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Essay

Mechanism of the Effects of Phyllostachys edulis Invasion on the Soil Microbial Community in Quercus acutissima Forests

by
Lu Fang
,
Haibo Hu
*,
Jianyu Chen
,
Yuyang Gong
and
Ziyi Zhu
Co-Innovation Center for Sustainable Forestry in Southern China of Jiangsu Province, Key Laboratory of Soil and Water Conservation and Ecological Restoration of Jiangsu Province, Nanjing Forestry University, Nanjing 210037, China
*
Author to whom correspondence should be addressed.
Forests 2024, 15(7), 1170; https://doi.org/10.3390/f15071170
Submission received: 26 April 2024 / Revised: 7 June 2024 / Accepted: 30 June 2024 / Published: 5 July 2024
(This article belongs to the Section Forest Soil)
Browse Figures
Versions Notes

Abstract

:
In recent years, the invasion of Phyllostachys edulis has intensified. This study selected Phyllostachys edulis, Phyllostachys edulis–Quercus acutissima mixed and Quercus acutissima forest areas and analyzed the changes in soil bulk density, porosity, water-holding capacity, pH, soil organic carbon (SOC), soil readily oxidized organic carbon (ROC), soluble organic carbon (DOC) and microbial biomass carbon (SMBC). Then, we obtained the Chao index, Shannon index and Simpson index and analyzed the relationship between microbial α diversity, β diversity and community composition, abundance and structure and between microbial community changes and environmental factors. The results showed that soil with the invasion of Phyllostachys edulis, soil pH, water content, capillary water capacity and total porosity of surface soil increased significantly by 4.35%, 18.71%, 16.37% and 14.56%, respectively, compared with the Quercus acutissima forest, while soil bulk density, organic carbon, microbial biomass carbon and soluble organic carbon decreased significantly by 7.27%, 18.43%, 61.12% and 61.90%. Soil readily oxidized organic carbon decreased at first and then increased. The soil community richness and diversity of the Phyllostachys edulis forest were significantly greater than those of the other two stands. Soil pH and organic carbon content were the main factors affecting the changes in the soil microbial community. Therefore, the invasion of Phyllostachys edulis improved soil water retention, while soil pH had the greatest effect on the microbial community, followed by soil bulk density and organic carbon, and water content had the smallest effect.

1. Introduction

In recent years, with the rapid development of the economy and society, biological invasions have caused serious interference and damage to original ecosystems [1,2,3,4,5,6,7]. The world currently faces a high probability of plant invasion over a large area that will cause a lot of harm [1,3,4,5,6,7]. Studies have shown stand succession caused by plant invasion and expansion can significantly change the decomposition of organic matter and nutrient cycling, leading to a reduction in biodiversity and accelerating species extinction, which has become one of the major environmental problems in the world [2,3,4,5,6,7]. Plant invasion can affect the soil carbon and nitrogen cycling processes by changing the species composition of the original forest and the ability of apomictic material to decompose [8]. The interaction between plants and soil nutrients is an important link affecting invasion. Invasive plants change the soil microbial community, and this change also leads to changes in soil nutrient cycling [9]. Phyllostachys edulis belongs to the grass family Poaceae and is a widely distributed plant in China [10]. It is also an important forest resource [11]. In recent years, the Phyllostachys edulis invasion phenomenon is significant [12,13,14,15]; its rapid growth and strong reproduction pose a threat to the species diversity, carbon sink function and ecosystem stability of neighboring forests.
Soil is an important part of forest ecosystems. Phyllostachys edulis invasion has a great influence on soil properties and microbial communities. Phyllostachys edulis expansion also affects the ecological functions of forest ecosystems, such as soil and water conservation, biomass and carbon storage [16,17,18]. For example, forests in southwestern Japan have been expanded by Phyllostachys edulis, and the loss of vegetation has increased the probability of landslides [19]. Phyllostachys edulis invasion significantly alters the vegetation community, litter input and habitat, and reduce plant diversity, but it has a strong carbon sink capacity [20]. The invasion of Phyllostachys edulis can also affect the soil nutrient cycle, microbial diversity and community structure, which is different from that of broad-leaved and coniferous forests [21,22,23,24,25,26]. Wu JS et al. found that the expansion of bamboo in Tianmushan affected the physical and chemical properties of forest surface soil [27]. The soil microbial structure is greatly affected by the environment in forest ecosystems [28]. Xu et al. showed that soil microbial communities changed significantly with the invasion process of Phyllostachys edulis [29].
The invasion of Phyllostachys edulis has a great impact on the soil and water conservation, climate, carbon fixation and oxygen release of the original habitat, and has very important practical significance for promoting sustainable development and improving the environment. Studying this scientific question can help us to better predict and mitigate the negative effects of invasive organisms, and protect and restore native ecosystems. Under global warming, Phyllostachys edulis is expanding into deciduous broad-leaved forest in the north subtropical region of China, so the current research focused on the impact of Phyllostachys edulis on evergreen broad-leaved/coniferous forests in South Asia and Central Asia [25,26,27,28,29]. However, the effect of Phyllostachys edulis invasion on soil properties and the microbial structure of typical subtropical forests is still unclear.
Based on this, the study investigated the basic physical and chemical properties of the soil and the evolution of the microbial community in the expansion area of Phyllostachys edulis in the middle and lower reaches of the Yangtze River. The purpose of this study was to solve the problem of insufficient research on the effects of Phyllostachys edulis invasion on soil microbial communities in deciduous broad-leaved forests, supplementing the research on soil microbial characteristics in northern subtropical China. The relationship between Phyllostachys edulis invasion, environmental factors and soil microbial communities was revealed; the results also can provide a theoretical basis for the study of subtropical forest vegetation invasion in China.

