The Application of Tomato Plant Residue Compost and Plant Growth-Promoting Rhizobacteria Improves Soil Quality and Enhances the Ginger Field Soil Bacterial Community

: Treating and utilizing vegetable residues may reduce waste and improve rhizosphere soil. This study explored the effects of tomato plant residue compost and plant growth-promoting rhizobacteria (PGPR) on the physicochemical properties and microbial community of ginger ﬁeld soil. Four treatment procedures were adopted: no compost or PGPR (CK), compost (TC), compost + Bacillus subtilis (TC-BS), and compost + Bacillus amyloliquefaciens SQR9 (TC-BA). The results showed that compared with the CK, TC signiﬁcantly increased soil organic matter, alkali hydrolyzable nitrogen, available phosphorus, and available potassium by 17.34%, 21.66%, 19.56%, and 37.20%, respectively. Soil urease activity, neutral phosphatase activity, and sucrase activity increased by 55.89%, 35.59%, and 57.21%, respectively. Chloroﬂexi, Gemmatimonadetes , and Bacillus abundances increased by 1.40%, 1.80%, and 0.68%, respectively, while Firmicutes decreased by 0.80%. TC-BS signiﬁcantly improved soil bacterial diversity than CK and TC, and relative abundance of Beneﬁcial Proteobacteria, Acidobacteria, Chloroﬂexi , and Bacillus microorganisms dominated. Principal coordinate analysis revealed signiﬁcant differences in bacterial community structure among different treatments. Redundancy analysis indicated total potassium ( p = 0.002), pH ( p = 0.0012), and available phosphorus ( p = 0.016) as the main community composition driving factors. In conclusion, B. subtilis inoculation in ginger ﬁeld soil supplemented with tomato compost enhanced bacterial diversity, altered bacterial community structure, enriched beneﬁcial microorganisms, and promoted a healthy rhizosphere.


Introduction
With the rapid development of vegetable production in China, explosive growth has occurred in protected vegetable crop cultivation, bringing with it an increase in the amount of vegetable residues that lack economic value, such as roots, stems, and leaves [1].Without a rational and effective treatment method, these vegetable residues are often disposed of carelessly, causing serious resource waste and environmental pollution [2].In ginger production, growers apply large quantities of synthetic fertilizers in pursuit of yield and profit, resulting in environmental issues such as reduced soil organic matter (SOM), increased soil acidification and degradation, the destruction of microbial populations, and soil hardening [3,4].Vegetable residue is usually high in water content, has a low C/N ratio, and can carry harmful pathogens [5].Thus, the formation of pollution-free organic matter through high-temperature composting and the fermentation of vegetable residue for improving the soil environment of ginger roots has been demonstrated to be an effective way to utilize vegetable wastes as resources.
Soil microorganisms form the most biodiverse communities in the biosphere.They provide important ecosystem services, including services related to soil fertility, carbon sequestration, and plant productivity and health.Bacteria and fungi usually comprise the dominant species in terms of soil microbial biomass and diversity, and participate in processes including soil nutrient cycling and decomposition [6,7].Soil enzymes secreted by soil microorganisms also play important roles in nutrient cycling, soil structure, and crop production [8].Studies have shown that the long-term application of organic fertilizers increases the total microbial biomass and fungal abundance of alluvium in northern China but reduces bacterial abundance [9].Crop residue, however, has a significant influence on the structure of bacterial communities, and helps to increase the relative abundances of Bacteroidetes, Betaproteobacteria, Cyanobacteria, and Gemmatimonadetes [10].Meng et al. [11] have demonstrated that in the intensively managed agricultural areas of northern China, the combined use of nitrogen fertilizer and straw return promotes organic carbon sequestration and the retention of nitrogen from fertilizer.In addition, Kong et al. [12] reported that the joint application of straw and pig manure could significantly improve soil fertility and bacterial diversity in arid lands.
In recent years, an increasing number of plant rhizosphere bacteria have been commercialized [13].These bacteria directly or indirectly promote plant growth and yield [14], and compete with pathogenic microbial populations for ecological niches.Changes in microbial diversity and species abundance can also be achieved via the inoculation of beneficial microorganisms (such as Bacillus subtilis and Trichoderma spp.) in soil [15].Bacillus spp.have been found to secrete extracellular metabolites antagonistic to plant pathogens, including antibiotics, cell wall hydrolases, and iron carriers [16,17].For example, in a previous study the foliar spraying of water-soluble fertilizers containing Bacillus amyloliquefaciens SQR9 on pepper and cabbage plants significantly increased the bacteria count and reduced the fungal count [18].The combined application of B. subtilis NCD-2 and broccoli residue has been shown to effectively suppress the occurrence of potato Verticillium wilt, achieving a 37.85% prevention of the disease and a 27.12% increase in potato yield, while reducing the fungal diversity in the rhizosphere soil [19].In a short-term study, Hui et al. [20] revealed that Firmicutes, Actinobacteria, Lysobacter, and Sphingopyxis became enriched after B. amyloliquefaciens DT inoculation, while Ascomycota was consumed, these results validated the disease inhibition role played by B. amyloliquefaciens DT in community structure.
Several plant growth-promoting rhizobacteria (PGPR) strains have been reported in recent years, but few have broad-spectrum efficacy and strong adaptability, possibly since PGPR require a complex living environment and are sensitive to external factors, such as soil fertility, soil physical and chemical properties, and crop species [21].As singlestrain, single-function fertilizers no longer meet the needs of modern-day agriculture, the combined use of PGPR and organic fertilizers could supply multiple functions and improve the environmental adaptability of PGPR [22].
At present, most studies have focused on the return of tomato residues from field crops or greenhouse vegetable [23,24].No reported studies are available on soil environment improvement in ginger fields through the application of high-temperature composted tomato plant residue, and it still remains unclear whether the combined application of tomato residue compost and PGPR has a positive effect on the ginger field soil environment.In order to reduce the adverse effects of continuous cropping and vegetable waste on soil quality, we explored the effects of tomato plant residue compost and PGPR on the physicochemical properties, soil enzyme activity and microbial community of ginger field soil.We found that B. subtilis inoculation in ginger field soil supplemented with tomato compost enhanced bacterial diversity, altered bacterial community structure, enriched beneficial microorganisms, and promoted a healthy rhizosphere, providing a better understanding of the technical feasibility of the application of in resource utilization of vegetable residues and sustainable cultivation of ginger.

