Litsea cubeba cover modified the diversity and structure of bacteria community

Rarefaction analysis was performed on each soil sample, and the rarefaction curves tended to the plateau phase, indicating that the sequencing was almost saturated, and the bacterial community diversity in the soil sample can be truly reflected. The sampling was reasonable (SubFig. 1).

The linear mixed model analysis indicated that OTUs number, Chao1, and Shannon index of soil microbes in tea plantation were significantly affected by the covering of Litsea cubeba (Fig. 1). The number of OTUs in the soil of tea plantation was 1470 ± 25.53 in control plot (Fig. 1a). Litsea cubeba cover significantly decreased the OTUs number in soil of tea plantation. The OTUs number was decreased from 1470 ± 25.53 to 943 ± 13.75 and 727 ± 15.58 in the F-Ab and F-Pr treatment, respectively (Fig. 1a). In addition, the change in bacterial diversity was analyzed with the Shannon index, and the total species richness was estimated by the Chao1 index in the tea plantation soil. Shannon and Chao1 indices showed that covering Litsea cubeba reduced the bacterial diversity and the total species richness compared to control soil (Fig. 1b and c).

Fig. 1
figure 1

Estimated number of observed a OTUs counts, b Shannon index, c Chao1 index of soil microbiome of tea plantation across all the covering Litsea cubeba plots. Values are means ± SE (n = 4). The star (*) indicates significant difference between treatments at 0.05 level; CK, control; F-Ab, the vegetative branches cover treatment; F-Pr, the fruit-bearing branches cover treatment

Nonmetric multidimensional scaling was used to detect variation in the community pattern. The two-dimensional figure showed that the bacterial communities were distributed separately, exhibiting differences in the bacterial community pattern in control, F-Ab, and F-Pr treatment (Fig. 2). These results suggest that the structure and diversity of bacterial community changed with the covering treatment.

Fig. 2
figure 2

Nonmetric multidimensional scaling (NMDS) of bacterial communities based on OTUs for all soil samples from covering Litsea cubeba plots of tea plantation. The NMDS1 stress was 0.00024. CK, control; F-Ab, the vegetative branches cover treatment; F-Pr, the fruit-bearing branches cover treatment

Litsea cubeba cover showed opposite effects on different bacterial communities

Analysis of the taxonomic groups detected in the soil samples showed that the most dominant phyla across all samples were Proteobacteria, Acidobacteria, Chloroflexi, and Planctomycetes, accounting for about 70% of the phyla (Fig. 3). In addition, Bacteroides, Verrucomicrobia, Gemmatimonadetes, Firmicutes, Saccharibacteria, Armatimonadetes, GAL15, Chlorobi, TM6-Dependentiae, and Deinococcus-thermus were detected in all the samples with low abundance, while the unclassified and rare phyla accounted for about 4.12% in the samples. Litsea cubeba cover had significant impact on the relative abundance of bacterial community (Fig. 3). The relative abundance of Proteobacteria and Acidobacteria was 33.48% and 33.54% in control, respectively (Fig. 3). Compared with the control, the relative abundance of Proteobacteria significantly increased in both Litsea cubeba cover treatment. And an obvious decrease in the relative abundance of Acidobacteria, Chloroflexi, and Planctomycetes was detected in the F-Pr treatment (Fig. 3). Litsea cubeba cover had little effect on the relative abundance of Bacteroides, Verrucomicrobia, Gemmatimonadetes, Firmicutes, Saccharibacteria, Armatimonadetes, GAL15, Chlorobi, TM6-Dependentiae, and Deinococcus-Thermus (Fig. 3).

Fig. 3
figure 3

The relative abundance of phyla of soil microbiome in tea plantation across all the plots covered with Litsea cubeba plots (% of total sequence). CK, control; F-Ab, the vegetative branches cover treatment; F-Pr, the fruit-bearing branches cover treatment

Litsea cubeba cover antagonized plant pathogens

We conducted a functional analysis of bacterial community with the software of Tax4Fun. The functions mainly involve plant pathogens, carbon cycle and mineral enrich, plant-derived biopolymers degradation, and so on (Table 1). The OTUs number of plant pathogens community in the CK was 907 ± 81 while in the F-Ab and F-Pr treatment was 658 ± 92 and 337 ± 25, respectively (Table 1); the OTUs number of nutrition metabolism and transport were 850 ± 79, 625 ± 48, and 165 ± 37 in CK and F-Ab and F-Pr treatment, respectively. The change trends of other functional group, such as carbon cycle and mineral enrich, nitrate reduction, and carbohydrate utilization, were similar with that of nutrition metabolism and transport (Table 1).

