Introduction
Potato has become the fourth major food crop after
rice, maize and wheat in China. China is the largest potato producer,
accounting for 1/4 of total planting area and 1/5 of total production in the world (Jin et al. 2018). However, due to limited cultivated lands,
blind pursuit of economic interests and lack of scientific planting concept, a
large amount of potato is generally planted by a continuous cropping pattern
that results in continuous-cropping
obstacle with poor growth as well as yield and quality reduction of potato (Qin et al. 2017a). Changes
in soil physical properties, enrichment of soil-borne
pathogens, and the autotoxicity of root exudates have been recognized as
the key influencing factors to lead to the continuous-cropping obstacle (Bennett et al. 2012; Huang et al. 2013). At present, rhizosphere microenvironment of plant is
commonly considered as the most important influencing factor (Dias et al. 2015). Several
studies have
compared the soil microenvironment between rotational cropping and continuous
cropping patterns of some crops (Govaerts et
al. 2007; Xuan et al. 2012).
Therefore, understanding how rhizosphere microenvironment of potato is
influenced by continuous cropping pattern can contribute to soil health and
potato production.
The composition of soil microbial community and relative abundance of soil microorganism play essential roles in enhancing soil quality, improving soil ecosystem functions, and maintaining plant health and growth (Helgason et
al. 2009; Kong et al. 2011). Soil microbial diversity, community
composition and structure are influenced by multiple factors, including soil types,
planting patterns and management practices (Garbeva et
al. 2004; Berg and Smalla 2009;
Jangid et al. 2011). Therefore, it is critically necessary to employ appropriate planting
pattern to enhance soil ecosystem
functions (Acosta-Martínez et al. 2010; Bender et al. 2016). Different planting patterns of crops change soil properties, leading to the changes of soil microbial communities (Helgason et
al. 2009; Lienhard et al. 2013). Li et al. (2018)
have showed that the bacterial community is altered in the sweet potato soils
with continuous cropping
pattern for 1, 2, 3, and 4 years by
pyrosequencing approach. Liu et al. (2014) have found that soil bacterial communities exhibit differential responses to the potato with
continuous cropping
pattern using pyrosequencing approach. In
addition, previous studies have mainly revealed that different crop planting
patterns (mainly including continuous cropping and rotational cropping
patterns) and different planting histories
affect soil community composition, diversity and structure (Govaerts
et al. 2007; Kong et al. 2011; Hurisso et al. 2013). However, the questions, such as how continuous cropping of potato affects soil microbial
communities; which species in soil are changed; and what are the key factors of soil properties influencing corresponding alteration of microbial communities, remain largely unexplored.
In the present study, we aimed to (i) investigate the differences of soil microbial diversity, community composition and
structure among continuous cropping, rotational
cropping and new planting potato soils by utilizing high-throughput 16S rRNA gene Illumina sequencing; (ii) compare the differences of the soil physicochemical properties among
continuous cropping, rotational cropping and new
planting potato soils; (iii) and establish the correlation between soil properties and
microbial community diversity and structure. Moreover, we attempt to more
systematically reveal what changes would continuous cropping pattern bring to
soil properties and microbial community. Our results provided data support and theoretical basis for promoting
sustainable development of potato industry through appropriate planting
pattern and improving soil microecological
environment of continuous cropping potato
soil.
Materials and Methods
Experimental Design and Soil Sampling
The experimental field was located in Experimental Base of Hunan Agricultural University, Hunan Province,
China. Field experiments of continuous cropping and rotational cropping patterns began in 2012. The field was divided into nine plots, each of 18×13 m2
in size. Three plots were selected for continuous cropping and rotational cropping patterns each year, respectively. The other three plots of newly cultivated soil
were designed for an new planting potato experiment in 2017. After 6 years of continuous cropping and rotational cropping
patterns and 1 year of new planting
potato experiment, nine plots were used
up (three replicates). Soil samples were simultaneously collected from plots in
the harvesting period of potato in May 2017. From each plot, 30 potato plants were randomly selected, one
core for 0–20 cm layer soil was collected from each plant, and one soil sample
consisted of 15 cores. All 18 soil samples
(three plots × 2) were placed into sterile plastic bags. Each
composite sample was passed through a 2 mm
sieve and divided into two parts. One part was air-dried for soil property
analysis, and the other part was stored at −80°C for DNA extraction.
