3.4 Bacterial community structure and composition
As shown in Fig. 1, the bacterial community structure at the phylum level in the four N applications consisted of 12 bacterial phyla with relative abundances greater than 1%., i.e., Proteobacteria, Actinobacteria, Chloroflexi, Acidobacteria, Gemmatimonadetes, Bacteroidetes, Planctomycetes, WPS-2, Patescibacteria, Firmicutes, Verrucomicrobia and others. However, their proportions in different N application treatments were quite different. Table 4 lists the compositions and relative proportions of the dominant soil bacterial at phylum level which were identified in this study. The numbers of identified bacterial phyla in the H, M, L and CK treatments were 11, 10, 9 and 9, respectively. All these results showed that the N applications not only changed the proportions of dominant soil bacterial phyla, but also altered the compositions of soil bacterial communities (Table 4).
All the above results indicate that Proteobacteria, Actinobacteria, Chloroflexi and Acidobacteria are the four most abundant soil bacterial phyla in sugarcane fields. Proteobacteria are easily enriched under high N application conditions, and Actinobacteria and Acidobacteria sensitively responded to low or moderate N applications. By contrast, Chloroflexi could be enriched in the soil of sugarcane fields without N application.
Notes: H: high N application in the sugarcane soil (964 kg ha− 1), M: moderate N application in the sugarcane soil (482 kg ha− 1), L: low N application in the sugarcane soil (96 kg ha− 1), CK: no N application in the sugarcane soil (0 kg ha− 1).
Table 4
The proportion of dominant bacterial communities at phylum level under four N application treatments (%)
Phylum
|
H
|
M
|
L
|
CK
|
Actinobacteria
|
28.54
|
32.57
|
25.15
|
22.65
|
Proteobacteria
|
29.44
|
25.52
|
23.33
|
25.10
|
Chloroflexi
|
15.31
|
21.48
|
24.26
|
26.16
|
Acidobacteria
|
13.62
|
10.56
|
15.98
|
12.71
|
WPS-2
|
1.59
|
1.93
|
-
|
4.58
|
Gemmatimonadetes
|
2.35
|
1.48
|
2.67
|
-
|
Planctomycetes
|
1.87
|
1.37
|
1.77
|
1.68
|
Bacteroidetes
|
1.95
|
1.01
|
1.20
|
1.91
|
Patescibacteria
|
1.41
|
-
|
-
|
1.21
|
Firmicutes
|
1.11
|
1.25
|
-
|
-
|
Verrucomicrobia
|
-
|
-
|
1.03
|
-
|
others
|
2.01
|
1.79
|
2.81
|
1.96
|
Notes: H: high N application in the sugarcane soil (964 kg ha− 1), M: moderate N application in the sugarcane soil (482 kg ha− 1), L: low N application in the sugarcane soil (96 kg ha− 1), CK: no N application in the sugarcane soil (0 kg ha− 1). |
As shown in Fig. 2, at the genus level, there were 28, 22, 26 and 25 dominant bacterial genera with relative abundances greater than 1% in the high, moderate, low and no N application treatments, respectively. Compared to the CK, the dominant bacterial genera all increased in the high- or low-nitrogen treatments, but they decreased in the moderate-nitrogen application treatments. Meanwhile, there were special dominant bacterial genera in every N application treatment, Mycobacterium, norank_f__SC-I-84, norank_f__norank_o__Saccharimonadales, norank_f__Micropepsaceae, norank_f__norank_o__Subgroup_2and norank_f__Acetobacteraceae were the unique dominant genera in the H treatment. norank_f__JG30-KF-CM45 and Jatrophihabitans were the unique dominant genera in the M treatment; norank_f__norank_o__norank_c__Subgroup_6, HSB_OF53-F07, Streptomyces, norank_f__67 − 14, norank_f__norank_o__SBR1031 and norank_f__norank_o__norank_c__KD4-96 were the unique dominant genera in the L treatment. FCPS473, Actinospica, 1921-2, Sinomonas and norank_f__Ktedonobacteraceae were the unique dominant genera in the CK treatment. All the above results indicate that the soil bacterial community structure in sugarcane fields could be significantly affected by N input. In particularly, more sensitive effects are triggered by low or high N application (Table 5).
Notes: H: high N application in the sugarcane soil (964 kg ha− 1), M: moderate N application in the sugarcane soil (482 kg ha− 1), L: low N application in the sugarcane soil (96 kg ha− 1), CK: no N application in the sugarcane soil (0 kg ha− 1).
