Effects of Successive Planting of Eucalyptus Plantations on Tree Growth and Soil Quality

Abstract

The ultra-short-cycle successive planting of Eucalyptus plantations has caused environmental and social problems, and changing the rotation cycle is a very good option to solve this issue. However, the effects of successive planting on Eucalyptus growth and soil quality after changing the cultivation period are unclear. This study evaluated the effects of successive Eucalyptus planting on growth, soil nutrients, and bacterial and fungal community structure with an eight-year cultivation period. Eucalyptus plantations with different succession generations (first, second and third generation) were selected, and tree height and diameter at breast height were measured. Ten indicators of soil nutrients in different soil layers (0–20 cm and 20–40 cm) were measured, and soil bacteria and fungi were sequenced in high throughput. Results show that there is an upward trend in tree growth after three successive generations, reaching the highest timber yield in the third-generation plantation. Soil nutrients also showed changes, in the 0–20 cm soil layer, with decreased TN, NH₄⁺-N, NO₃⁻-N and AK and increased AP, particularly for OM and TP content. In the 20–40 cm soil layer, the content of NH₄⁺-N and NO₃⁻-N increased slightly and the soil’s OM, TP, and TK content increased significantly. The diversity of bacterial and fungal communities in different soil layers increased significantly, and the community structure composition changed. Bacterial and fungal community structures were mainly driven by pH, NH₄⁺-N, TP and AP factors and by OM, NH₄⁺-N, TP and TK factors, respectively. Thus, successive plantings of Eucalyptus plantations with a cultivation period of eight years is conducive to the growth of trees. Some nutrients of the soil were returned, and the soil microbial diversity increased. Successive planting has brought efficiency and economic benefits while maintaining the soil’s fertility.

1. Introduction

Eucalyptus has been widely planted in southern China due to its fast growth, adaptability, high yield, and excellent pulping properties. By 2018, the planting area had exceeded 5.4 million hm² [1]. To address market demands and timber shortages, short-cycle multi-generation successive planting has been widely applied in Eucalyptus plantation management. This management model allows trees to sprout and regrow on their own stumps after felling, without replanting. In China, the cultivation period is usually between 5 to 7 years, or even 4 to 5 years, generally shorter than in other countries where these periods typically range from 6 to 14 years [1,2], due to limited forest land resources and timber scarcity [3].

Eucalyptus plantation cultivation has significantly contributed to the timber supply security in China [4]. Planting fast-growing species helps in restoring degraded forestland quickly and generates employment opportunities in afforestation, forest management, the wood pulp industry, and fiberboard production. Short cycle rotation offers more employment opportunities than traditional, long cycle rotation and increases the income of forestry farmers, which in turn stabilizes the prosperity of the rural population. However, this commercial management model has created ecological and environmental issues [5,6]. This form of land use conversion often exhausts soil nutrients and decreases soil quality [7], which consequently influences the sustainability of plantations.

Multi-generational succession in Eucalyptus plantation management has attracted considerable attention regarding its impact on tree growth, soil nutrients, and microorganisms. Soil nutrients play a crucial role in sustaining Eucalyptus plantation growth. The nature of the soil, its fertility and the conditions of cultivation and management determine the growth state of the stand. Moreover, the decay of trees through the litter material constantly drives new nutrient replenishment back into the forest soil, and soil nutrients are constantly updated [8,9]. Research has shown that successive plantations of Eucalyptus with a cultivation period of 5 years had a significant negative impact on soil nutrients, leading to a decrease in total nitrogen (TN) and available phosphorus (AP) [10]. Furthermore, as soil acidification pH increased with the increase in successive planting generations, soil organic matter (OM) exhibited a significant decreasing trend [11].