Test Field Conditions
The study was conducted in the field at the Nankou Pilot Base of the Chinese Academy of Agricultural Sciences in Beijing, China from April to November 2021.The test site is located in an area with a semi-humid, semi-arid temperate continental monsoon climate, with an annual average temperature of 11.8 • C and precipitation of 550.3 mm.The soil type is loam, the previous crop planted was onion.Before ginger cultivation, the conditions of the topsoil (0-20 cm) were as follows: 23.00 g/kg soil organic matter (SOM), 1.29 g/kg total nitrogen (TN), 1.56 g/kg total phosphorus (TP), 24.80 g/kg total potassium (TK), 107.1 mg/kg alkali hydrolyzable nitrogen (AN), 296 mg/kg available potassium (AK), 234.2 mg/kg available phosphorus (AP), 7.66 mg/kg arsenic (As), 0.168 mg/kg hydrargyrum (Hg), 29.3 mg/kg lead (Pb), 7.61 pH, and 3.6 ms/m electrical conductivity (EC).

Experimental Materials
A cultivar of Burmese ginger was used in this study.Tomato plant residue obtained from the solar greenhouse of the Nankou Pilot Base of the Chinese Academy of Agricultural Sciences was used for composting.After the fall harvest, the plant roots were removed, and the plants were piled in the fermentation tank and covered with plastic film for hightemperature fermentation.During this period, the temperature was maintained at above 55-60 • C for 15 days.The fermentation system was ventilated with air blowers every morning, noon, and evening to purge CO 2 and replenish oxygen, thereby accelerating the fermentation process.Water was sprayed on the fermentation tank regularly to increase the moisture content of the compost.The tank was turned once a week to ensure the thorough fermentation of the compost.The temperature at the center of the compost and the ambient temperature were monitored with an agro-environmental monitor (Beijing Qishuo Foundation Technology Co., Ltd., Beijing, China).Ventilation and compost turning were performed until the temperature became stable, after which the compost was left in the fermentation tank for later use.The nutrient composition of the compost was as follows: 35.2% total carbon (TC), 1.65% TN, 0.763% TP, 3.97% TK, and 2.13 C: N.
Two kinds of commercial PGPR B. subtilis and B. amyloliquefaciens SQR9 were used in this study.The dry PGPR inoculants was produced by Jining Jinshan Biotechnology Co., Ltd., (Jining, China) via three-stage liquid fermentation, (The preparation method was).With a total viable bacteria count of 10 × 10 10 bacteria/g, microorganism contamination ≤1%, a sporulation rate ≥98%, and water content ≤8%.