Table 5
The proportion of dominant bacterial communities at genus level under four N application treatments (%)
Genus
|
H
|
M
|
L
|
CK
|
Acidothermus
|
6.02
|
6.3
|
1.68
|
4.66
|
norank_f__norank_o__Gaiellales
|
5.25
|
4.76
|
4.49
|
2.73
|
norank_f__norank_o__norank_c__AD3
|
1.94
|
6.03
|
2.05
|
6.16
|
norank_f__Xanthobacteraceae
|
3.27
|
2.97
|
4.20
|
1.36
|
norank_f__norank_o__norank_c__TK10
|
1.89
|
2.94
|
4.78
|
2.12
|
norank_f__norank_o__Acidobacteriales
|
2.88
|
2.56
|
2.72
|
3.13
|
Bradyrhizobium
|
2.97
|
2.89
|
1.7
|
3.2
|
Conexibacter
|
1.33
|
3.55
|
1.37
|
3.35
|
norank_f__norank_o__norank_c__norank_p__WPS-2
|
1.59
|
1.93
|
-
|
4.58
|
norank_f__norank_o__Elsterales
|
2.38
|
2.38
|
1.73
|
1.83
|
norank_f__JG30-KF-AS9
|
3.34
|
2.25
|
-
|
2.1
|
norank_f__norank_o__norank_c__Subgroup_6
|
-
|
-
|
5.32
|
-
|
Candidatus_Solibacter
|
2.17
|
2.22
|
1.89
|
-
|
Sphingomonas
|
1.66
|
1.63
|
1.37
|
2.27
|
norank_f__norank_o__B12-WMSP1
|
-
|
-
|
1.08
|
4.58
|
Bryobacter
|
1.73
|
1.93
|
1.31
|
1.53
|
norank_f__norank_o__IMCC26256
|
1.41
|
1.56
|
1.75
|
-
|
Burkholderia-Caballeronia-Paraburkholderia
|
1.18
|
1.87
|
-
|
1.95
|
unclassified_f__Micromonosporaceae
|
1.58
|
1.54
|
1.47
|
-
|
norank_f__Gemmataceae
|
1.28
|
-
|
1.43
|
1.16
|
unclassified_f__Acidobacteriaceae_Subgroup_1
|
1.90
|
-
|
-
|
2.00
|
norank_f__norank_o__norank_c__JG30-KF-CM66
|
1.39
|
1.12
|
-
|
1.31
|
unclassified_f__Ktedonobacteraceae
|
-
|
-
|
1.02
|
2.16
|
norank_f__Gemmatimonadaceae
|
1.62
|
-
|
1.93
|
-
|
Acidibacter
|
1.18
|
1.11
|
-
|
-
|
norank_f__Roseiflexaceae
|
-
|
-
|
2.71
|
1.42
|
norank_f__JG30-KF-CM45
|
-
|
1.26
|
-
|
-
|
norank_f__SC-I-84
|
1.67
|
-
|
-
|
-
|
Gaiella
|
-
|
1.00
|
2.00
|
-
|
Mycobacterium
|
1.14
|
-
|
-
|
-
|
FCPS473
|
-
|
-
|
-
|
1.16
|
norank_f__Micropepsaceae
|
1.34
|
-
|
-
|
-
|
norank_f__norank_o__Saccharimonadales
|
1.13
|
-
|
-
|
-
|
Jatrophihabitans
|
-
|
1.28
|
-
|
-
|
HSB_OF53-F07
|
-
|
-
|
1.06
|
-
|
Actinospica
|
-
|
-
|
-
|
1.10
|
norank_f__norank_o__Subgroup_2
|
1.13
|
-
|
-
|
-
|
Streptomyces
|
-
|
-
|
1.08
|
-
|
1921-2
|
-
|
-
|
-
|
1.04
|
norank_f__67 − 14
|
-
|
-
|
1.13
|
-
|
Sinomonas
|
-
|
-
|
-
|
1.09
|
norank_f__Acetobacteraceae
|
1.04
|
-
|
-
|
-
|
norank_f__Ktedonobacteraceae
|
-
|
-
|
-
|
1.12
|
norank_f__norank_o__SBR1031
|
-
|
-
|
1.45
|
-
|
norank_f__norank_o__norank_c__KD4-96
|
-
|
-
|
1.32
|
-
|
others
|
33.69
|
29.41
|
36.51
|
30.75
|
Note. H: high N application in the sugarcane soil (964 kg ha− 1), M: moderate N application in the sugarcane soil (482 kg ha− 1), L: low N application in the sugarcane soil (96 kg ha− 1), CK: no N application in the sugarcane soil (0 kg ha− 1). |
The number of bacteria obtained at the OTU (Operational Taxonomic Units) level under the H, M, L and CK treatments was 3237, 3318, 3923 and 2576, respectively. The numbers of unique bacteria in the H, M, L and CK treatments at the OTU level were 222, 152, 852 and 254, respectively (Fig. 3A). In addition, the numbers of bacteria in the H, M, L and CK treatments at the genus level were 587, 588, 631 and 508, respectively. Moreover, the numbers of unique bacteria in the H, M, L and CK treatments at the genus level were 18, 4, 59 and 11, respectively (Fig. 3B). All the above results suggested that the soil bacterial community structure could be significantly altered by nitrogen application. However, higher nitrogen input (964 kg ha− 1 and 482 kg ha− 1) was not helpful for improving the number of unique soil bacteria in sugarcane fields. On the contrary, low nitrogen application (96 kg ha− 1) was more efficient for improving soil bacterial diversity and richness in sugarcane fields.
Note H: high N application in the sugarcane soil (964 kg ha− 1), M: moderate N application in the sugarcane soil (482 kg ha− 1), L: low N application in the sugarcane soil (96 kg ha− 1), CK: no N application in the sugarcane soil (0 kg ha− 1).
The top 50 most abundant soil bacteria at the genus level in sugarcane fields under different nitrogen applications were selected to form the heat map (Fig. 4). The horizontal level represents the different treatments, and the longitudinal direction shows the abundance of bacterial species. As seen in Fig. 4, the distribution of soil dominant bacteria under low, high and moderate nitrogen applications was different from the distribution under the CK, and there was also a difference between each treatment. However, the distribution of soil dominant bacteria was quite similar between the CK and high or moderate nitrogen application treatments. However, the composition and abundance of dominant soil bacteria under low nitrogen application changed significantly between CK. This phenomenon indicates that the response of the soil bacterial community structure to nitrogen application is more sensitive to low nitrogen input at 96 kg ha− 1.
Note A color zone represents relative abundance. H: high N application in the sugarcane soil (964 kg ha− 1), M: moderate N application in the sugarcane soil (482 kg ha− 1), L: low N application in the sugarcane soil (96 kg ha− 1), CK: no N application in the sugarcane soil (0 kg ha− 1).