Soil microbes are essential for maintaining soil function and ecosystem sustainability through the regulation of nutrient cycling via the decomposition of litter and organic matter [12,13]. The growth, activities, and structures of forest soil microbial communities are influenced by various factors, including the quality and quantity of litter and organic matter inputs, soil nutrient effectiveness, and anthropogenic disturbances [14,15]. Multi-generational succession of plantations has caused significant impacts to soil bacterial and fungal community diversity, richness and metabolic activities in Eucalyptus plantations [11,16]. Studies have found that soil fungal and bacterial community diversity significantly decreased in Eucalyptus plantations with a 5-year cultivation period, leading to a negative impact on soil multi-functionality and microbial communities [11,17].

The operation mode of short-cycle, multi-generational successive plantations of Eucalyptus plantations is highly controversial [5]. Addressing soil quality issues associated with short-cycle successive plantations is necessary for the sustainable development of Eucalyptus plantations. Extending the age of Eucalyptus plantations and maintaining a near-natural state of operation can improve soil quality, but this may not meet market demand and may result in job losses. For some foresters and Eucalyptus investors, fund recovery is especially important. Balancing ecological problems and practical production needs is crucial when extending the cultivation period. Therefore, the cultivation period can only be reasonably extended from the existing ultra-short cultivation period. However, current studies have focused mainly on the 4–5-year ultra-short cultivation period, and it is still uncertain whether successive plantation is beneficial to tree growth and soil quality beyond the 5–7-year cultivation period.

Based on the above background, in this study, the first, second, and third generations of 8-year-old Eucalyptus plantations in the Guangxi state-owned Dongmen Forest Farm was used as the research object, in consideration of the fact that 8 years is not very short compared with the prevailing rotation cycle, but it is also not a particularly long period for some foresters who are under pressure to attain a cash flow. Tree growth and soil nutrient indicators in two soil layers were assessed and soil bacteria and fungi were identified with high-throughput sequencing to determine the effects of successive planting on tree growth, soil nutrients, and bacterial–fungal community structure and diversity. Our hypotheses were that (1) successive Eucalyptus plantations with an 8-year cultivation period had effects on stand growth, soil physical and chemical properties, and soil bacterial and fungal structure; (2) with successive generations, tree growth did not decline, some soil nutrients were replenished, and soil bacterial and fungal diversity and richness increased; and (3) changes in soil bacterial and fungal community structure were linked to changes in one or more of the soil’s physical and chemical properties. The study aimed to determine the effects of successive planting on tree growth, soil nutrients, and bacterial–fungal community structure and diversity of Eucalyptus plantations with an 8-year cultivation period, and to establish the relationship between soil nutrients and microbial community diversity and structure. It also aimed to promote the sustainable development of Eucalyptus plantation forests.

2. Materials and Methods

2.1. Study Area

The experimental site was located in the Guangxi state-owned Dongmen Forest Farm in Dongmen Town, Fusui County, Guangxi, and the test area was about 5 ha. The distance between the three stands of first generation plantation, second generation plantation and third generation plantation did not exceed 1 km, and the plantation conditions, planting density and forestry measures were the same. The initial planting density was 1665 trees/ha. All three stands were fertilized with 622.5 kg/ha of compound fertilizer (composition: total nutrients ≥ 30%, N-P₂O₅-K₂O: 15-6-9, organic matter ≥ 15%) per year until 2020. After 2020, annual application of 1245 kg/ha organic fertilizer (composition: effective live bacteria ≥ 200 million/g, organic matter ≥ 40%, nitrogen, phosphorus and potassium ≥ 8%, plus amino acids, humic acid and trace elements) was added. The soil of the three stands is a typical brick red loam red soil in the Dongmen area, with a pH value between 4.5 and 5.6. The topography is low hills, with an elevation of around 100–130 m and a slope of 8–12°. The average annual temperature is 21.2–22.3 degrees Celsius, with extreme maximum temperature of 38–41 degrees Celsius and extreme minimum temperature of −4–1.9 degrees Celsius. The annual frost-free period is 346 days, the annual rainfall is 1000–1300 mm, and the relative humidity is 74–83%.