Table 1 The potential function of soil microbia (n = 4)

Litsea cubeba cover changed the soil-enriched metabolites composition

The soil-enriched metabolites were obtained by ethyl-acetate extraction from air-dried soil and subsequently analyzed by GC-MS. The identified components with their relative percentages were reported in Table 2. In the F-Pr treatment, (+)-2-bornanone (8.25 ± 2.62%), alpha-terpineol (10.38 ± 4.79%), citronellol (14.2 ± 3.32%), cyclohexanol (20.97 ± 5.64%), and oleamide (35.80 ± 8.99%) were the main constituents (Table 2). The cyclohexene (0.49 ± 0.25%), nickel tetracarbonyl (0.57 ± 0.33%), and phosphine oxide (0.84 ± 0.14%) were the minimum oil (Table 2). The main constituents of control and F-Ab treatment were extraction solvent-ethyl acetate (95.97%) (Table 2).

Table 2 Chemical composition of soil-enriched metabolites in control and treatment (n = 4)

Litsea cubeba cover reduced soil aluminum and increased phosphorus content

Covering with Litsea cubeba plants had significant effects on the content of aluminum and phosphorus (Fig. 4a and b). The aluminum content in F-Pr treatment was lowest, the content was 23.60 ± 3.54 mg/kg while in the CK and F-Ab treatment was 48.28 ± 3.32 and 26.73 ± 2.17 mg/kg, respectively. In contrast, the phosphorus content increased in Litsea cubeba cover treatment. In F-Pr and F-Ab treatments, the phosphorus content was 18.41 ± 2.42 mg/kg and 5.29 ± 0.97 mg/kg, respectively, which were higher than that in control plots (1.53 ± 0.29 mg/kg).

Fig. 4
figure 4

Effect of Litsea cubeba cover on soil mineral nutrition of tea plantation. a Aluminum. b Phosphorus. Data are means ± SE (n = 4). The star (*) indicates significant difference between treatments at 0.05 level; CK, control; F-Ab, the vegetative branches cover treatment; F-Pr, the fruit-bearing branches cover treatment

Litsea cubeba cover changed soil pH and aluminum forms

Litsea cubeba cover affected the soil pH and different aluminum forms content (Table 3). Compared with the control, Litsea cubeba cover increased the soil pH. In the F-Ab and F-Pr treatment, the soil pH index was 4.76 ± 0.02 and 5.28 ± 0.05, increased by 0.30 and 0.82, respectively. For the different aluminum forms, covering with Litsea cubeba decreased the exchangeable aluminum content. Compare with the control, the content of exchangeable aluminum of F-Ab treatment was 16.32 ± 3.34 mg/kg, and that of F-Pr treatment was 11.27 ± 2.57 mg/kg, reduced by 45.28% and 62.21%, respectively. The trend of aluminum humate was opposite to that of exchangeable aluminum. The aluminum humate content in F-Ab and F-Pr treatments increased by 68.94% and 56.51%, respectively (Table 3). The change trends of aluminum hydrous oxide and hydroxide were similar to that of aluminum humate, while the inorganic adsorption aluminum content was not obviously changed (Table 3).

Table 3 Different forms of aluminum content in experiment (n = 4)

Soil-enriched metabolites and mineral nutrition affected soil bacteria structure and diversity

RDA analysis results showed that the bacterial community was affected by mineral nutrition and soil-enriched metabolites. The first quadrant showed that the phosphorus distribution affected the structure and diversity of Gemmatimonadetes, Saccharibacteria, and Deinococcus-Thermus; the soil-enriched metabolites, such as oleamide (Ole), cyclohexene (Cyc), and (+)-2-bornanone (Bor), mainly distribute in the second quadrant and affected the structure and diversity of Proteobacteria and Actinobacteria; in the four quadrant, the structure and diversity of Firmicutes, Chloroflexi, Armatimonadetes, and Verrucomicrobia were mainly affected by the aluminum (Fig. 5a).

Fig. 5
figure 5

Redundancy analysis of soil bacterial and environmental factors. Al, aluminum; P, phosphorus; Ole, oleamide; Cyc, cyclohexene; Bor, (+)-2-bornanone; Alp, alpha-terpineol; Cit, citronellol; Amm, ammonia; Al-1, exchangeable aluminum; Al-2, inorganic adsorption aluminum; Al-3, aluminum hydrous oxide and hydroxide; Al-4, aluminum humate. Note: In the plot, red hollow arrows represent environmental factors; solid arrows stand for bacterial community structure information; cosine of the angle between the extension lines of environmental factor and bacterial species equals to the correlation coefficient between the two in numerical value

The environment factors explain showed that aluminum (Al), oleamide (Ole), cyclohexene (Cyc), (+)-2-bornanone (Bor), phosphorus (P), and alpha-terpineol (Alp) were the main impact factors, the value of explains > 95%, and P < 0.05 (Table 4). The redundant analysis of different forms of aluminum and bacterial community showed that the exchangeable aluminum was the main form of aluminum that affects the structure and diversity of soil bacteria (Fig. 5b).

Table 4 Redundancy analysis (RDA)

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