Physicochemical Characteristics of Soils
The following physicochemical
properties of collected soil samples were determined: pH, total organic carbon,
nitrate nitrogen, ammonia nitrogen, total nitrogen, available phosphorus,
available potassium, and total phosphorus. Analysis of soil properties was
performed by the Department of Nanjing Institute of Geography, CAS, using the previously published methods (Tan et al. 2019).
DNA Extraction and Amplicon Sequencing
DNA
extraction of soil
samples and amplicon sequencing were performed following Tan et al. (2019). The V4 region of the 16S rRNA gene was amplified
using the purified DNA as a template with
following primers: 515F (5'-GTGCCAGCMGCCGCGGTAA-3)/806R (5'-GGACTACHVGGGTWTCTAAT-3'). PCR
amplification was conducted on a SelectCycler II (Select BioProduct).
The purified amplicons were quantified by using a Qubit fluorometer, and the library was constructed with VAHTSTM Nano DNA
Library Prep Kit for Illumina® (Vazyme Biotech Co., Ltd., Nanjing, China). The samples were
sequenced by Miseq-sequencing machine (Illumina).
Sequence Preprocessing
Sequence
preprocessing was consistent with the
methods of Gu et al. (2019). The barcodes were assigned to the raw reads. After removal of
barcodes and primers. The pair-ended sequences were quality-filtered by using
Flash program (Feng et al. 2017).
UPARSE algorithm was applied to remove chimeras and generate OTU table (Magoc
and Salzberg 2011). All the sequence preprocessings were performed in an in-house pipeline
(http://mem.rcees.ac.cn:8080) with various bioinformatics tools (such as FLASH, Btrim and UPARSE program).
Ecological and Statistical Analysis
Shannon and Richness diversity
indices were calculated according to the methods of Gu et al. (2019). Unweighted
%20721-729_files/image002.png)
Fig. 1: Diversity indices
based on 16S rRNA gene of bacterial community in continuous cropping, rotational cropping and new planting potato soils. (a) Shannon index; (b) Richness index. Different letters represent the significant difference among soils at P = 0.05. CC:
continuous cropping, RC: rotational cropping, NP: new planting, similarly
hereinafter
%20721-729_files/image004.jpg)
Fig. 2: Principal component analysis
(PCA) of bacterial community structures in continuous cropping, rotational cropping and new planting potato soils
Table 1: Physicochemical
properties of continuous cropping, rotational cropping and new planting potato soils. CC:
continuous cropping, RC: rotational cropping, NP: new planting. TOC, total organic
carbon; TN, total nitrogen; NO3-N, nitrate nitrogen; NH4-N,
ammonia nitrogen; AP, available phosphorus; AK, available potassium; WC, water content; TP, total phosphorus.
|
Sample |
TP (mg kg-1) |
TN (mg kg-1) |
NH4-N (mg g-1) |
NO3-N (mg kg-1) |
AP (mg kg-1) |
TOC (%) |
pH |
AK (mg kg-1) |
|
CC |
759.56 ± 41.78b |
873.96 ± 32.76b |
15.30 ± 4.53a |
121.51 ± 5.24a |
91.75 ± 6.45b |
0.91 ± 0.06c |
4.86 ± 0.09c |
80.66 ± 2.80b |
|
RC |
891.12 ± 89.45a |
1107.68 ± 133.69a |
19.46 ± 14.97a |
124.62 ± 15.02a |
127.4 ± 16.71a |
1.41 ± 0.20a |
6.22 ± 0.25a |
106.57 ± 5.99a |
|
NP |
336 ± 26.48c |
930.29 ± 57.24b |
14.51 ± 3.18a |
116.17 ± 6.78a |
15.45 ± 8.46c |
1.13 ± 0.14b |
5.54 ± 0.08b |
46.4c ± 3.28c |
Different letters represent the significant difference among soils at P = 0.05
principal coordinate analysis
(PCoA) was performed to evaluate the alteration of microbial community
structure (Gu et al. 2019). The
Pearson correlation approaches were used to construct the relationship between
microbial diversity and soil properties (PCC: Pearson correlation coefficient). Mantel
test and canonical correspondence analysis (CCA) were used to analyze the
contributions of soil properties to bacterial community.
