1. Introduction
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.
2. Materials and methods