytic bacteria were related to the organic carbon and water content of the litter, endophytic bacteria were related to litter pH and nitrogen content, and fungi were correlated tonitrogen and phosphorus contents. In summary, the community structures of bacteria and fungi in the half-mature forest were more uniform, which was beneficial to the growth of Casuarina equisetifolia plants, and the changes observed in the physical and chemical factors of litter were one of the key factors driving microbial community structure.