2. Materials and Methods

2.1. Study Area

The research was carried out in the Xiashu Practice Forest Farm of Nanjing Forestry University, Jiangsu Yangtze River Delta Forest Ecological Station of National Forestry and Grassland Administration, which is located in the middle part of the low hills of the Ningzhen Mountain Range; the geographic coordinates are 119°14′ E, 31°58′ N, and the forest farm occupies an area of 389.1 hm2. The average annual temperature is 15.2 °C, the extreme maximum temperature is 39.6 °C, and the extreme minimum temperature is −16.7 °C. The average annual rainfall is 1055.6 mm, the average annual relative humidity is 79%, and the terrain is hilly, mountainous and gently undulating. The soil is mainly mountain yellow-brown loam, with depths of 40–60 cm, and the soil is acidic, with pH values between 4.0 and 6.0 [30].

2.2. Experimental Design

A typical mixed interface for the expansion of Phyllostachys edulis into a Quercus acutissima forest was selected and set up as a sample plot for long-term observation and research in the Xiashu Internship Forest of Nanjing Forestry University. Three sample strips with basically the same initial topography and elevation conditions were selected to form three replicates, each with a length of 60 m and a width of 20 m. The Phyllostachys edulis forests were all mature forests. Three 20 m × 20 m standard square plots were set in each transect, so that three stand types were continuously distributed on each sample strip—a pure Phyllostachys edulis forest, mixed Phyllostachys edulis–Quercus acutissima forest (Phyllostachys edulis/Quercus acutissima basal area = 1:1) and Quercus acutissima forest (hereinafter referred to as “Phyllostachys edulis forest”, “mixed forest” and “Quercus acutissima forest”)—to ensure that the first and last two sample plots were single vegetation-type communities; the middle sample plots cover the interface of the Phyllostachys edulis expansion, and the corners of the sample plots are permanently marked with wooden stakes. A total of nine sample plots were used for the forest stand surveys.
The stand survey included the latitude, longitude, topography and elevation of the test site. The forest stand survey (Table 1) was conducted as follows: each 20 m × 20 m sample plot was divided into four 10 m × 10 m sample plots to survey the plant community in the tree layer (height > 5 m), and indicators such as tree species, number of plants, tree height and diameter at breast height were recorded. At the same time, five 2 m × 2 m small sample plots were selected from each larger sample plot to survey the diversity of species in the shrub layer (height < 5 m), including the young trees, as well as the shrubs and vines, and the tree species, number of plants and height of shrubs were recorded. Five 1 m × 1 m small sample squares were selected for the herbaceous layer plant survey to record the species, number of plants and cover. The stand density was calculated as the ratio of the number of stand trees to sample plot area. The average tree height of the stand was calculated as the ratio of the sum of the tree height in the quadrat to the number of trees, and the average chest diameter was calculated as the ratio of the sum of chest diameter in the quadrat to the number of trees. Crown density is the degree to which the canopy of trees in a forest shades the ground: total canopy/total sample area. Plant diversity was calculated using the Shannon—Wiener diversity index [31].
The pure forest of Phyllostachys edulis is an artificial planting of a secondary forest (Table S1). In addition to the main species of Phyllostachys edulis, there are a few Celtis sinensis and Symplocos paniculata, and the shrub layer has Fortunearia sinensis and Camellia sinensis. The understorey (Table S1) is simple vegetation, which is mainly composed of Pseudostellaria heterophylla, Trachelospermum jasminoides, Ophiopogon bodinieri and Ardisia japonica. The dominant species of the mixed forest (Table S1) are Phyllostachys edulis and Quercus acutissima, mixed with other tree species such as Symplocos paniculata and Liquidambar formosana, and the main shrub species are Lindera glauca, Photinia villosa, Camellia sinensis, Rosa sertata, Rubus innominatus and Ilex cornuta. The understorey herbs (Table S1) are Amana Honda, Semiaquilegia adoxoides, Euscaphis japonica, Liriope muscaria and Euscaphis japonica. The Quercus acutissima forest (Table S1) is also an artificial secondary forest; the main tree species in addition to Quercus acutissima is mixed with Liquidambar formosana; the shrub layer species include understorey vegetation, such as Photinia villosa, Lindera glauca, etc.; and the herb layer mainly includes Pseudostellaria Pax and Ophiopogon bodinieri.

2.3. Sample Collection and Analytical Methods

2.3.1. Sample Collection

Soil samples were collected in March 2023 (spring), July 2023 (summer), October 2023 (fall) and January 2024 (winter) after 1 week of consecutive sunny days. Three soil sampling points were set up at each sample site, and debris and litter were removed from the ground during sampling. Soil profiles were dug near each sampling point each time sampling was conducted, and soil samples were collected along the soil profiles from bottom to top. Each sampling point was sampled by tape measure in layers at depths of 0–10 cm, 10–20 cm and 20–40 cm, and the soil samples were placed in self-sealing bags for the determination of soil physicochemical properties, with each sampling point constituting three replicates. A total of 81 soil samples were collected from all stands per season, totaling 324 samples over the four seasons.
Soil samples were collected for the determination of the microbial community composition and structure. In the set sampling area, we removed litter from the ground surface; according to the five-point sampling method [32], the central intersection and four points of the diagonal were selected as five points. Five surface soil samples (0–10 cm) were collected. The depth of the soil layer was measured using a tape measure and a sealing bag of the same size was used to ensure that the sampling depth and sampling volume were uniform at each sampling point. The sample was placed in a dry ice box after passing through a 2 mm steel sieve, freeze dried immediately, and stored in a refrigerator at −80 °C until use.