Experimental Design
Ginger planting furrows of 7 m (length) × 0.35 m (wide) × 0.15-0.2m (depth) were plowed 0.7 m apart.Ginger plants were grown at a 0.2-m interval to achieve a planting density of 6.65 plants/m 2 .Four treatment procedures were used on the plants: no tomato plant residue compost or PGPR (CK), tomato plant residue compost (TC), tomato plant residue compost +B.subtilis (TC-BS), and tomato plant residue compost +B.amyloliquefaciens SQR9 (TC-BA).Five plots of land (Five biological repeats), each with an area of 6.4 m 2 , were randomly assigned under each treatment procedure.Before planting, 15 t/hm 2 of refined organic fertilizer and 750 kg/hm 2 of ternary fertilizer were applied to the land.The soil was thoroughly mixed with fertilizers and prepared for furrowing.Then, 15,000 kg/hm 2 of tomato plant residue compost was applied to the furrows, covered with a thin layer of soil, and 150 kg/hm 2 of B. subtilis and B. amyloliquefaciens SQR9 were applied evenly.

Soil Sample Collection
Soil sampling took place one day before ginger harvest (19 October 2021).Soil samples were taken from six sampling sites arranged in an 'S' shape for each treatment procedure.
Visible contaminants, such as gravel and roots, were removed.The collected soil was placed into a sterile zip bag and mixed thoroughly.The soil content of each bag was divided into four 5-10 g portions, which were placed in zip bags and wrapped with aluminum foil.These samples were immediately transferred back to the lab in an ice box and stored in a freezer at −80 • C for DNA extraction and sequencing.Any leftover soil was kept in a cool and dry place to air-dry, ground, and passed through a 1-mm sieve for soil nutrient content and enzyme activity measurement.

Soil Nutrient Content and Enzyme Activity Determination
Soil pH and EC were measured with a multi-parameter water quality analyzer (Models: HQ440D, Manufacturer: HACH, Shanghai).SOM was determined by dichromate oxidation with external heating.TN was determined by the semi-micro Kjeldahl method.AN was quantified by boric acid absorption and the conductometric titration of the ammonia liberated from the NaOH hydrolysis of the soil sample.TP was determined by the molybdenum blue method following perchloric acid-sulfuric acid digestion.AP was determined by the molybdenum blue method following sodium bicarbonate extraction.TK and AK were both measured by atomic absorption spectrometry [25].
Soil urease activity was determined by the sodium phenoxide-sodium hypochlorite method.Neutral phosphatase activity was determined by the disodium phenyl phosphate method.Sucrase activity was determined by the 3, 5-dinitrosalicylic acid method [26].

Soil DNA Extraction, 16S rRNA Gene Amplicon Sequencing
Total microbial genomic DNA samples were extracted using the DNeasy PowerSoil Kit (QIAGEN, Shanghai Paiseno Biotechnology Co., Ltd., Shanghai, China), following the manufacturer's instructions and stored at −20 • C prior to further analysis.The quantity and quality of extracted DNAs were measured using a NanoDrop ND-1000 spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA) and agarose gel electrophoresis, respectively [27].

Soil Microbial Community Analysis
QIIME2 software was used in Paiseno cloud platform to remove primers, quality filtering, denoising, stitching and chimerism, and obtain high-quality sequences for subsequent analysis.Five bacteria alpha diversity indices (Chao1 richness estimator, Observed_species, Goods _Coverage, Shannon diversity index, and Simpson diversity index) were calculated by QIIME OTU tables [29].Beta diversity analysis was performed based on the Bray-Curtis dissimilarity index (Bray and Curtis, 1957), Visualization was performed through principal coordinate analysis (PCoA) [30].The relationships between soil environmental factor and soil microbial communities were analysed using redundancy analysis (RDA).CANOCO 5.0 software was used to extract the physicochemical characteristics which had the most decisive influence on microbial communities.