Results
Physicochemical Properties of Continuous Cropping,
Rotational Cropping and New Planting Potato Soils
ANOVA showed that there were significant differences in soil properties (P
= 0.05) among continuous cropping, rotational cropping and new
planting soils, except for NO3-N and NH4-N contents (Table
1). TP, AP, TOC and AK contents as well as pH value in rotational cropping soil were significantly higher than those in continuous cropping and new planting soils. However, the TOC content and
pH value in continuous cropping soil were lower than those in other samples. The lowest TP, AP, TOC and
AK contents were observed in new planting soil. In addition, TP, TN, AP, TOC
and AK contents as well as pH value were significantly lower in continuous cropping soils when only compared with rotational cropping soil.
Bacterial Diversity and Community Structure of Continuous
Cropping, Rotational Cropping and New Planting Potato Soils
After data processing, 716,221 valid reads were obtained from the 18 soil samples. All the
reads were classified into 31,167 OTUs. The Shannon (Fig. 1a) and Richness (Fig. 1b) indices were used to analyze the alpha-diversity of
bacterial communities in continuous cropping, rotational cropping and new planting soils. We found that continuous cropping soil had the lowest microbial diversity, while rotational cropping and new planting soils had the similar levels of
bacterial diversity. Moreover, we further analyzed
beta-diversity of microbial communities in continuous cropping, rotational cropping and new planting soils. The
PCoA showed a perfect separation among bacterial community structures of continuous cropping, rotational cropping and new planting potato soils (Fig. 2). In addition, the dissimilarity tests of bacterial communities were
carried out using ANOSIM and MRPP based on Bray-Curtis distance, and the results showed that the
significant differences also existed in soil bacterial communities among continuous cropping, rotational cropping and new planting soils (Table 2). In summary, we concluded that there were significant
differences for alpha- and beta-diversity among continuous cropping, rotational cropping and new planting soil bacterial communities.
Microbial Community Compositions of Continuous Cropping,
Rotational Cropping and New Planting Potato Soils
All
bacterial operational taxonomic units (OTUs) of 18 soil samples were identified
into 949 genera and 36 phyla at a similarity level of 97%. The
cladogram indicated the
phylogenetic distribution of bacterial lineages among continuous cropping, rotational cropping and new planting Table 2: Dissimilarity test of bacterial community structure in continuous cropping, rotational cropping and new planting potato soils based on Bray-Curtis distance. CC:
continuous cropping, RC: rotational cropping, NP: new planting
|
Comparison |
MRPP |
ANOSIM |
||
|
Delta |
P |
R |
P |
|
|
NP VS. CC |
0.6017 |
0.002** |
1 |
0.004** |
|
NP VS. RC |
0.6074 |
0.001*** |
1 |
0.002** |
|
CC VS. RC |
0.6094 |
0.004** |
0.987 |
0.003** |
**Difference
is significant at P = 0.01 level.
***Difference is significant at P =
0.001 level.