2.3.2. Soil Physicochemical Index Determination

The soil physical properties were determined as follows: according to the industry standard of the “Determination of Forest Soil Moisture-Physical Properties” (GB/T 1215-1999) [33], the soil water content (SWC) was determined using the drying method; using the ring-knife method, the soil bulk density (BD), total porosity (TPO), capillary porosity (CPO), noncapillary porosity (NPO), water content (SWC), capillary water capacity (CWHC) and maximum water capacity (MWC) were measured and calculated. The soil chemical properties were determined as follows: pH was measured using a pH meter, and soil organic carbon (SOC) was measured using an elemental analyzer after acidification [34]. For soil readily oxidized organic carbon (ROC), the potassium permanganate oxidation method was adopted [35]. Soil microbial biomass carbon (SMBC) was determined by the chloroform fumigation—K2SO4 extraction method using a TOC analyzer [36]; the same procedure using unfumigated soil was used to determine soil soluble organic carbon (DOC). The calculation formulas are as follows:
BD   ( g / cm 3 ) = M 2 − M 1 V
SWC   ( g / kg ) = M 2 − M 1 M 1   ×   1000
CWHC   ( g / kg ) = M 3 − M 1 M 1   ×   1000
MWC   ( g / kg ) = M 4 − M 1   M 1  
CPO (%) = 0.1 × CWC × BD
NPO (%) = 0.1 × (MWC − CWC) × BD
TPO (%) = CP + NP
M1: Dry soil mass inside the ring knife, g;
M2: Mass of wet soil inside the ring knife, g;
M3: Mass of wet soil in the ring knife after 2 h on dry sand, g;
M4: Mass of wet soil in the ring knife after wetting for 12 h, g;
V: Ring tool volume, cm3.

2.3.3. DNA Extraction and PCR Amplification

Total microbial genomic DNA was extracted from 108 samples using the E.Z.N.A.® soil DNA Kit (Omega Bio-tek, Norcross, GA, USA) according to the manufacturer’s instructions. The quality and concentration of the DNA were determined by 1.0% agarose gel electrophoresis and a NanoDrop2000 spectrophotometer (Thermo Scientific, Waltham, MA, USA) and the DNA was kept at −80 °C until further use. The hypervariable region V3-V4 of the bacterial 16S rRNA gene was amplified with primer pairs 338F (5′-ACTCCTACGGGAGGCAGCAG-3′) and 806R (5′-GGACTACHVGGGTWTCTAAT-3′) [37] using a T100 Thermal Cycler PCR thermocycler (BIO-RAD, Hercules, CA, USA). The PCR reaction mixture included 4 μL of 5 × Fast Pfu buffer, 2 μL of 2.5 mM dNTPs, 0.8 μL of each primer (5 μM), 0.4 μL of Fast Pfu polymerase, 10 ng of template DNA, and ddH2O to a final volume of 20 µL. The PCR amplification cycling conditions were as follows: initial denaturation at 95 °C for 3 min, followed by 27 cycles of denaturing at 95 °C for 30 s, annealing at 55 °C for 30 s and extension at 72 °C for 45 s, and a single extension at 72 °C for 10 min, and end at 4 °C. The PCR product was extracted from a 2% agarose gel and purified using the PCR Clean-Up Kit (YuHua, Shanghai, China) according to the manufacturer’s instructions and quantified using a Qubit 4.0 (Thermo Fisher Scientific, Waltham, MA, USA).

2.3.4. Statistical Analysis

The soil physical and chemical property indices were statistically analyzed using Excel 2023 (Microsoft, Redmond, WA, USA) and SPSS 25.0 software (SPSS Inc., Chicago, DE, USA). The data were tested for homogeneity of variance and normal distribution, then one-way ANOVA was performed to compare the significance of differences between different forest soil properties (p < 0.05), and correlation plots were construct with Origin 2022 software (OriginLab, Northampton, MA, USA). The data in the bar graphs are presented as the means ± standard deviations.
A bioinformatic analysis of the soil was carried out using the Majorbio Cloud platform (https://cloud.majorbio.com (accessed on 1 March 2024)). Based on the ASV information, alpha diversity indices including observed ASVs, Chao1 richness, Shannon index and Good’s coverage were calculated using Mothur v1.30.1 [38].
Stacked bar plots generated using R (v3.3.1) (Miseq, MA, USA) were used to identify the most abundance bacterial communities at both the phylum and genus levels. Taxonomic groups with a relative abundance of under 1% in all samples were combined into others.
Comparisons of taxonomic data at the phylum levels among three groups were tested using one-way ANOVA with post hoc Tukey’s HSD tests using the stats package in R (v3.3.1). Statistical significance was accepted as p < 0.05.
The similarity among the microbial communities was determined by PCoA (principal coordinate analysis) based on Bray–Curtis distances using the vegan package in R (v3.3.1). Analysis of similarities (ANOSIM) based on Bray–Curtis distances was carried out to test for significant differences among the microbial communities of different samples using the vegan package in R (v 3.3.1).
Redundancy analyses (RDAs) were performed to investigate the relationships between environmental variables and the soil bacterial community structure of each treatment at the phylum level of bacteria using the vegan package in R (v3.3.1). Forward selection was based on Monte Carlo permutation tests (permutations = 9999). Values of the x- and y-axes and the length of the corresponding arrows represented the importance of each soil environmental variable in explaining the distribution of taxa across communities.