Data Analysis
Data compilation and computation were performed with Microsoft Excel 2016.DPS 9.01 was used for data analysis, and Duncan's new multiple range test was used as the significance test for sample differences (α = 0.05).GraphPad Prism 8.4.2 was used to plot the data, and the Pearson correlation test in SPASS 26.0 was used for correlation analysis.

Effect of Applying Tomato Plant Residue Compost and PGPR on the Nutrient Content of Ginger Field Soil
As shown in Table 1, compared to CK, the TC, TC-BS, and TC-BA treatments significantly enhanced the AP content (increases of 19.54%, 31.09%, and 18.91%, respectively) and AK content (increases of 37.20%, 37.38%, and 18.78%, respectively) of ginger field soil.In addition, the TC and TC-BS treatments significantly enhanced the AN (increases of 21.66% and 11.05%, respectively) and TN content.Compared to CK, the TC treatment significantly increased the amount of SOM, while the TC-BS treatment significantly lowered the amount of TK and improved the EC of ginger field soil.

Effect of Applying Tomato Plant Residue Compost and PGPR on the Enzyme Activity of Ginger Field Soil
As shown in Figure 1, different treatment procedures had different effects on soil enzyme activities.Compared to CK, the TC and TC-BS treatments significantly increased soil urease activity, neutral phosphatase activity, and sucrase activities (TC treatment: 55.89%, 35.59%, and 57.27% increases in the activity of these three enzymes, respectively; TC-BS treatment: 29.57%, 27.03%, and 25.37% increases in activity, respectively).TC-BA only significantly increased the activity of sucrase.Compared to TC, the TC-BS and TC-BA treatments inhibited the activity of soil urease, neutral phosphatase, and sucrase.Thus, the effects of different treatments on the activity of the three soil enzymes followed the order of TC > TC-BS > TC-BA > CK.

Effect of Applying Tomato Plant Residue Compost and PGPR on the Alpha Diversity of Ginger Field Soil Bacteria
After chimera removal, a total of 249,467 high-quality target sequences were obtained from the four treatments.Sparse curves indicated the gradual flattening of the operational taxonomic unit count in the samples as the number of sequences increased (Figure 2), demonstrating that the amount of sequencing data was large enough to reflect the data of most species in the samples.For a comprehensive evaluation of the alpha diversity in microbial communities, the Chao1 index and Observed_species was selected to characterize the abundance in these communities, the Good's coverage index was selected to characterize the coverage, while the Shannon index and Simpson index were chosen to characterize the diversity.As shown in Table 2, compared to CK and TC, the TC-BS treatment led to significantly increases in the Chao1 index, observed_species, Shannon index, and Simpson index, while the TC-BA treatment significantly decreased the Chao1 index, ob-served_species and Shannon index.