%20721-729_files/image006.png)
Fig. 3: Comparison of
microbial community compositions in continuous cropping, rotational cropping and new planting potato soils. (a) Cladogram indicating the
phylogenetic distribution of the most differentially abundant taxa in continuous cropping, rotational cropping and new planting potato soils. Each
circle’s diameter is relative to the abundance of taxa in different
potato soil communities, different colors represent the
differences of the most differentially abundant taxa in continuous cropping, rotational cropping and new planting potato soil
communities (red indicates CC, green indicates RC, blue indicates NP), circles
represent phylogenetic levels from domain to genus; (b) Comparison of the soil community composition at the phylum level (relative
abundance higher than 1%) ; (c) Comparison of the
soil community composition at the genus level (relative
abundance higher than 1%)
soils
under different agricultural management practices (Fig. 3a), suggesting that there were
significant differences of the microbial community composition among continuous cropping, rotational cropping and new planting
soils from phylum to genus levels (P <
0.05). The 12 dominant phyla (relative
abundance >1%) across 18 samples
were Proteobacteria, Acidobacteria, Bacteroidetes, Firmicutes,
Actinobacteria, Planctomycetes, Verrucomicrobia,
Chloroflexi, Gemmatimonadetes, candidate division WPS-1, Thaumarchaeota
and unclassified bacteria. The relative
abundances (Table 3) of phyla Firmicutes (4.043%), Chloroflexi
(3.19%) and candidate division WPS-1 (2.92%)
in rotational cropping
soil were significantly higher than those of continuous cropping and new planting soils (P <
0.05), while
the lowest abundances of Acidobacteria (10.447%), Planctomycetes
(0.926%), Verrucomicrobia (1.948%) and candidate division WPS-1 (0.424%) were observed in continuous cropping soil (P <
0.05; Fig. 3b).
Table 3: Relative
abundances of dominant phyla and genera in soil bacterial communities of continuous cropping, rotational cropping and new planting potato soils. CC:
continuous cropping, RC: rotational cropping, NP: new planting
|
Dominant phyla (>1%) |
Relative abundance in potato soil communities |
||
|
CC |
RC |
NP |
|
|
Proteobacteria |
48.88a |
37.199a |
43.656a |
|
Acidobacteria |
10.447b |
12.8ab |
15.344a |
|
Bacteroidetes |
17.337a |
15.403a |
9.912b |
|
Actinobacteria |
7.435a |
5.96a |
6.242a |
|
Unclassified |
2.474b |
5.319a |
3.921ab |
|
Firmicutes |
1.583b |
4.043a |
0.861b |
|
Planctomycetes |
0.926b |
1.374a |
1.227a |
|
Verrucomicrobia |
1.948b |
5.016a |
4.267a |
|
Chloroflexi |
1.39b |
3.144a |
0.835b |
|
Gemmatimonadetes |
1.889a |
1.946a |
1.382a |
|
Candidate division WPS-1 |
0.424b |
1.544a |
0.957b |
|
Thaumarchaeota |
3.928b |
3.629b |
9.324a |
|
Dominant genera (>1%) |
|
|
|
|
Sphingomonas |
5.006b |
3.929b |
10.884a |
|
Nitrososphaera |
3.926b |
3.612b |
9.32a |
|
Gp6 |
0.472c |
1.431b |
3.916a |
|
Gp4 |
0.049b |
0.147b |
2.738a |
|
Terrimonas |
0.081b |
0.715b |
2.658a |
|
Spartobacteria genera incertae sedis |
0.389c |
1.045b |
2.145a |
|
Arthrobacter |
0.971b |
1.002b |
1.953a |
|
Enterobacter |
0.291b |
1.068ab |
1.593a |
|
Subdivision3 genera incertae sedis |
0.936c |
2.139a |
1.53b |
|
Flavisolibacter |
0.184b |
0.686ab |
1.414a |
|
Gemmatimonas |
1.889a |
1.946a |
1.382a |
|
Gp3 |
1.119a |
1.428a |
1.313a |
|
Luteimonas |
0.199b |
0.779b |
1.294a |
|
Rhodanobacter |
9.604a |
0.835b |
1.226b |
|
Gp1 |
1.728ab |
2.486a |
1.198b |
|
WPS-1 genera
incertae sedis |
0.424b |
1.544a |
0.957ab |
|
Gaiella |
2.443a |
1.781ab |
0.883b |
|
Flavobacterium |
6.817a |