3. Results

3.1. Effects of Phyllostachys edulis Invasion on Soil Physicochemical Properties

The invasion of Phyllostachys edulis affected the soil physicochemical properties to different degrees. Table 1 shows that the soil bulk density of the three forest stands increased with increasing soil depth, indicating that the looseness of the top layer of soil was the greatest and that the forest stand had the greatest ameliorating effect. The soil bulk density of the 0–10 cm soil layer of the three forest stands was significantly lower than that of the other two layers, and the soil bulk density of the Phyllostachys edulis forest was significantly lower than that of the mixed forest and the Quercus acutissima forest.
The soil mass water content and capillary water capacity can effectively reflect the water-holding performance of forest soil. Table 2 shows that the soil mass water content and capillary water capacity of the three kinds of forest stands that formed during the expansion of Phyllostachys edulis into Quercus acutissima forest decreased with increasing soil depth, and the water-holding performance of the different forest stands at the same soil depth also differed. The soil water content and capillary water holding capacity of the Phyllostachys edulis forest at soil depths of 0–10 cm were 289.46 g/kg and 432.78 g/kg, respectively, which were significantly greater than the values of 265.61 g/kg and 402.27 g/kg, respectively, in mixed forest and 243.83 g/kg and 371.89 g/kg, respectively, in Quercus acutissima forest. The same was true at the 10–20 cm soil depth and the 20–40 cm soil depth, which was in line with the change in the total soil porosity.
In general, the expansion of Phyllostachys edulis into the Quercus acutissima forest effectively increased the water-holding capacity of the soil, which was conducive to the growth of the root system.
Soil porosity can effectively reflect soil aeration [39,40,41,42]. As shown in Table 3, the total porosity, noncapillary porosity and capillary porosity of the three stands that formed during the expansion process decreased with increasing soil depth. There was a significant difference between the 0–10 cm soil depth of the Phyllostachys edulis forest and the 10–20 cm and 20–40 cm soil depths in Phyllostachys edulis forest (p < 0.05). The distributions of noncapillary porosity, capillary porosity and total porosity in different stands at the same soil depth were also different, and the rankings of the total porosity and capillary porosity at soil depths of 0–10 cm, 10–20 cm and 20–40 cm were as follows: Phyllostachys edulis forest > mixed forest > Quercus acutissima forest.
Generally, the expansion of Phyllostachys edulis improved the total porosity and capillary porosity of the soil, especially in the surface layer, which was also related to the well-developed bamboo whip root system in the shallow soil layer in the Phyllostachys edulis forest.
As shown in Table 4, the pH of the soil in the Phyllostachys edulis forest was greater than that in the other two forest stands, and the average values at the three soil depths were 0.19 and 0.21 greater than those in the mixed forest and the Quercus acutissima forest, respectively. The soil pH of the Phyllostachys edulis forest at the 0–10 cm soil depth was 4.11% and 4.35% greater than that of the other two forest types. The soil pH of the Phyllostachys edulis forest at the 10–20 cm soil depth was 4.72% and 5.21% greater, and that at the 20–40 cm soil depth, it was 4.49% and 5.0% greater. The soil organic carbon (SOC) content of the Phyllostachys edulis forest was the lowest among the three forest types. The soil organic carbon content of the Phyllostachys edulis forest was significantly lower than that of the other forest types, with average decreases of 20.65% and 37.82%, respectively, compared with those of the mixed forest and the Quercus acutissima forest. Compared with that of the Phyllostachys edulis forest (Table 4), the soil organic carbon content of the mixed forest and the Quercus acutissima forest increased, and the pH decreased significantly (p < 0.05). The invasion of Phyllostachys edulis increased the soil pH, and the soil organic carbon content decreased significantly due to the greater consumption of soil organic matter by asexual propagation (p < 0.05).
With the invasion process of Phyllostachys edulis, the ROC decreased first and then increased, which had a significant effect on the top soil, but there were small differences in the bottom soil. There were significant differences among the different stands in the same soil layer. In 0–10 cm soil layer, the ROC of the Phyllostachys edulis forest was 0.68~1.08 g/kg, and the average value was 0.96 g/kg, which was significantly higher than that of the mixed forest and Quercus acutissima forest by 11.63% and 4.35%. In the 10–20 cm soil layer, the ROC content of the mixed forest was 0.69~0.87 g/kg. The mean value of ROC in the mixed forest was 0.78 g/kg, which was significantly lower by 8.97% and 12.82% than that of the Phyllostachys edulis forest and Quercus acutissima forest. In the 20–40 cm soil layer, the ROC content of the mixed forest was 0.69~0.82 g/kg, and the mean value was 0.75 g/kg, which was significantly lower (by 12% and 14.67%) than that of the Phyllostachys edulis forest and the Quercus acutissima forest.
As shown in Figure 1, the soil microbial biomass carbon content decreased with increasing soil depth, and the order of the soil microbial biomass carbon content at the three soil depths was Quercus acutissima forest > mixed forest > Phyllostachys edulis forest. The SMBC content of the Quercus acutissima forest at 0–10 cm was 143.22% and 61.12% greater than that of the Phyllostachys edulis forest and mixed forest, respectively, and the differences reached a significant level; the same was true for the 10–20 cm and 20–40 cm soil depths. Therefore, the invasion of Phyllostachys edulis into the Quercus acutissima forest reduced the soil microbial biomass carbon content.
As shown in Figure 2, soil soluble organic carbon (DOC) decreased gradually with the invasion process of Phyllostachys edulis, and the DOC at 0–10 cm was significantly higher than that at 10–20 cm and 20–40 cm. In the 0–10 cm soil layer, the average DOC of the Phyllostachys edulis forest was 0.021 mg/g, which was 38.10% and 61.9% lower than that of the mixed forest and Quercus acutissima forest. The DOC of the Phyllostachys edulis forest in the three soil layers was significantly lower than that of the other stands. Therefore, the invasion of Phyllostachys edulis into the Quercus acutissima forest reduced the DOC.

