3.2 Effect of amendments on soil MBC, MBN and MBC/MBN
Amended treatments and incubation time had significant effects on MBC,
MBN (P <0.001) and MBC/MBN (P <0.01),
and had no significant interactions between the two factors. During the
entire incubation period, MBC and MBN in all amended treatments were
significantly higher than those in CK treatment, while MBC/MBN only in
C+M+N treatments were significantly lower than that of CK treatment
(Table 2). Specifically, MBC and MBN contents increased significantly
after addition of M (in C-M-N and C-M treatments), in which the M2 and
N2 level supported the higher contents. MBC/MBN decreased significantly
in N2 levels (C+M2+N2 and C+M1+N2)(Fig.3). With the increasing
incubation time (Fig.3), MBC, MBN and MBC/MBN in CK treatment remained
almost unaltered, and with the average value of 42 mg/kg, 5 mg/kg and
9.52, respectively. MBC and MBN (P <0.01) increased
significantly up to day 45, and then decreasing during the remaining
incubation time(day 45 to 90). MBC/MBN in all amended treatments
exhibited significant changes with incubation time but no obvious
regularity.
3.3Diversity and
composition of soil bacterial community
A total of 1,532,205 bacterial sequences were obtained from the complete
data set, of which 13,769 bacterial OTUs belonged to 33 phyla, 257
classes, 257 orders, 475 families and 1,000 genera. The rarefaction
curves of bacteria showed clear asymptotes, which indicated a near
complete and true sampling of the community. The dominant phyla
(relative abundance >1%) were Proteobacteria
(48.26-82.32%), Actinobacteria (16.59-4.09%), Bacteroidetes
(4.49-15.47%), Firmicutes (0.55-9.20%), Gemmatimonadetes
(1.76-8.18%), Chloroflexi (0.89-5.21%), Patescibacteria (0.87-5.17%),
and Acidobacteria (0.09-2.72%), together accounting for >
98% of bacterial sequences across all samples (Fig. S1a). Notably, the
relative abundance of Bacteroidetes increased significantly in C-M-N
treatments (P <0.001), in which N2 level increased the
higher value (Fig. S1a). Firmicutes also increased significantly in
C-M-N treatments (P <0.01), in which N1 level increased
the higher value. However, the relative abundance of Acidobacteria
decreased significantly (P <0.001) with addition of N
fertilizer treatments, in which N1
level (ordered by C+N1, C+M1+N1, C+M2+N1) decreased obviously (Table 3).
Alpha-diversity estimated by Chao1 estimator, and Shannon and Simpson
indices showed significant differences in species richness and diversity
of soil bacterial community between different treatments
(P <0.05). Chao1 estimator was significantly higher in C
(C-N0), C+M (C-M1-N0, C-M2-N0) and C+M+N2 (C-N2, C-M1-N2, C-M2-N2)
treatments than in CK treatment (P <0.05), in which N2
level treatment supported higher value (highest in C-M2-N2 treatment).
Simpson and Shannon indices were significantly higher in all amended
treatments especially at the N2 level (P <0.05), but no
significant difference observed between amended treatments (Table 4).
PCoA analysis based on Bray-Curtis
distances accounted for 47.3% of total variance among bacterial
communities, with axes 1 and 2 explaining 25.4 and 21.9% of the
variance, respectively (Fig. 4a). PCoA analysis showed that bacterial
communities were divided into three major groups. Treatments with N1 and
N2 level (C-N1, C-N2, C-M1-N1, C-M1-N2, C-M2-N1 and C-M2-N2) tended to
group together, N3 level (C-N3, C-M1-N3 and C-M2-N3) clustered into
another group, and treatments without N addition (C-N0, C-M1-N0,
C-M2-N0) grouped together with CK (R2 = 0.555
> 0.5, P < 0.001; PERMANOVA, Test statistic=
5.0189, P = 0.001). Overall, three groups exhibited significant
differences in bacterial community composition and were separated mainly
by N level.