3.000b |
0.728c |
|
Gp2 |
1.904b |
3.32a |
0.565c |
|
Rhizomicrobium |
1.747a |
1.241a |
0.507b |
|
Burkholderia |
1.986a |
0.54b |
0.464b |
|
Pseudomonas |
0.444b |
2.634a |
0.381b |
|
Terriglobus |
1.126a |
0.094b |
0.123b |
|
Others |
55.797 |
61.488 |
50.485 |
Different letters represent the significant difference among soils at P = 0.05
The
23 dominant genera (relative abundance >1%) across all samples were
Spartobacteria genera
incertae sedis, Arthrobacter, Enterobacter,
Subdivision 3 genera incertae sedis, Flavisolibacter, Gemmatimonas,
Gp3, Luteimonas, Rhodanobacter, Gp1, WPS-1 genera incertae sedis,
Gaiella, Flavobacterium, Gp2, Rhizomicrobium, Burkholderia,
Pseudomonas and Terriglobus
(Fig. 3c). The relative abundances (Table 3) of genera Subdivision3 genera
incertae sedis (2.139%), Gp1 (2.486%), WPS-1 genera incertae
sedis (1.544%), Gp2 (3.32%) and Pseudomonas (6.817%) in rotational cropping soil were significantly higher
than those of continuous cropping and
new plantingsoils (P=0.05)
and the
highest relative abundances of genera Rhodanobacter
(9.604%), Gaiella (2.443%),
Flavobacterium (2.443%), Burkholderia (1.986%) and Terriglobus (1.126%)
and the
lowest relative abundances of genera Flavisolibacter (0.184%), Gp6 (0.472%),
Spartobacteria genera
incertae sedis (0.389%), Enterobacter
(0.291%), Subdivision3 genera incertae sedis (0.936%) and WPS-1
genera incertae sedis (0.424%) were observed in continuous cropping soil. Overall, the microbial
composition and relative abundance of bacterial phyla and
genera were significantly different among continuous cropping, rotational cropping and new planting soils.
The Relationship Between Soil Properties and Bacterial
Community
Table 4 shows the relationship analysis between soil properties and five
alpha-diversity indices of bacterial communities by Pearson correlation
approach. The results showed that TP content was significantly positively correlated
with Shannon (PCC = 0.519, P = 0.027)
and Pelou_evenness (PCC = 0.593, P =
0.010). AP content was significantly positively correlated
with Simpson (PCC = 0.592, P = 0.010)
and Chao1 (PCC = 0.471, P = 0.048).
TOC content was significantly positively correlated
with Shannon (PCC = 0.687, P = 0.002) and
Richness (PCC = 0.701, P = 0.001) as well as Pelou_evenness (PCC =
-0.630, P = 0.005). Moreover, pH was significantly positively correlated
with Shannon (PCC = 0.628, P = 0.005) and Richness (PCC = 0.633, P =
0.005) as well as Pelou_evenness (PCC = 0.583, P = 0.011). Table 4: Relationship
between soil properties and alpha-diversity of continuous cropping, rotational cropping and new planting potato soils
|
Soil properties |
Shannon |
Simpson |
Richness |
Pelou_evenness |
Chao1 |
|||||
|
PCC |
P |
PCC |
P |
PCC |
P |
PCC |
P |
PCC |
P |
|
|
TP |
0.519 |
0.027* |
0.681 |
0.002** |
0.269 |
0.280 |
0.593 |
0.010** |
0.002 |
0.994 |
|
TN |
-0.010 |
0.970 |
-0.302 |
0.223 |
0.156 |
0.536 |
-0.074 |
0.771 |
0.295 |
0.234 |
|
NH4-N |
0.195 |
0.437 |
0.099 |
0.697 |
0.197 |
0.432 |
0.188 |
0.455 |
0.163 |
0.519 |
|
NO3-N |
0.225 |
0.368 |
0.383 |
0.117 |
0.001 |
0.998 |
0.309 |
0.213 |
-0.126 |
0.619 |
|
AP |
0.135 |
0.593 |
0.592 |
0.010** |
0.186 |
0.459 |
0.263 |
0.292 |
0.471 |
0.048* |
|
TOC |
0.687 |
0.002** |
0.220 |
0.380 |
0.701 |
0.001*** |
0.630 |
0.005** |
0.436 |
0.071 |
|
pH |
0.628 |
0.005** |
0.343 |
0.163 |
0.633 |
0.005** |
0.583 |
0.011* |
0.462 |
0.054 |
|
AK |
0.372 |
0.128 |
0.739 |
0.001*** |
0.059 |
0.816 |
0.479 |
0.044* |
-0.260 |
0.297 |
PCC= Pearson
correlation coefficient. *Difference is significant at P = 0.05 level. **Difference is
significant at P = 0.01 level.