3.2. Effects of Phyllostachys edulis Invasion on Soil Microbial Communities

The α diversity analysis of the soil samples revealed (Table 5) that the Chao index and Shannon index differed significantly (p < 0.05) between the Phyllostachys edulis forest and the mixed forest, and the community richness and diversity were the highest in the Phyllostachys edulis forest and lowest in the mixed forest, indicating that the richness and diversity of the flora in the mixed forest were significantly lower than those in the Phyllostachys edulis forest. The α diversity of soil microorganisms in the mixed forest decreased compared with that of the Phyllostachys edulis forest in terms of the community diversity index and richness, with the Chao index decreasing by 16.1%, the Shannon index decreasing by 1.9% and the Simpson index decreasing by 7.1%. The microbial diversity and abundance of the Quercus acutissima forest were significantly different from those of the Phyllostachys edulis forest, with the Chao index decreasing by 15.1% and the Shannon index decreasing by 1.9%. The invasion of Phyllostachys edulis into the Quercus acutissima forest changed the α diversity of the soil microorganisms, and the Chao and Shannon indices of the Phyllostachys edulis forest increased significantly.
The PCoA showed that in the aggregation of soil microorganisms in the three forest stands, principal component 1 and principal component 2 explained 25.68% and 16.39% of the variation in the soil microbial communities, respectively, and the cumulative contribution rate of the two principal components was 42.07% (Figure 3). Most of the soil microorganisms gathered on the left side of the mixed forest, while the soil microorganisms gathered on the right side of the Phyllostachys edulis forest. The distance between the mixed forest and the Quercus acutissima forest was relatively close but there was no overlap. There was no overlap in the soil microbial aggregation of the three stands, thus the microbial community structure was different. At the same time, the similarity of the microbial communities in the three forest stands was examined; it was found that the soil microbial community structure of the three stands differed significantly after the expansion of Phyllostachys edulis into the Quercus acutissima forest (R2 = 0.3617, p = 0.001).
According to the relative abundance of major phyla of microorganisms (Figure 4), the main microbial phyla in the three stands were Proteobacteria, Acidobacteriota, Actinobacteria, Chloroflexi, WPS-2, Verrucomicrobiota, Firmicutes and Planctomycetota. Proteobacteria accounted for 32.27%, 30.36% and 31.02% of the bacteria in the Phyllostachys edulis forest, mixed forest and Quercus acutissima forest, respectively; Acidobacteriota accounted for 25.23%, 22.08% and 24.66% of the bacteria in the Phyllostachys edulis forest, mixed forest and Quercus acutissima forest, respectively; and Actinobacteria accounted for 15.96%, 21.31% and 18.38% of the bacteria in the Phyllostachys edulis forest, mixed forest and Quercus acutissima forest, respectively. Therefore, the dominant strains in the Phyllostachys edulis forest were Proteobacteria and Acidobacteriota, the dominant strains in the mixed forest were Actinobacteria, and the dominant strains in the Quercus acutissima forest were Acidobacteriota.
The multigroup analysis shown in Figure 5 shows that the abundances of Actinobacteria, the WPS-2 phylum, Verrucomicrobiota and Planctomycetota were significantly different in the different forest stands. The abundance of Actinobacteria and WPS-2 in the main groups of microorganisms was the lowest in the Phyllostachys edulis forest, and the highest in the mixed forest; however, the abundance of Verrucomicrobiota and Planctomycetota in the main groups of microorganisms was the highest in the Phyllostachys edulis forest, and the lowest in the mixed forest. This is indicating that Actinobacteria and WPS-2 increased first and then decreased, while the Verrucomicrobiota and Planctomycetota decreased at first and then increased, when Phyllostachys edulis invaded the Quercus acutissima forest.

3.3. Association between the Soil Microbial Community and Environmental Factors in Phyllostachys edulis-Invaded Soils

The effects of the seven factors on microbial community changes were analyzed by determining the pH, water content, soil bulk density, SOC, ROC, SMBC and DOC in all seasons. As shown in Figure 6, 34.74%, 35.44%, 28.71% and 36.88% of the variation in microbial structure were accounted for by the first two sorting axes in spring, summer, autumn and winter, respectively. It can be seen that microbial community structure changed the most in winter and the least in autumn. ROC had the greatest influence on the soil microbial community in spring and winter, while soil water content, pH and ROC had a great influence on soil bulk density in summer. The effects of SMBC on the soil microbial community were equal in all four seasons. Soil pH had the greatest influence on the soil microbial community structure in autumn, followed by soil bulk density and SOC, while soil water content, ROC, DOC and SMBC had little effect. In general, soil pH, SOC, ROC and SMBC had significant effects on the soil microbial community structure.

4. Discussion

4.1. Effects of Phyllostachys edulis Invasion on Soil Physicochemical Properties

Soil bulk density is an important index reflecting the degree of soil looseness and tightness [43,44,45,46]. In this study, the soil bulk density of the Phyllostachys edulis forest was significantly lower than that of the other two stands, especially in the surface soil, possibly because the Phyllostachys edulis whip root system was well developed and concentrated in the surface layer, thus improving the soil structure. An increase in the soil porosity had a greater impact on the soil water-holding capacity. The rankings of the total porosity and capillary porosity of the three soil layers were Phyllostachys edulis forest > mixed forest > Quercus acutissima forest, and there was a significant difference among the Phyllostachys edulis forest, mixed forest and Quercus acutissima forest. The soil mass water content and capillary water capacity of the three kinds of forests decreased with increasing soil depth from the Phyllostachys edulis to Quercus acutissima forest. The water-holding capacity of the different forests in the same soil layer also differed. At the 0–10 cm soil depth, the mass water content and capillary water capacity of the Phyllostachys edulis forest were significantly greater than those of the mixed forest and Quercus acutissima forest, consistent with the changes in soil porosity. This is consistent with the study of Shinohara et al. [47], where a comparison of soil water content measurements between Phyllostachys edulis forests and adjacent broadleaf evergreen forests revealed that Phyllostachys edulis forests had a greater soil water content than broadleaf forests, which suggests that the expansion of Phyllostachys edulis leads to an increase in the soil water content. Cui Cheng et al. [48] showed that the root secretion of Phyllostachys edulis crushed soil particles, improved soil aeration, enhanced water storage capacity, provided a water base for Phyllostachys edulis growth, accelerated the rate of Phyllostachys edulis invasion and enhanced the soil water-holding capacity. This finding is consistent with the results of most studies, such as those of Zhao Yuhong’s study [49], which showed that the expansion of Phyllostachys edulis can effectively improve the soil structure of evergreen broadleaf forest species, thus improving the soil bulk density, porosity and water content. However, the noncapillary porosity was still lower than that of evergreen broadleaf forests, which may be related to soil disturbances caused by the artificial disturbance and Phyllostachys edulis shoot digging. In this study, we also found that although the arborvitae layer of the Phyllostachys edulis forest had the lowest species diversity index, it significantly improved the soil bulk density, and it was hypothesized that Phyllostachys edulis might have promoted soil aggregation through its large root secretions; this improved the soil structure and increasing the utilization and absorption of nutrients in the soil, thus further improving the expansion capacity. This finding is consistent with the studies of Bai Shangbin et al. [29,50], in which species richness and the Simpson diversity index were significantly reduced in the tree and shrub layers of the forest community after the invasion of Phyllostachys edulis.
Previous studies have produced many similar results on the increase in soil pH due to the invasion of Phyllostachys edulis. Umemura et al. [51] also reported that the soil pH of mixed forests formed by the Phyllostachys edulis invasion of Japanese cypress was significantly greater than that of uninvaded forests, and there was a significant positive correlation between pH and exchangeable calcium ions in the soil, while there were no significant correlations with potassium and magnesium ions, suggesting that Phyllostachys edulis may adjust the pH by regulating exchangeable calcium ions in the soil. Xu Daowei [52] also reported a significant increase in soil pH in Phyllostachys edulis forests during the expansion of Phyllostachys edulis to Cunninghamia lanceolata on Daiyun Mountain. There are many dominant factors that lead to seasonal changes in soil pH, but the seasonal dynamics of pH in the three forest stands in this study were the greatest in summer and weakest in spring, indicating that the expansion of Phyllostachys edulis into the Quercus acutissima forest had no effect on the seasonal dynamics of soil pH. In this study, it was concluded that the expansion of Phyllostachys edulis led to a significant decrease in the soil organic carbon content, which is consistent with the results of Zhao Yuhong et al. [53], who studied Phyllostachys edulis invasion in broadleaf evergreen forests in Dagangshan, Jiangxi Province. In this study, it was hypothesized that the Phyllostachys edulis invasion of the Quercus acutissima forest led to a decrease in the soil organic carbon content due to the high consumption of soil organic matter by asexual reproduction.
In the invasion process of Phyllostachys edulis, the SMBC and DOC were in the order of Quercus acutissima forest > mixed forest > Phyllostachys edulis forest, and the surface soil showed the highest significance. DOC is closely related to SMBC, and litter is one of the main sources of DOC [54]. The SMBC and DOC were higher in the mixed forest and broad-leaved forest, because the variety and total amount of organic matter imported into the soil increased significantly compared with the Phyllostachys edulis forest. Zhang Yu et al. [55] found that adding leaf litter increased the DOC, while removing leaf litter and root decreased the DOC, which is consistent with the findings in this study.

