3.1 Tillage on physical and chemical properties
The physical and chemical properties of the black soil are the primary factors affecting soil microbial activity and diversity. The contents of soluble salts (SS), pH value, organic matter (OM), total nitrogen (TN), available phosphorus (AP) and available potassium (QP) in DFBS and PBS were monitored. No significant effect of tillage disturbance on the SS in soil was observed over the experimental. Compared with NDBS, the OM decreased significantly and AP increased significantly for both DFBS and PBS (p < 0.05). The pH value decreased significantly in DFBS (p < 0.05), while increased significantly in PBS (p < 0.05). In contrast, tillage led to a significant decrease of the TN in PBS (p < 0.05), but not in DFBS. On the contrary, tillage caused the QP significantly decreased in DFBS (p < 0.05), but not in PBS.
Table 1 Soil physical and chemical properties
Sample
|
SS/g·kg-1
|
pH value
|
OM/g·kg-1
|
TN/g·kg-1
|
AP/mg·kg-1
|
QP/mg·kg-1
|
NDBS
|
2.6±1.5 a
|
6.40±0.00 b
|
45.43±0.91 a
|
2.36±0.31 a
|
5.37±1.24 c
|
233.67±18.56 a
|
DFBS
|
1.8±0.6 a
|
5.93±0.12 c
|
38.67±2.55 b
|
2.36±0.42 a
|
29.50±5.47 b
|
203.33±4.16 b
|
PBS
|
2.3±0.8 a
|
7.40±0.10 a
|
34.17±1.24 c
|
1.64±0.16 b
|
45.40±2.52 a
|
229.67±15.50ab
|
- The same lowercase letters in the table represent no significant difference between different samples at the P = 0.05 level.
- SS: soluble salt, OM: organic matter, TN: total nitrogen, AP: available phosphorus, QP: available potassium.
3.2 Tillage on the diversity of bacteria and archaea
The Chao1 index is an index reflecting the richness of species. The larger the index, the higher the richness of the microbial community. The Shannon index is an indicator reflecting the diversity of the microbial community, and the higher the index, the higher the diversity of the microbial community. The Chao1 index and Shannon index of bacteria at DFBS were lower than NDBS. The Chao1 index of bacteria at PBS was lower than NDBS, the Shannon index was higher than the NDBS (Table 2). The Chao1 index and Shannon index of archaea disturbed by DFBS were lower than the NDBS, the Chao1 index and Shannon index of archaea at PBS were higher than the NDBS (Table 3). The diversity of archaea decreased significantly after arable land farming (p < 0.05). After conversion from arable land farming to rice paddy, the diversity of bacteria increased sinificantly, while the diversity and richness of archaea increased significantly (p < 0.05).
Table 2 Alpha diversity index of bacteria
Samples
|
Chao1
|
Shannon
|
NDBS
|
3742.45±190.00 a
|
9.61±0.14 ab
|
DFBS
|
3097.32±113.83 a
|
9.42±0.30 b
|
PBS
|
3453.89±386.54 a
|
9.94±0.26 a
|
Table 3 Alpha diversity index of archaea
Samples
|
Chao1
|
Shannon
|
NDBS
|
1209.10±362.00 b
|
4.51±2.17 a
|
DFBS
|
961.14±259.39 b
|
3.91±0.76 b
|
PBS
|
1855.66±199.43 a
|
6.68±0.54 a
|
Based on the data of high-throughput sequencing, the effect of tillage on the bacterial and archaea community were further analyzed with respect to the relative abundance at the phyla and genus levels.
The dominant bacteria phyla included Proteobacteria, Chloroflexi, Acidobacteria, Gemmatimonadetes, Bacteroidetes, Planctomycetes, Actinobacteria, Nitrospirae, Firmicutes and Armatimonadetes accounted for > 87% of the total bacteria species in all sample types, among which half of the phyla were sensitive to the tillage disturbance (Fig. 1). At the phylum levels, the diversity of dominant bacteria was decreased significantly in DFBS (p < 0.05), but not affected in PBS. After soil was disturbed, the relative abundance of Gemmatimonadetes was increased significantly in DFBS (p < 0.05), and the relative abundance of Chloroflexi was increased significantly in PBS (p < 0.05). Compared with NDBS, the relative abundance of Proteobacteria and Armatimonadetes exhibited significantly increased in DFBS (p < 0.05), but Actinobacteria and Nitrospirae significantly decreased in both soil samples (p < 0.05).
Similarly, the relative abundance of most bacteria at the genus levels was also dramatically affected by tillage disturbance (p < 0.05). At the genus level, Bacillus, Flavobacterium, Anaerolinea, Bradyrhizobium, Variovorax, Flavisolibacter, Sporosarcina, Rhodoplanes, Kaistobacter and Methylibium were the dominant genera (Fig. 2). The bacterial genus level was analyzed and found the diversity of the dominant ones were increased dramatically in both soil samples after tillage disturbance comparing with the non-disturbed virgin soil (p < 0.05). The relative abundance of Kaistobacter, Flavobacterium and Rhodoplanes were increased significantly at DFBS (p < 0.05), yet Bacillus and Sporosarcina were decreased significantly (p < 0.05). Unlike in DFBS, Anaerolinea, Variovorax and Methylium abundance were increased significantly in PBS (p < 0.05), yet the Bacillus was decreased significantly (p < 0.05).
The dominant archaea phyla included Crenarchaeota, Euryarchaeota and Parvarchaeota, the sum of the relative abundance of three archaea phyla were more than 74% of the total in all sample types (Fig. 3), among which most phyla were sensitive to the tillage disturbance. The diversity of dominant archaea was increased significantly after tillage at the phylum level (p < 0.05). The relative abundance of Euryarchaeota and Parvarchaeota were increased significantly in PBS (p < 0.05).
There were four dominant archaea genera, namely Methanobacterium, Methanosaeta, Candidatus Nitrososphaera and Candidatus Methanoregula (Fig. 4). In PBS, the number of dominant archaea genera increased from 1 to 3, and the affected was considerably (p < 0.05), but there was no affected in DFBS. The relative abundance of Methanobacterium, Methanosaeta and Candidatus Methanoregula were considerably increased in PBS (p < 0.05), yet the relative abundance of Candidatus Nitrososphaera decreased considerably (p < 0.05).
3.3 The relationship between bacterial and archaea community and soil properties
The environmental factors with the most considerable correlation with Variovorax, Bacillus and Sporosarcina were AP, OM and SS respectively, and the relationship was positive (The smaller the angle between the arrows, the larger the correlation)(Fig. 5). However, the changes of Flavisolibacter and Kaistobacter were most closely related to QP, Rhodoplanes was negatively correlated with pH, TN had the most correlation with Anaerolinea and Methylibium, and the relationship was negative. Ranked the influence of environmental factors on bacterial community structure: pH> AP> OM> TN> QP> SS.
Methanobacterium was positively correlated with AP, and Methanosaeta was negatively correlated with TN (Fig. 6). In addition, Candidatus Methanoregula was positively correlated with pH. On the contrary, Nitrososphaera was negatively correlated with pH. Ranked the influence of environmental factors on bacterial community structure: pH> TN> OM> AP> QP> SS.