Effect of Applying Tomato Plant Residue Compost and PGPR on the Alpha Diversity of Ginger Field Soil Bacteria
After chimera removal, a total of 249,467 high-quality target sequences were obtained from the four treatments.Sparse curves indicated the gradual flattening of the operational taxonomic unit count in the samples as the number of sequences increased (Figure 2), demonstrating that the amount of sequencing data was large enough to reflect the data of most species in the samples.For a comprehensive evaluation of the alpha diversity in microbial communities, the Chao1 index and Observed_species was selected to characterize the abundance in these communities, the Good's coverage index was selected to characterize the coverage, while the Shannon index and Simpson index were chosen to characterize the diversity.As shown in Table 2, compared to CK and TC, the TC-BS treatment led to significantly increases in the Chao1 index, Observed_species, Shannon index, and Simpson index, while the TC-BA treatment significantly decreased the Chao1 index, Observed_species and Shannon index.Sequences in each operational taxonomic unit were analyzed at 97% similarity.It was found that 98.68% of the sequences belonged to the phylum level, and 34 bacterial phyla were identified.As shown in Figure 3a, the dominant bacterial phyla in the ginger field soil were Proteobacteria (35.53-40.45%),Actinobacteria (15.23-17.15%),Acidobacteria (14.52-15.91%),Chloroflexi (7.60-9.85%),Firmicutes (5.42-8.49%),and Gemmatimonadetes (3.60-5.26%).Compared to CK, the TC treatment increased the relative abundances of Chloroflexi and Gemmatimonadetes by 1.40% and 1.80%, respectively, and reduced the relative abundance of Firmicutes by 0.80%.The TC-BS treatment led to increases in the relative abundances of Proteobacteria, Acidobacteria, and Chloroflexi 2.80%, 0.40%, and 1.30%, respectively, and a reduction in the relative abundance of Firmicutes of 2.90%.The TC-BA treatment had no significant influence on the relative abundances of bacterial phyla.A total of 504 bacterial genera were identified by sequence analysis, as shown in Figure 3b.The top 10 bacterial genera accounted for about 25.00% of the total genus count.Other than the undefined genera, the dominant bacterial genera in the ginger field soil were Subgroup_6 (7.71-8.42%),Bacillus (2.33-4.11%),MND1 (2.64-3.44%),KD4-96 (1.69-2.15%),Skermanella (1.60-1.92%),and RB41 (1.39-1.75%).Compared to CK, the TC treatment increased the relative abundance of Bacillus by 0.68%; the TC-BS treatment increased the relative abundances of Subgroup_6, MND1, Bacillus, and Skermanella by 0.14%, 0.41%, 1.12%, and 0.30%, respectively; and the TC-BA treatment increased the relative abundances of the above genera by 0.51%, 0.43%, 0.59%, and 0.32%, respectively.

Principal Coordinate Analysis (PCoA) of the Ginger Field Soil Bacterial Community
PCoA based on Bray-Curtis dissimilarity metrics was used to compare the structural differences in bacterial communities across different treatments (Figure 4).The explained variances of samples on PCo1 and PCo2 were 19.3% and 14.3%, respectively.In the analysis results, each sample formed a cluster by itself on PCo1 and PCo2, indicating good reproducibility of the treatments.Samples from different treatments aggregated in four different regions of the plot, an indication of significant changes in the bacterial community structure of ginger field soil after the application of tomato plant residue compost and PGPR.

Redundancy Analysis (RDA) of the Soil Bacterial Community of Ginger Field Soil and Environmental Factors
RDA was applied to reveal the correlations between the community composition of ginger field soil bacteria at the phylum level and environmental factors (Figure 5).The bacterial communities significantly affected by environmental factors.PCo1 and PCo2 was able to explain 50.55% and 20.36% of the variance seen in bacterial communities, respectively.By ranking the constraints in Canoco5, five environmental factors making the greatest contributions to the composition of bacterial communities were selected: TK (30.60%), pH (24.20%), AP (22.80%),AK (14.80%), and AN (12.40%).Proteobacteria and Actinobacteria, the dominant bacterial phyla, were positively correlated with pH, AP, and AK, and negatively correlated with AN and TK.Acidobacteria was positively correlated with TK and AN, and negatively correlated with AP and pH, while showing no correlation with AK.The above results showed that TK (p = 0.002), pH (p = 0.0012), and AP (p = 0.016) were the main driving forces behind the composition of bacterial communities in the ginger field soil.

Effect of Applying Tomato Plant Residue Compost and PGPR on Ginger Field Soil Nutrient Content
Vegetable residue compost contains a large amount of cellulolytic bacteria.In the composting process, these bacteria decompose residue into nutrients that are easily absorbed by plants [31].could inhibit pathogens by producing plant hormones, iron carriers, volatile organic compounds, and antibiotics [32,33], while accelerating the decomposition of organic substances and dissolving the phosphorus and potassium in soil to promote nutrient absorption by plants [34][35][36].The results of this study confirmed the synergic interaction between vegetable residue compost and PGPR.The joint application of tomato plant residue compost and PGPR led to significantly increased levels of SOM, TN, AN, and AK in ginger field soil.However, the best effect was achieved when tomato plant residue compost was applied alone.Consistent results were obtained by Saqib et al. [37] in their effort to restore polluted soil using vegetable waste compost.In addition, compared to the application of tomato plant residue compost alone, or no application of tomato plant residue compost or PGPR, B. subtilis inoculation in soil supplemented with tomato plant residue compost increased the AP content by 9.66%-31.09%.These results were consistent with those obtained by Babalola [38], who reported that B. subtilis could dissolve bound phosphorus, mineralize organic phosphorus, and release soluble inorganic phosphate into soil by decomposing phosphate-rich organic compounds.Other than the potassium directly provided by the decomposition of vegetable residue, the increase in AK in ginger field soil may have originated from the transformation of mineralized potassium into free potassium by acid hydrolysis [39].