***Difference is significant at P = 0.001
level
AK was significantly positively correlated
with Simpson (PCC = 0.739, P = 0.001) and Pelou_evenness (PCC = 0.479, P =
0.044). Therefore, the TP, AP, TOC and AK contents
as well as pH value were were
important physicochemical factors to affect alpha-diversity of bacterial
communities in continuous cropping, rotational cropping and new planting soil. To investigate the contribution of above-mentioned soil factors to
microbial community of continuous cropping, rotational cropping and new planting potato soils, a CCA was conducted to study the relationship between soil
properties and microbial community structures (Fig. 4). The CCA plots also
clearly showed that pH, TP, AP, AK and TOC
contents were the five long vectors, and pH was the longest vector. To identify
the most important factors influencing the soil bacterial community, the Mantel test based on both Bray-Curtis and Jaccard
distances was adopted. The results further confirmed that pH, TP, AP, AK and
TOC contents were Table 5: Correlation
analysis between soil properties and bacterial community structures based on
Mantel test
|
Soil properties |
r.BC |
p.BC |
r.JC |
p.JC |
|
TP |
0.1962 |
0.016* |
0.1930 |
0.029* |
|
TN |
-0.0099 |
0.479 |
0.0566 |
0.192 |
|
NH4_N |
0.0012 |
0.395 |
-0.0313 |
0.698 |
|
NO3_N |
-0.0085 |
0.478 |
-0.0981 |
0.919 |
|
AP |
0.6436 |
0.001*** |
0.7175 |
0.001*** |
|
TOC |
0.2978 |
0.005** |
0.2636 |
0.007** |
|
pH |
0.5054 |
0.001*** |
0.4534 |
0.001*** |
|
AK |
0.635 |
0.001*** |
0.7138 |
0.001*** |
*Difference is significant at P = 0.05 level. **Difference is significant at P = 0.01 level. ***Difference is significant at P = 0.001 level
%20721-729_files/image008.jpg)
Fig. 4: CCA plots of bacterial community
structures correlated with soil physicochemical properties
significantly positively correlated
with microbial community (Table 5). Overall, it was concluded that pH, TP, AP, AK and TOC contents were the
important soil factors to powerfully drive the bacterial community assembly in continuous cropping, rotational cropping and new planting potato soils.