4.2. Effects of Phyllostachys edulis Invasion on the Soil Microbial Community

The invasion of Phyllostachys edulis to form a pure forest had a significant effect on the structure of the soil microbial community. The α diversity analysis revealed that the Chao index and Shannon index differed significantly (p < 0.05) between the Phyllostachys edulis forest and the mixed forest. By comparing the three stands, the community richness and diversity were the highest in the Phyllostachys edulis forest and the lowest in the mixed forest, and the microbial community diversity index and richness decreased in the mixed forest compared with the Quercus acutissima forest. There was a significant difference in the microbial diversity and richness in the Quercus acutissima forest and the Phyllostachys edulis forest, with a decrease in the Chao index of 15.1%, and the Shannon index decreased by 1.9%. This is similar to the findings of Li Weicheng et al. [45] on the change in the soil flora diversity associated with the Phyllostachys edulis invasion of Pinus massoniana forests and is consistent with the findings of Xu et al. [29] who reported that the soil microbial biomass increased after Phyllostachys edulis invaded natural broadleaf woodlands. The β diversity analysis revealed that the microbial community structure changed significantly (p < 0.05) after the expansion of Phyllostachys edulis into the Quercus acutissima forest; the overlap of the Phyllostachys edulis forest with the mixed forest and Quercus acutissima forest decreased, and the distance between the mixed forest and Quercus acutissima forest increased, which indicated that the soil microbial structure of the mixed forest and Quercus acutissima forest significantly differed from that of the Phyllostachys edulis forest.
This study found that under the influence of environmental factors such as soil pH, soil bulk density, water content, SOC, ROC, DOC and SMBC, the soil microbial community structure changed the most in winter and the least in autumn. Due to the decrease in temperature in winter, soil microorganisms enter dormancy, and the microbial community structure changes greatly. In summer, the soil water content, pH and ROC had a great influence, and soil bulk density had the smallest effect. The summer soil water content, pH and ROC all had a great influence. This study area is located in a north subtropical monsoon climate with high temperatures and amounts of rain in summer, so the summer water content and pH have great influence on soil microbial communities. It has been shown [56,57,58] that the soil microbial biomass increases with pH and that soil organic carbon is the material basis for supporting the community biomass [59]. Although the RDA analysis found that the effect of soil water content was not significant except in summer, the cover and surface apoplastic stock of the Phyllostachys edulis forest were lower than those of the other two forest stands, which may significantly affect the soil water content. It has been pointed out [60] that this is because the extension of the root whip and the growth of its aboveground parts are not completely synchronized during the invasion of Phyllostachys edulis, which increases soil heterogeneity, suggesting that Phyllostachys edulis roots and apoplastic matter do not have consistent impacts on the soil and that experiments with different variables can be conducted in future research to determine the main environmental factors that affect the soil microbial biomass to further explore the impacts of the invasion of Phyllostachys edulis on soil microorganisms.

5. Conclusions

The soil physicochemical properties and microbial communities changed significantly during the invasion of Phyllostachys edulis. The study showed that in soil invaded by Phyllostachys edulis, the soil pH, water content, capillary water capacity and total porosity of surface soil increased significantly by 4.35%, 18.71%, 16.37% and 14.56%, respectively, compared with the Quercus acutissima forest, while the soil bulk density, organic carbon, microbial biomass carbon and soluble organic carbon decreased significantly by 7.27%, 18.43%, 61.12% and 61.90%. The richness and diversity of the soil microbial community in the Phyllostachys edulis forest were significantly higher than those of the other two stands. Soil pH, SOC, ROC and SMBC had significant effects on the soil microbial community structure, and environmental factors had the greatest impact on the microbial community structure in winter.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/f15071170/s1, Table S1: Tree survey details sheet.