2.2. Site Setup and Stand Growth Survey

The experiment was selected from three different stands of the first, second, and third generation plantations of the fast-growing Eucalyptus DH32-26 asexual line at the age of 8 years. The first-generation stands were planted in 2014, the second and third generation stands were obtained from felled stump sprouts after felling of the previous generation stand. Three standard sample squares with an area of 20 m × 20 m were selected from each of the three generations, for a total of nine sample squares. The height of the trees in each sample square was measured with a height meter and recorded as H. The diameter at breast height of the trees in each sample square was measured with a diameter at breast height ruler at 1.3 m and recorded as D. The wood volume was estimated according to the Guangxi Eucalyptus binary material accumulation table. Visual inspection and recording of Eucalyptus leaves in each sample plot revealed no obvious disease or pest infestation.

2.3. Soil Sampling

Three standard sample squares (9 in total) with an area of 20 m × 20 m were selected from each of the three generations, and a near-center point was selected within each sample square. Six sampling points were established at a distance of 5 m from the near-center point along the 0°, 60°, 120°, 180°, 240°, and 300° direction lines. After removing fresh and semi-decomposed litter residues from the soil surface, the original soil was collected in the 0–20 cm and 20–40 cm soil layers. Careful measures were taken to avoid soil clods and disturbance from external soil during the digging process. Soil from the six sampling points in each sample plot were mixed, and 10–25 g was collected and stored on dry ice in sterile centrifuge tubes for soil bacterial and fungal DNA extraction and sequencing. The remaining soil samples were packed in square aluminum boxes and transported back to the laboratory for sieving and storage at 4 °C to facilitate testing of soil physical and chemical properties.

2.4. Soil Nutrient Analysis

Soil pH was determined by mixing the 10 g air-dried soil sample and deionized water (1:2.5 soil:water ratio) and, after the soil solution had clarified, the supernatant was tested [18]. Organic matter (OM) was quantified using the potassium dichromate-sulfate colorimetric method [19]. The total nitrogen (TN) and the soil’s ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃⁻-N) were determined by the continuous flow analyzer method. Total phosphorus (TP) was determined by the HCLO₄–H₂SO₄ method. Available phosphorus (AP) was determined by the double acid leaching–molybdenum antimony anti-colorimetric method [20,21]. Total potassium (TK) and available potassium (AK) were determined by flame spectrophotometric method [22].

2.5. Soil Bacterial and Fungal DNA Extraction and Sequencing

The DNA was extracted from the soil subsamples stored at −80 degrees Celsius with the TGuide S96 Magnetic Soil /Stool DNA Kit (Tiangen Biotech (Beijing, China) Co., Ltd., Beijing, China) according to manufacturer’s instructions. The DNA concentration of the samples was measured with the Qubit dsDNA HS Assay Kit and Qubit 4.0 Fluorometer (Invitrogen, Thermo Fisher Scientific, Portland, OR, USA). The bacterial 16S rRNA V3–V4 region was amplified using the primer pair 338F (5′-ACTCCTACGGGAGGCAGCAG-3′) and 806R (5′-GGACTACHVGGGTWTCTAAT-3′). The fungal rDNA ITS2 region was amplified using PCR with the primers ITS2F (5′-GCATCGATGAAGAACGCAGC-3′) and ITS2R (5′-TCCTCCGCTTATTGATATGC-3′). After amplification, PCR products were detected by electrophoresis using agarose of 1.8% (manufacturer: Boomerang Fuxin Technology (Beijing, China) Co., Ltd., Beijing, China). The total of PCR amplicons were purified with Agencourt AMPure XP Beads (Beckman Coulter, Indianapolis, IN, USA) and quantified using the Qubit dsDNA HS Assay Kit and Qubit 4.0 Fluorometer. After the individual quantification steps, amplicons were pooled in equal amounts to construct sequencing libraries, and the libraries that passed quality control were sequenced using an Illumina novaseq 6000 (Illumina, Santiago, CA, USA). The raw data were quality filtered using Trimmomatic (version 0.33). Primers were identified and removed using Cutadapt (version 1.9.1), followed by the splicing of double-end reads and the removal of the chimera (UCHIME, version 8.1) using USEARCH (version 10) to provide high quality sequences. The amplicon sequence variants (ASVs) were obtained by denoising the sequences using the dada2 method included in QIIME2 software, and finally the feature sequences were annotated taxonomically using a plain Bayesian classifier based on the ASVs sequence information with Silva.138 as the reference database.