Discussion
Continuous-cropping problem, also called the
continuous-cropping obstacle, is common phenomenon in the cultivation of potato
in China. It has been reported that long-term continuous cropping pattern
brings negative effects to soil micro-ecosystem, leading to the deterioration
of soil physicochemical properties, occurrence of various soil-borne diseases
and decline of the crop yield (Huang et
al. 2013; Zhang et al. 2018). Recently, the comparative studies on soil properties of crop under continuous cropping and rotational cropping pattern have been reported (Liu et
al. 2017; Li et al. 2018). TOC content
and pH value were significantly lower in continuous cropping
soils than those in rotational cropping and
new planting soils which were consistent
with the previous studies that continuous cropping of sweet potato causes soil
acidification and a significant decline in soil organic carbon (Li et
al. 2018). In addition, when only compared
with rotational cropping
soil, TP, TN, AP, TOC and AK contents as well as pH value were significantly
lower in continuous cropping
soils, which was consistent with the previous studies that long-term continuous cropping pattern leads to the decreased
soil available nutrients in potato (Liu et al. 2015; Zhou et al. 2018). Moreover, several studies have
revealed that the accumulation of allelochemicals in plant exudates may
contribute to continuous-cropping obstacle (Song et al. 2018),
while it is a far more controversial idea (Wu et
al. 2016). Thus, more studies tend to focus
on the response of soil microbial communities and their correlations with the
changes of soil properties to continuous-cropping obstacle (Xiong et
al. 2015).
Soil microbial diversity and
community structure are closely associated with soil quality,
ecosystem functions, and sustainable development (Govaerts et al. 2007; Kong et al.
2011), thereby impairing the plant growth, development,
health and productivity. Our results showed that continuous cropping soil had the lowest level of
alpha-diversity compared with rotational cropping and
new planting potato soils according to
Shannon and Chao1 indices. This finding was consistent with some previous reports that long-term continuous cropping pattern decreases microbial
diversity and alters community structure compared with the crop rotation system
of tobacco (Chen et
al. 2018),
soybean (Liu et al. 2017), cotton (Zhang et al. 2013), rice (Xuan et
al. 2012) and sweet potato (Li et al. 2018).
The soil of higher microbial diversity was more likely to maintain the function
of soil ecosystems, enhance the resistance of abiotic and biotic environmental stresses and
give defense against soil-borne diseases (Garbeva et al.
2004; Li et al. 2010). However, rotational cropping and new planting soils had similar level of
alpha-diversity, indicating that crop rotation was an effective planting
pattern for maintaining the function of soil ecosystems. It has been reported
that long-term continuous cropping
pattern leads to soil-borne diseases in potato (Qin et al. 2017b),
indicating that lower level of soil microbial diversity may be associated with
the occurrence of soil-borne diseases (Shi et al. 2019). In
addition, beta-diversity analysis based on PCoA and dissimilarity tests showed
that the microbial community
structure of continuous cropping, rotational cropping and new planting
potato soils was significantly different. Microbial community structure is also
regarded as an indicator of soil health (Valadares-Pereira
et al. 2017).
Overalls, continuous cropping
pattern of potato decreased soil microbial diversity and altered community
structure that affected directly or indirectly the soil quality and ecosystems,
consequently resulting in negatively influenced potato production.
The cladogram analysis showed that
the microbial community components of continuous cropping, rotational cropping and new planting
soils had significant differences from phylum to genus levels (P <
0.05). This finding was consistent with that the microbial community
composition is significantly different among continuous cropping, rotational cropping and new planting soils of other plants according to
Sun et al. (2014) and Liu et al. (2017). In
addition, the relative abundance of some bacterial phyla and genera was
significantly altered among continuous cropping, rotational cropping and new planting
potato soils. The relative abundances of phyla Firmicutes
(4.043%), Chloroflexi
(3.19%) and candidate division WPS-1 (2.92%) in rotational cropping soil were significantly higher
than those of continuous cropping and
new planting soils, while the lowest abundances of Acidobacteria (10.447%), Planctomycetes (0.926%), Verrucomicrobia (1.948%)
and candidate division WPS-1 (0.424%) were observed in continuous cropping soil (P = 0.05).
Previous reports have indicated that phylum Firmicutes may be involved in the defense
against vanilla Fusarium wilt disease in
soils of sugar beet (Li et al. 2018).
The phyla Verrucomicrobia and Acidobacteriacan
play important roles in soil biogeochemical cycling processes (Shen
et al. 2017) and
nutrient cycling (Yang et al. 2019a),
respectively. The phylum Planctomycetes is involved in the absorption of
plant nutrients, especially in the utilization of carbon and nitrogen sources (Bhattacharyya
et al. 2017).