Effect of Applying Tomato Plant Residue Compost and PGPR on Ginger Field Soil Enzyme Activity
Soil enzymes exhibit rapid responses to stress [40].Hydrolases are one of the two major enzyme families involved in the bioremediation of polluted soil [41].The main representatives of hydrolases (i.e., urease, sucrase, and phosphatase) play important roles in the decomposition of exogenous organic matter [42].In this study, the application of tomato plant residue compost and PGPR significantly increased the activity of these three enzymes in soil.Similar results have been obtained in a number of studies.For example, Chen et al. [43] showed that increasing crop residue application significantly improved the activity of soil sucrase, acid phosphatase, and urease in Loess Plateau soil.Asghar et al. [44] find that Trichoderma spp.RW309 inoculation in organic compost could significantly improve soil phosphatase activity.This was largely due to the increase in microorganism diversity in the ginger rhizosphere by PGPR, as the inducing compounds secreted by this class of enzymes could improve soil enzyme activity [45].Vegetable residue compost, however, increases the SOM and humus levels and supplies soil enzymes with abundant binding sites and protective sites, thus enhancing their activity [46].Lu et al. [47] found that Bacillus velezensis inoculation in tomato field soil significantly reduced soil urease activity, but had no significant effect on sucrase activity.Similar results were also obtained in the present study.Compared to the control, the inoculation of B. amyloliquefaciens had no significant effect on urease or phosphatase activity.However, compared to the application of tomato plant residue compost alone, this treatment significantly reduced soil urease activity, phosphatase activity, and sucrase activity, indicating a less-than-optimum response of soil enzyme activity to the combined application of tomato plant residue and PGPR.

Effect of Applying Tomato Plant Residue Compost and PGPR on the Ginger Field Soil Bacterial Community
A diverse bacterial community helps to promote the healthy and sustained growth of the soil ecosystem and is an indicator of soil health [48].In the sand dunes of the Netherlands, rhizosphere soil bacteria possess more functional genes related to transport proteins, glycolysis, and hydrogen metabolism compared to non-rhizosphere soil bacteria [49].In this study, tomato plant residue compost and PGPR were applied to ginger field soil and soil bacterial communities in the proximity of ginger roots were sequenced and analyzed using the Illumina MiSeq system (Shanghai Paiseno Biotechnology Co., Ltd.Shanghai, China).The results showed that the Chao1 index and Shannon index of ginger root soil increased after the application of tomato plant residue compost.Significant increases in these two bacterial diversity indices were also observed after B. subtilis inoculation.These results were consistent with those obtained by Du et al. [50], who reported that the addition of organic fertilizers or bio-fertilizer was beneficial to soil fertility, crop growth, and microbial diversity, and that these fertilizers helped to enrich the amounts of Bacillus and Clostridium bacteria in walnut orchard soil, while reducing the number of Fusarium bacteria.Tomato plant residue compost is rich in nutrients such as nitrogen, phosphorus, and potassium [51].B. subtilis contains a large quantity of organic substances, such as amino acids and sugars.The active strains of B. subtilis also secrete bioactive factors such as growth-promoting hormones after they have colonized the soil, stimulating plant root secretion [52] and enhancing the proliferation of beneficial soil bacteria.
The spores of B. subtilis and their multi-layer membranes allow for the long-term survival of bacteria [53].Due to the presence of the 16S rRNA gene, Proteobacteria, Actinobacteria, and Acidobacteria are the most abundant bacterial phyla in the soil of coniferous forest, shrub grassland, and tropical forest ecosystems [54].In the present study, it was found that B. subtilis inoculation in soil following the application of tomato plant residue compost significantly increased the relative abundance of dominant bacterial phyla (Proteobacteria, Acidobacteria, and Chloroflexi) and dominant bacterial genera (Subgroup_6, Bacillus, and MND1).These results are similar to the findings of Sui et al. [55].The inoculation of B. amyloliquefaciens SQR9, however, had no such noticeable effect on the bacterial diversity and species abundance of ginger field soil, possibly as a result of bacterial antagonism and competition.The antibiotics, secondary metabolites, and degradative enzymes released by Actinobacteria may have an active effect on soil microbiota by inhibiting the growth of some soil microorganisms, including plant pathogens [17].Wei et al. [56] noticed the increased abundance of Actinobacteria after returning vegetable residue to the field.Similar results were also observed in this study.In contrast, no significant change in Actinobacteria abundance was observed after PGPR inoculation.
Soil environmental factors either directly or indirectly affect the structure of the soil bacterial community.Xun et al. [57] have suggested that soil properties, rather than the number of microorganisms inoculated, determine the composition and function of these microorganisms, and that soil pH is the dominant factor behind changes in bacterial community structure.The results of RDA in the present study confirmed these observations.After the application of tomato plant residue compost and PGPR, the pH, TK, and AP were identified as the main driving factors of bacterial community composition in ginger field soil.