Author Contributions

Conceptualization, L.F. and H.H.; methodology, L.F.; software, J.C.; validation, Y.G. and Z.Z.; formal analysis, L.F.; investigation, L.F.; resources, L.F.; data curation, J.C.; writing—original draft preparation, L.F.; writing—review and editing, L.F.; visualization, J.C.; supervision, H.H.; project administration, H.H.; funding acquisition, H.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the 2023 Geological Survey Project in Jiangsu Province (2200113); Jiangsu Province Carbon Peak and Carbon Neutrality Technology Innovation Special Fund Project in 2021 (BE2022305); Jiangsu Yangtze River Delta Forest Ecosystem Localization Research Project, National Forestry and Grassland Administration (2022132077); and Changzhou Ecological Green City Construction Research Project in 2021 (2021-108). This research was supported by the Comprehensive Survey on Ecological and Geological Environment of Important Ecological Function Areas in the Taihu Lake Basin, and the Changzhou project’s basic research program of 2022 (second batch), “Characterisation of soil carbon pools of major forest types in Changzhou city” (CJ20220194).

Data Availability Statement

Data contained in this study are available on request from the corresponding authors. The data is not publicly.

Acknowledgments

We would like to thank all the colleagues who participated in the study or the authors who participated in the experiment.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Distribution characteristics of soil microbial biomass carbon (SMBC) in the three stands. Note: Different capital letters indicate significant differences (p < 0.05) at different soil depths within the same stand; different lowercase letters indicate significant differences (p < 0.05) at the same soil depth within different stands.
Figure 1. Distribution characteristics of soil microbial biomass carbon (SMBC) in the three stands. Note: Different capital letters indicate significant differences (p < 0.05) at different soil depths within the same stand; different lowercase letters indicate significant differences (p < 0.05) at the same soil depth within different stands.
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Figure 2. Distribution characteristics of soil soluble organic carbon (DOC) in the three stands. Note: Different capital letters indicate significant differences (p < 0.05) at different soil depths within the same stand; different lowercase letters indicate significant differences (p < 0.05) at the same soil depth within different stands.
Figure 2. Distribution characteristics of soil soluble organic carbon (DOC) in the three stands. Note: Different capital letters indicate significant differences (p < 0.05) at different soil depths within the same stand; different lowercase letters indicate significant differences (p < 0.05) at the same soil depth within different stands.
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Figure 3. Soil microbial community structure in different stands. Z: Phyllostachys edulis forest; H: mixed forest; M: Quercus acutissima forest.
Figure 3. Soil microbial community structure in different stands. Z: Phyllostachys edulis forest; H: mixed forest; M: Quercus acutissima forest.
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Figure 4. Relative abundance of major microbial phyla in different stands. Z: Phyllostachys edulis forest; H: mixed forest; M: Quercus acutissima forest.
Figure 4. Relative abundance of major microbial phyla in different stands. Z: Phyllostachys edulis forest; H: mixed forest; M: Quercus acutissima forest.
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Figure 5. Average abundance of major soil microbial phyla in different stands. Z: Phyllostachys edulis forest; H: mixed forest; M: Quercus acutissima forest. The transverse axis represents the mean percentage of microbial phylum abundance, and the vertical axis represents the major phylum of microorganisms. * represents significant differences: * 0.01 < p ≤ 0.05, ** 0.001 < p ≤ 0.01, *** p ≤ 0.001.
Figure 5. Average abundance of major soil microbial phyla in different stands. Z: Phyllostachys edulis forest; H: mixed forest; M: Quercus acutissima forest. The transverse axis represents the mean percentage of microbial phylum abundance, and the vertical axis represents the major phylum of microorganisms. * represents significant differences: * 0.01 < p ≤ 0.05, ** 0.001 < p ≤ 0.01, *** p ≤ 0.001.
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Figure 6. Relationships between environmental factors in different stands in four seasons. Figures (a–d) represent the redundancy analysis of soil microorganisms in spring, summer, autumn and winter respectively. Z: Phyllostachys edulis forest; H: mixed forest; M: Quercus acutissima forest; BD: soil bulk density; SOC: soil organic carbon; ROC: soil readily oxidized organic carbon; DOC: soil soluble organic carbon; SMBC: soil microbial biomass carbon.
Figure 6. Relationships between environmental factors in different stands in four seasons. Figures (a–d) represent the redundancy analysis of soil microorganisms in spring, summer, autumn and winter respectively. Z: Phyllostachys edulis forest; H: mixed forest; M: Quercus acutissima forest; BD: soil bulk density; SOC: soil organic carbon; ROC: soil readily oxidized organic carbon; DOC: soil soluble organic carbon; SMBC: soil microbial biomass carbon.