The phylum candidate division WPS-1 shows a significantly positive relationship
with soil nutrient cycling (Ji et al. 2018). Phylum Chloroflexi can also play an important role in the
biogeochemical chlorine cycle (Krzmarzick
et al. 2012), which may explain why the pH value was increased in rotational cropping soil. In addition, the highest relative abundances of
genera Rhodanobacter (9.604%),
Gaiella (2.443%), Flavobacterium (2.443%), Burkholderia (1.986%)
and Terriglobus (1.126%) were observed in continuous cropping soil. Some of these genera are regarded as
non-beneficial bacteria in soils according to previous studies. For example, Rhodanobacter
and Flavobacterium have the denitrification
ability that is defined as the reduction
of nitrate or nitrite to gaseous nitrogen (Prakash et al. 2012; Hatayama et
al. 2016), thus leading to the decreased content of
nitrogen nutrition in soil (Green et al. 2012; Liu
et al. 2019), which is unfavorable to
agricultural production. Gaiella is negatively correlated with
microbial activity and biomass in soils that plays the important role in
nutrient cycling and ecosystem sustainability (Yao et
al. 2003; Yang et al. 2019b). Some species of Burkholderia
are recognized as pathogens of plants and animals (Bergmark
and Poulsen 2012). Overall, rotational cropping pattern significantly
increased the relative abundances of
beneficial bacteria that could significantly promote the absorption of
nutrients, improve the utilization efficiency
of organic matter and enhance the resistance against diseases. However, continuous cropping pattern increased the relative abundances of non-beneficial bacteria that were unfavorable for soil quality
development and agricultural production.
Furthermore, evaluation of the relationship between the soil
physicochemical properties and bacterial community in continuous cropping, rotational cropping and new planting soils provides direct insights into the mechanisms of
continuous-cropping obstacle (Li et
al. 2018), which can contribute to improvement of soil
productivity and health for continuous cropping potato soil. In this study, pH value as well as TP, AP, AK and TOC contents
were the important soil factors to powerfully drive the bacterial community
assembly in continuous cropping, rotational cropping and new planting potato soils, and the CCA plots also showed that pH was the longest
vector. Previous studies have indicated that the microbial community and
diversity of the bacterial communities in continuous cropping and rotational cropping soybean soils are affected by pH, TN, TP, AK, AN and AP contents (Liu et
al. 2017), and soil pH has also been proved to be the most important influential factor (Fierer and Jackson 2006). In addition, the alterations
of bacterial community in the sweet potato soils with continuous cropping pattern are mainly driven by soil pH and soil
organic matter (Li et al. 2018). Our results showed that TOC
content and pH value were significantly lower in continuous cropping soils than those in rotational cropping and new planting
soils. Overall
analyses of relationship between the soil physicochemical properties and bacterial
community revealed that the pH value and TOC content are the most important
physicochemical factors to alter the microbial community in continuous cropping potato soils.
Conclusion
This
study investigated the divergence of bacterial communities among continuous
cropping, rotational cropping and new planting potato soils. In general, the
continuous cropping pattern of potato decreased the microbial diversity,
increased the abundances of non-beneficial bacteria, and altered the community
structure compared with rotational cropping and new planting potato soils. In
addition, the pH value and TOC content were
determined as the most important physicochemical factors to alter the microbial community in continuous cropping potato soils. This study provided valuable
insights into the occurrence mechanisms of potato continuous-cropping obstacle,
which might contribute to improving soil microbial diversity and enhancing soil
productivity of continuous cropping potato soil.
Acknowledgement
Financial supports from the National Key Research and Development Program of China
(2018YFD0200800 and 2018YFD1000400), the project of Hunan Provincial Key
Laboratory of Crop Germplasm Innovation and Utilization, Hunan Agricultural
University (2018KFXM02), and the National Natural Science Foundation of China
(31772352).
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