With the increasing soil degradation and growing population, soil salinization has been a critical problem to agricultural production and ecosystem sustainability in not only arid and semi-arid areas, but also the coastal ecosystem (Pitman and Läuchli, 2004). This is particularly the case in the marine-terrestrial interlaced zone of the Delta region, where large amount of mudflats were developed from marine sediments and alluvial deposits, and these mudflats are naturally saline and can be used as wet lands or cultivated as reserve land resources (Li et al., 2014; Long et al., 2016). However, soil salts cause high osmotic stress and constrains the water and nutrient uptake by plants (Hagemann, 2011), restrict microbial growth and biochemical functioning which plays a pivotal role in soil organic matter and nutrient cycle (Wichern et al., 2006; Yan and Marschner, 2013). Therefore, improving the microbial biomass and activity in saline soil contributes to increasing soil organic matter input, promoting microbial C mineralization and nutrient cycling, and enhancing nutrient utilization efficiency (Elmajdoub and Marschner, 2015; Meena et al., 2016).

It has been shown that soil salinity is important in shaping bacterial communities in saline soils under halophytic vegetation, irrigation, fertilization regimes and even amendment residue application (Zhao et al., 2018; Rath et al., 2019). Recently, many efforts have been devoted to linking soil bacterial community composition to soil salinity along environmental gradients. Rousk et al. (2011) suggested that soil salinity was not a decisive factor for bacterial growth, and for structuring the decomposer community in an arid agroecosystem. Ren et al. (2018) found that soil salinity shaped microbial communities and contributed to nitrogen cycling and carbon fixation,Thaumarchaeota and Proteobacteria were crucial for nitrogen cycle and Proteobacteria and Crenarchaeota played important roles in dicarboxylate-hydroxybutyrate cycle. Study of Zhang et al. (2019) exhibited the importance of environmental filtering in microbial community assembly and suggested that soil salinity was a key determinant for soil microbial community composition and assembly processes in a desert ecosystem. In addition to terrestrial ecosystem, this relationship was also observed for saline lake sediments and wetland. Hollister et al. (2010) found that soil microbial community structure shifted along an ecological gradient of hypersaline sediments, and the greater depth of sequencing resulted in the detection of taxa not described previously. Cong et al. (2014) concluded that soil microbial community structure evolved along halophyte succession in Bohai Bay wetland and the belowground processes were strongly related with aboveground halophyte succession. Under the saline environment, soil microbial community was also found to be well responsive to interactive effect of amendment measurements, including biochar-manure compost (Lu et al., 2015), biogas residue (Shi et al., 2018), flue gas desulfurization gypsum by-products (Li et al., 2012), crude oil contamination (Gao et al., 2015), saline water irrigation (Chen et al., 2017) and even cultivation year (Cui et al., 2018). Furthermore, Baumann and Marschner (2013) stated that the microbial tolerance to soil drying and rewetting stress was salt level dependent, and the adaptation to salt stress could reduce the influence of water stress on microbial community composition only when salt stress was beyond a critical salinity level.

Fertilization is also capable of shaping soil microbial community structure in different scenarios of soil environment and planting patterns. A recent meta-analysis of soil microbial metabolic activity reported that the shift in microbial activity was a crucial mechanism for the change of N transformation rates in N-limited ecosystems with N addition (Zhou et al., 2017). Ikeda et al. (2014) assessed urea-formaldehyde (UF) fertilizer on the diversity of bacterial communities in onion and sugar beet, and revealed that that the community structures in both planting patterns shifted unidirectionally in response to the UF fertilizer. Li et al. (2016) found that nitrogen fertilization rate was one of the main factors influencing rhizosphere microbial community in continuous vegetable cropping within an intensive greenhouse ecosystem. For the alkaline soil, Zhou et al. (2016) revealed that the change in straw chemical properties had impact on the bacterial communities associated with the decomposition of straw in agro-ecosystems. Interactions between soil fertilization and saline water irrigation or precipitation were also observed on soil bacterial community and microbial metabolic activity, and the findings showed similarity under such circumstances, i.e., long-term saline water irrigation altered the bacterial composition of soil in an N-dependent manner (Guo et al., 2018), and alleviated the adverse effects of irrigation salinity on microbial metabolic activity (Chen et al., 2017). However, the non-synergistic effects of N fertilization and precipitation regimes on the microbial functional groups was reported by Sun et al. (2018), and it also showed the negative effect of lower pH induced by N enrichment would be alleviated by precipitation regimes. Dong et al. (2015) discovered that combined additions of N and P fertilizer could promote soil fertility and microbial activity in fir plantations of subtropical area, and suggested β -glucosidase (β G) and N-acetyl-β -D-glucosaminidase (NAG) as useful indicators of the biogeochemical transformation and metabolic activity of soil microbes. More recently, Nguyen et al. (2018) discussed the legacy impacts of extreme weather events and N fertilizer addition on soil bacterial communities and the key processes involved in carbon cycling, and summarized that nitrogen addition did not improve the resilience (rate of recovery) of soil bacterial communities and functions to prolonged-drought event, and a long time was needed for the recovery of the soil microbial community historically exposed to extreme weather events.

With the above reviews, soil salinity and fertilization have been demonstrated to be the most important influencing factors on microbial composition at a global scale (Lozupone and Knight, 2007; Zhou et al., 2013). The importance of understanding bacterial community evolution in deterministic and stochastic processes is broadly recognized in microbial ecology (Evans et al., 2017), and recent literatures mostly focused on community assembly processes along natural salinity, pH, moisture, nitrogen fertilization, and irrigation water volume gradients (Van Horn et al., 2014; Zhang et al., 2019). However, little is known about soil characteristics and bacterial community assembly processes along a nitrogen addition gradient under saline environment. In this study, effect of N fertilization rates on soil characteristics and bacterial community structure was examined for coastal salt-affected Fluvo-aquic soil. This work was performed in a marine-terrestrial interlaced area, and the soil was reclaimed from mudflats and exposed to seawater immersion before reclamation for cultivation. The main objectives of this study were: (1) to investigate the shifts of soil chemical and microbial properties with cultivation years and N fertilization rates; (2) to determine how N fertilization rates affect the soil bacterial richness and diversity under saline environment; (3) to explore how the composition of bacterial community vary along N fertilization gradients at phylum and class levels; and (4) to determine which environmental factors are responsible for the alteration of the bacterial community structure at the class level.