Impacts of enhanced nitrogen load on microbial community composition and assembly in a typical subtropical estuarine marsh, Southeast China

Soil microbial community is easily modified by environmental changes, but little is known regarding the effects of nitrogen (N) enrichment on composition and assembly of microbial community in estuarine marshes. This study aimed to explore the impacts of N enrichment on soil microbial composition, assembly processes and co-occurrence network. A field experiment with four N load levels (0.0, 37.5, 50.0 and 100.0 g exogenous N m -2 yr -1 , respectively) was conducted in a typical subtropical estuarine marsh. After 7 and 19 months of sustained N enrichment (represented by T7 and T19 periods, respectively), soil microorganisms were determined by high-throughput sequencing technique. Results showed that although the microbial community composition showed causality with N load, its variation was primarily driven by time-dependent effect rather than N-dependent effect. Except for the Shannon indices of fungi, the microbial α-diversity in different treatments significantly decreased with prolonged the experiment ( P <0.05). Compared with the T7 period, the relative abundances of Proteobacteria, Bacteroidota and Actinobacteriota at T19 period significantly declined while those of Chloroflexi and Nitrospirota markedly increased ( P <0.05). Within each sampling period, significant N-dependent effect was occasionally observed as evidenced by the noticeably decreased abundances of Basidiomycota and the markedly increased values of Ascomycota with increasing N load levels ( P <0.05). With prolonged the experiment, the relative importance of homogeneous selection (HoS) in bacterial community and that of dispersal limitation (DL) in fungal community significantly elevated. Concurrently, the complexity of bacterial co-occurrence network was reduced while the complexity of fungal co-occurrence network was enhanced. This study found that time-dependent effect dominated the assembly processes of bacterial community by influencing environmental variables and diminishing soil bacterial functional potential through reducing α-diversity and complexity. Conversely, time-dependent effect significantly increased the diversity and complexity of fungal community, which ultimately enhanced its network stability.