2.6. Data Processing and Statistical Analysis

The differences between tree growth data and soil physical and chemical properties were analyzed using one-way ANOVA (LSD) in Excel and SPSS 26.0 (SPSS Inc., Chicago, IL, USA) software (p < 0.05 or p < 0.01). The results were then presented using histograms and box line plots created in Origin2022. Soil fungal and bacterial AVS Venn diagrams were produced using a highly customizable Venn and Euler diagram package in the R language, the visualization parameters were as follows: alpha = 0.50, fontfamily = ‘serif’, margin = 0.2. The inter-group variability of soil fungal and bacterial communities was compared using PCA analysis, and PCA plots were drawn using Origin 2022 based on the Bray–Curtis algorithm. Pearson correlation analysis was performed using SPSS 26.0 (SPSS Inc., Chicago, IL, USA) to examine the correlation between soil fungal and bacterial diversity and soil physicochemical properties. The relationship between soil physicochemical properties and fungal and bacterial community structure was determined using RDA analysis, RDA was conducted using CANOCO software for Windows (version 5.0). The bacterial and fungal community diversity index (Chao1) and richness index (Shannon) were analyzed by software QIIME2 (https://qiime2.org/, Date accessed: 1 October 2022).

3. Results

3.1. Tree Growth Status

The growth indexes of the trees were significantly different among different generations and showed an increasing trend with the addition of generations, as depicted in Figure 1. The average tree height of the first, second and third generation plantations were 24.63 m, 24.49 m and 28.53 m, respectively. The average tree height of the third generation plantation was significantly higher than that of first and second generation plantations (p < 0.05), with an increase of 15.87% and 16.53%, respectively. The mean diameter at breast height of the third generation plantation was significantly larger than that of the first generation plantation (p < 0.05), with an increase of 10.94%. The average individual-tree volume of the third generation plantation was 1.43 times that of the first generation plantation and 1.30 times that of the second generation plantation (p < 0.05).

3.2. Changes in Soil Nutrients

Differences in soil properties were observed among generations (p < 0.05,

Table 1. Effect of different successive planting generations on soil physicochemical properties of Eucalyptus plantations.

Indicator	Soil Layer/cm	First Generation	Second Generation	Third Generation
pH	0–20	4.76 ± 0.50 a	4.52 ± 0.10 a	4.51 ± 0.11 a
	20–40	4.62 ± 0.06 a	4.57 ± 0.06 a	4.50 ± 0.11 a
OM (g·kg−1)	0–20	27.91 ± 1.92 b	28.85 ± 1.08 b	39.30 ± 0.79 a
	20–40	26.60 ± 2.10 b	27.70 ± 1.26 b	36.96 ± 0.97 a
TN (g·kg−1)	0–20	1.06 ± 0.16 a	0.85 ± 0.01 ab	0.72 ± 0.04 b
	20–40	0.89 ± 0.09 a	0.85 ± 0.06 a	0.79 ± 0.20 a
NH4+-N (mg·kg−1)	0–20	17.62 ± 0.81 a	14.20 ± 0.42 b	14.44 ± 0.49 b
	20–40	14.64 ± 0.66 a	14.77 ± 0.84 a	16.31 ± 1.04 a
NO3−-N (mg·kg−1)	0–20	15.21 ± 0.0.48 a	15.94 ± 0.42 a	13.82 ± 0.20 b
	20–40	14.54 ± 0.26 a	15.31 ± 0.59 a	14.85 ± 0.59 a
TP (g·kg−1)	0–20	0.35 ± 0.01 b	0.70 ± 0.49 a	0.73 ± 0.30 a
	20–40	0.38 ± 0.05 b	0.61 ± 0.01 a	0.66 ± 0.04 a
AP (mg·kg−1)	0–20	1.07 ± 0.05 a	0.98 ± 0.05 a	1.29 ± 0.15 a
	20–40	1.03 ± 0.01 b	1.06 ± 0.01 ab	1.10 ± 0.03 a
TK (g·kg−1)	0–20	2.47 ± 0.09 a	2.62 ± 0.49 a	2.19 ± 0.20 a
	20–40	2.70 ± 0.18 a	2.32 ± 0.53 a	3.18 ± 0.57 a
AK (mg·kg−1)	0–20	33.28 ± 1.03 a	27.29 ± 1.34 b	22.78 ± 1.14 c
	20–40	24.12 ± 1.96 a	22.44 ± 1.21 a	19.56 ± 1.96 a