Conclusions
Compared to CK, the TC and TC-BS treatment significantly increased the level of SOM, AN, AP, AK, and TN, increased soil urease activity, neutral phosphatase activity, and sucrase activity, and enhanced the relative abundances of the phylum Chloroflexi, the phylum Gemmatimonadetes, and the genus Bacillus, while decreasing the relative abundance of Firmicutes.Compared to the TC treatment, the TC-BS and TC-BA treatments had no significant effects on soil nutrients and enzyme activity, but TC-BS significantly improved the diversity of soil bacteria, the beneficial microorganisms, Proteobacteria, Acidobacteria, Chloroflexi, and Bacillus, were dominant in all soil samples.PCoA revealed significant differences in bacterial community structure across different treatments.RDA indicated that TK (p = 0.002), pH (p = 0.0012), and AP (p = 0.016) were the main driving factors of bacterial community composition in ginger field soil.
In conclusion, the high-temperature composting and fermentation of tomato plant residue and its return to the field is a simple and economical way to improve the soil quality of ginger fields.This method effectively reduces the waste of tomato plant residue as a resource, and helps to address continuous cropping obstacle in ginger field soil such as the decrease of soil fertility.The synergic application of tomato plant residue compost and PGPR enhances the relative abundance of beneficial microorganisms in the soil, reduces the occurrence of soil-borne diseases, and helps to maintain a healthy ginger rhizosphere.

Figure 3 .
Figure 3. Relative abundance of soil bacteria in ginger field at the phylum and genus level.[(a) for phylum level; (b) for genus level].CK: no tomato plant residue compost or PGPR; TC: tomato plant residue compost; TC-BS: tomato plant residue compost +B.subtilis; TC-BA: tomato plant residue compost + B. amyloliquefaciens SQR9.

Figure 4 .
Figure 4. Principal coordinate analysis (PCoA) of the ginger field soil bacterial community.

Figure 5 .
Figure 5. Redundancy analysis (RDA) of the soil bacterial community of ginger field soil and environmental factors.

Table 1 .
Effect of applying tomato plant residue compost and PGPR on the nutrient content of ginger field soil.
Note: CK: no tomato plant residue compost or PGPR; TC: tomato plant residue compost; TC-BS: tomato plant residue compost +B.subtilis; TC-BA: tomato plant residue compost +B.amyloliquefaciens SQR9.(Average ± standard deviation, n = 3.The different letters in the same column indicate significant differences at level of p < 0.05).

Table 2 .
Effect of applying tomato plant residue compost and PGPR on the alpha diversity of ginger field soil bacteria.

Effective Sequence Number Chao1 Index Shannon Index Simpson Index Observed_Species Goods_Coverage
Note: The different letters in the same column indicate significant differences at level of p < 0.05.

Table 2 .
Effect of applying tomato plant residue compost and PGPR on the alpha diversity of ginger field soil bacteria.The different letters in the same column indicate significant differences at level of p < 0.05.3.4.Effect of Applying Tomato Plant ResidueCompost and PGPR on the Relative Abundance of Soil Bacteria in Ginger Field at the Phylum and Genus Level