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Table 1. Basic information on the forest stands.
Table 1. Basic information on the forest stands.
StripSample PlotSlope
(°)		DOSSPSD (Plants/hm2)ATH (m)ACD (cm)CDSpecies Diversity Index *
strip 1Phyllostachys edulis forest15eastmiddle360010.119.330.90.88
Mixed forest15eastmiddle220013.2816.730.81.76
Quercus acutissima forest15eastmiddle100013.7719.320.82.02
strip 2Phyllostachys edulis forest18eastmiddle440010.8310.100.91.73
Mixed forest18eastmiddle260013.1818.930.81.91
Quercus acutissima forest18eastmiddle160010.8519.670.82.19
strip 3Phyllostachys edulis forest17eastmiddle350011.0110.170.91.41
Mixed forest17eastmiddle180013.7317.220.81.79
Quercus acutissima forest17eastmiddle110011.9619.030.81.93
Note: DOS—direction of slope; SP—slope position; SD—stand density; ATH—average tree height; ACD—average chest diameter; CD—crown density. * refers to the Shannon index of the tree layer species.
Table 2. Distribution characteristics of the soil bulk density, soil mass water content and capillary water capacity.
Table 2. Distribution characteristics of the soil bulk density, soil mass water content and capillary water capacity.
Stand TypeSoil Depth (cm)Soil Bulk Density (g/cm3)Soil Water Content (g/kg)Capillary Water Capacity (g/kg)
Phyllostachys edulis forest0–101.10 ± 0.03 Bb289.46 ± 10.08 Aa432.78 ± 12.89 Aa
10–201.27 ± 0.08 Aa255.04 ± 14.17 Ba357.22 ± 10.11 Ba
20–401.35 ± 0.12 Aa220.86 ± 10.25 Ca328.96 ± 15.23 Ca
Mixed forest0–101.16 ± 0.02 Ba265.61 ± 9.45 Ab402.27 ± 10.64 Ab
10–201.29 ± 0.14 ABa229.11 ± 10.36 Bb324.28 ± 17.48 Bb
20–401.37 ± 0.07 Aa197.94 ± 10.46 Cb287.66 ± 18.14 Cb
Quercus acutissima forest0–101.18 ± 0.04 Ba243.83 ± 11.44 Ac371.89 ± 12.27 Ac
10–201.29 ± 0.09 ABa210.53 ± 7.73 Bc266.88 ± 15.33 Bc
20–401.38 ± 0.10 Aa173.55 ± 10.27 Cc226.29 ± 17.54 Cc
Note: Different capital letters indicate significant differences (p < 0.05) at different soil depths within the same stand; different lowercase letters indicate significant differences (p < 0.05) at the same soil depth within different stands.
Table 3. Distribution characteristics of soil porosity in the three stands.
Table 3. Distribution characteristics of soil porosity in the three stands.
Stand TypeSoil Depth (cm)Capillary Porosity (%)Noncapillary Porosity (%)Total Porosity (%)
Phyllostachys edulis forest0–1047.70 ± 0.83 Aa5.65 ± 0.11 Aa53.34 ± 0.19 Aa
10–2045.67 ± 0.76 Ba3.55 ± 0.57 Ba49.22 ± 1.89 Ba
20–4043.34 ± 1.77 Ba2.92 ± 0.14 Ba46.26 ± 2.26 Ba
Mixed forest0–1043.00 ± 0.54 Ab5.02 ± 0.41 Ab48.02 ± 1.41 Ab
10–2040.91 ± 0.80 Bb4.04 ± 0.36 Ba44.95 ± 0.40 Bb
20–4040.62 ± 0.26 Bb3.49 ± 0.33 Ba44.11 ± 0.86 Bab
Quercus acutissima forest0–1042.69 ± 0.46 Ab3.87 ± 0.66 Ab46.56 ± 1.59 Ab
10–2039.61 ± 0.19 Bc3.80 ± 0.32 Aa43.41 ± 1.21 Bbc
20–4038.71 ± 0.87 Bc2.39 ± 0.32 Bb41.1 ± 1.98 Bb
Note: Different capital letters indicate significant differences (p < 0.05) at different soil depths within the same stand; different lowercase letters indicate significant differences (p < 0.05) at the same soil depth within different stands.
Table 4. Soil pH and organic carbon content (SOC) distribution characteristics of the three stands.
Table 4. Soil pH and organic carbon content (SOC) distribution characteristics of the three stands.
Stand TypeSoil Depth (cm)pHSOC (g/kg)ROC(g/kg)
Phyllostachys edulis forest0–104.56 ± 0.11 Aa17.25 ± 0.17 Ab0.96 ± 0.10 Aa
10–204.44 ± 0.12 Aa9.04 ± 0.78 Bb0.85 ± 0.06 Ba
20–404.42 ± 0.08 Aa6.57 ± 0.30 Cb0.84 ± 0.07 Ba
Mixed forest0–104.38 ± 0.06 Ab18.34 ± 0.63 Aa0.86 ± 0.08 Ac
10–204.24 ± 0.05 Ab11.00 ± 0.25 Aa0.78 ± 0.06 Bb
20–404.23 ± 0.09 Ab8.80 ± 1.00 Ba0.75 ± 0.08 Bb
Quercus acutissima forest0–104.37 ± 0.06 Ab20.43 ± 1.70 Aa0.92 ± 0.09 Ab
10–204.22 ± 0.08 Ab13.42 ± 2.21 Ba0.88 ± 0.15 Ba
20–404.21 ± 0.11 Ab9.63 ± 1.24 Ba0.86 ± 0.12 Ba
Note: Different capital letters indicate significant differences (p < 0.05) at different soil depths within the same stand; different lowercase letters indicate significant differences (p < 0.05) at the same soil depth within different stands.
Table 5. Soil α diversity indices in different stands.
Table 5. Soil α diversity indices in different stands.
Stand TypeChao IndexShannon IndexSimpson Index
Phyllostachys edulis forest2411.00 ± 171.52 a6.82 ± 0.19 a0.0028 ± 0.0008 a
Mixed forest2076.04 ± 103.49 b6.47 ± 0.12 b0.0026 ± 0.0006 a
Quercus acutissima forest2095.63 ± 135.04 b6.47 ± 0.15 b0.0028 ± 0.0008 a
Note: Different lowercase letters after the data in the same column indicate significant differences between stands (p < 0.05).
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Fang, L.; Hu, H.; Chen, J.; Gong, Y.; Zhu, Z. Mechanism of the Effects of Phyllostachys edulis Invasion on the Soil Microbial Community in Quercus acutissima Forests. Forests 2024, 15, 1170. https://doi.org/10.3390/f15071170

AMA Style

Fang L, Hu H, Chen J, Gong Y, Zhu Z. Mechanism of the Effects of Phyllostachys edulis Invasion on the Soil Microbial Community in Quercus acutissima Forests. Forests. 2024; 15(7):1170. https://doi.org/10.3390/f15071170

Chicago/Turabian Style

Fang, Lu, Haibo Hu, Jianyu Chen, Yuyang Gong, and Ziyi Zhu. 2024. "Mechanism of the Effects of Phyllostachys edulis Invasion on the Soil Microbial Community in Quercus acutissima Forests" Forests 15, no. 7: 1170. https://doi.org/10.3390/f15071170

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