All results in the table are the mean ± standard error. OM: organic matter, TN: total nitrogen, NH₄⁺-N: ammoniacal nitrogen, NO₃⁻-N: nitrate nitrogen, TP: total phosphorus, AP: available phosphorus, TK: total potassium, AK: available potassium. Different lowercase letters indicate significant differences (p < 0.05).

). In the 0–20 cm soil layer, pH did not change significantly (p < 0.05) and OM content (27.91–39.30 g·kg⁻¹) showed a significant upward trend (p < 0.05). TN content decreased, with the third generation plantation TN content (0.72 g·kg⁻¹) significantly lower than that of the first generation plantation (1.06 g·kg⁻¹). NH₄⁺-N content was significantly lower than that of the first generation plantation (17.62 mg·kg⁻¹) in both the second and third generation plantations (p < 0.05). NO₃⁻-N content was the lowest at 13.82 mg·kg⁻¹ in the third generation plantation, significantly lower than that in the first and second generation plantations (15.21 mg·kg⁻¹, 15.94 mg·kg⁻¹) (p < 0.05). The TP content increased, and the first generation plantation of 0.35 g·kg⁻¹ was significantly lower than that of the second and third generation forests (p <0.05). AP content showed a slight decrease and then an increase. The highest TK content was 2.62 g·kg⁻¹ in the second generation plantation. The soil AK content (22.78–33.28 g·kg⁻¹) decreases significantly in each generation. In the 20–40 cm soil layer, the contents of pH, TN and AK decreased slightly, but the content of NH₄⁺-N, NO₃⁻-N and TK increased. The soil OM, TP and AP content increased significantly (p < 0.05) and reached the maximum value in the third generation plantation.

3.3. Diversity and Structure of Soil Bacterial and Fungal Communities

The number of amplicon sequence variants (ASVs) measured for bacteria in the 0–20 cm soil layer of the plantation was 910, 992, and 993, for the first, second and third generation plantations, respectively, and the number of ASVs measured in the 20–40 cm soil layer was 923, 978, and 960, respectively, which increased with the addition of successive planting generations (Figure 2). The number of specific ASVs of bacteria was greater than the common number among the three generations, indicating that there were large differences in bacterial communities among them. The number of ASVs of fungi in the 0–20 cm soil layer was 323, 489, and 471, for the first, second and third generation plantations, respectively, and the number of ASVs in the 20–40 cm soil layer was 352, 545, and 381, respectively, all of which were the smallest in the first generation plantation. The number of fungal-specific ASVs was greater than the common number among the three generations, indicating that there were large differences in the fungal communities among them.

PCA analysis of soil bacterial and fungal community structure in different successive plantation generations is presented in Figure 3. The sample points for soil fungal and bacterial groups in the first, second, and third generation plantations were located in the same quadrant, suggesting that the composition of these three generations was quite similar within their respective groups. Conversely, sample points between the groups were scattered and distributed in different quadrants, indicating significant differences between the groups in terms of the structure of the bacterial and fungal communities across the three generations.

Table 2. Analysis of bacterial and fungal community structure diversity in different generations of Eucalyptus plantation.

