4.3. Relationship between bacterial community structure and soil
chemical and microbial traits
Changes in soil bacterial community composition are more likely the
result of a direct influence of shifts in soil chemical and microbial
properties which were associated with different N fertilization rates,
whereas compositional changes that did not corresponded with N
fertilization were more likely to be the result of indirect factors
synergetically changing with N fertilization. According to Lau and
Lennon (2012) and Radhakrishnan and Baek (2017), bacterial community
composition could vary by replacing less adapted species with better
adapted ones, and this was one of the major mechanisms through which the
trait distribution of community changed. Relative abundance ofAnaerolineae , which belonged to phylum Chloroflexi and was
a type of carbohydrate and phenols degrading bacteria, showed negative
correlation with most soil chemical and microbial properties. This was
different from Zhang et al. (2019b) who reported that relative abundance
of Anaerolineae in coastal salt-affected mudflats tended to
increase with increasing years of rice cultivation. The possible reason
was that high N fertilization rates and low organic matter resulted in
low C/N and the richness of Anaerolineae which was important
organic matter degraders under anoxic condition. These results agreed
with Zhang et al. (2019a) who concluded that and the relative abundance
of Anaerolineae decreased by 8.1%~22.7% with
the increase of nitrogen application. Relative abundance ofAlphaproteobacteria and Gammaproteobacteria was positively
associated with most soil chemical and microbial properties. This showed
consistency with Iwaoka et al. (2018) who found that relative abundance
of Alphaproteobacteria and Gammaproteobacteria was
positively correlated with total N (TN) and net N mineralization rate
(NMR). Planctomycetia and Nitrospira , which played
important roles in soil nitrogen cycle (Fuerst and Sagulenko, 2013),
showed positive correlation with most soil attributes in this study,
especially class Nitrospira . Among them, Planctomycetiautilized and transformed ammonium and Nitrospira played pivotal
roles in nitrification as an aerobic chemolithoautotrophic
nitrite-oxidizing bacterium (Daims and Wagner, 2018). This was not
unexpected considering that consecutive N fertilization improved the
metabolic activities of some soil bacteria associated with N cycle, such
as the net N mineralization rate and potential nitrification rate in
this study. Interestingly, Cytophagia , Bacilli ,Gemmatimonadetes and Acidobacteria (includingAcidobacteria _Gp 6 and Acidobacteria _Gp 10)
were negatively influenced by soil chemical and microbial properties.
Actually, relative abundance of Cytophagia was noted for an
endophytic bacterial population closely associated with soil basic
cations, namely soil salinity level (Szymańska et al., 2018), whereas
soil salinity was negatively correlated with most of other soil
microbial attributes (Table 5). With regard to Bacilli , Sharma et
al. (2015) used 16S rRNA gene sequencing and fatty acid methyl ester
(FAME) for diversity analysis of Bacilli , and found that most
species possessed the ability to tolerate high salt, form endospores,
and withstand harsh environments. Moreover, previous researches reported
that flooding favored Actinobacteria and Gemmatimonadetesin salt-affected soil (de León-Lorenzana et al., 2017), and the relative
abundance of Gemmatimonadetes was negatively correlated with soil
enzyme activity and decreased significantly with cultivation year. This
could be ascribed to the halotolerant and oligotrophic characteristics
of these bacterial. Gad (2014) isolated halotolerantActinobacteria from hypersaline sediments in Great Salt Plains,
Oklahoma and analyzed the phylogenetic diversity and anti-MRSA activity.
Siles et al. (2014) also found that amendments to salt-affected soil
using crop residues decreased the relative abundance ofGemmatimonadetes .