Diversity	Index	Soil Layer/cm	First Generation	Second Generation	Third Generation
Bacterial	Chao1	0–20	417.67 ± 8.69 c	534.62 ± 11.59 a	487.73 ± 7.83 b
		20–40	436.58 ± 11.51 b	535.61 ± 15.32 a	507.33 ± 17.34 a
	Shannon	0–20	7.17 ± 0.21 B	8.27 ± 0.04 A	8.05 ± 0.07 A
		20–40	6.96 ± 0.02 B	8.21 ± 0.03 A	8.00 ± 0.09 A
Fungal	Chao1	0–20	142.78 ± 7.01 b	205.33 ± 8.86 a	205.50 ± 7.18 a
		20–40	163.25 ± 12.25 b	222.44 ± 11.33 a	172.61 ± 4.40 b
	Shannon	0–20	3.50 ± 0.30 b	5.40 ± 0.24 a	4.85 ± 0.19 a
		20–40	3.56 ± 0.31 b	6.19 ± 0.68 a	4.19 ± 0.36 b

All results in the table are the mean ± standard error. Different lowercase letters indicate significant differences (p < 0.05) and different capital letters indicate highly significant differences (p <0.01).

demonstrates that the structural richness and diversity of soil bacterial and fungal communities increased significantly with successive planting generations. The Chao1 indices of soil bacteria and fungi increased with successive planting generations in the 0–20 cm soil layer and were significantly lower in the first generation plantation compared with the second and third generation plantations (p < 0.05). In the 20–40 cm soil layer, the soil bacterial Chao1 index was significantly higher in the second and third generation plantations compared with the first generation plantation (p < 0.05), while the soil fungal Chao1 index was highest in the second generation plantation and significantly lower in the first and third generation plantation compared with the second generation plantation (p < 0.05). The Shannon index of soil bacteria in both the 0–20 cm and 20–40 cm soil layers increased with successive planting generations, with the largest index observed in the second generation plantation, and a significantly lower index in the first generation plantation compared with the second and third generation plantations (p < 0.05). The Shannon index of soil fungi in both the 0–20 cm and 20–40 cm soil layers initially increased and then decreased with successive planting generations. In the 0–20 cm soil layer, the first generation plantation had a significantly lower index compared with the second and third generation plantations (p < 0.05), while in the 20–40 cm soil layer, the second generation plantation had a significantly larger index than the first and third generation plantations (p < 0.05).

Bacterial and fungal communities showed significant variations at the phylum level across different generations of successive plantation. Figure 4 reveals that the relative abundance of Proteobacteria, the dominant bacterial phylum, differed significantly between soil layers in successive planting generations (p < 0.05). Moreover, the relative abundance of Acidobacteriota, another dominant bacterial phylum, showed a highly significant difference (p < 0.01). Similarly, Chloroflexi, Gemmatimonadota, Verrucomicrobiota, Myxococcota, and other bacterial phyla displayed significant differences (p < 0.05). As for fungi, the relative abundance of Ascomycota increased significantly in the 20–40 cm soil layer with successive planting generations (p < 0.05). Furthermore, the dominant phylum Basidiomycota showed a significant difference between the two soil layers (p < 0.05), while Chytridiomycota, Rozellomycota, and Glomeromycota also exhibited significant differences among generations (p < 0.05).

3.4. Relationship between Soil Nutrients and the Diversity and Structure of Bacterial and Fungal Communities

Soil nutrients were significantly correlated with bacterial and fungal community diversity. As shown in

Table 3. The Pearson correlation analysis of soil physicochemical properties and bacterial and fungal community diversity, n = 9.

Indicator	Soil Layer/cm	Bacterial		Fungal
		Chao1	Shannon	Chao1	Shannon
pH	0–20	−0.643	−0.614	−0.524	−0.629
	20–40	−0.591	−0.201	−0.258	−0.223
OM (g·kg−1)	0–20	0.076	0.229	0.439	0.321
	20–40	0.182	0.275	−0.067	−0.317
TN (g·kg−1)	0–20	−0.423	−0.380	−0.637	−0.492
	20–40	−0.046	−0.286	−0.001	−0.215
NH4+-N (mg·kg−1)	0–20	−0.847 **	−0.948 **	−0.726 *	−0.883 **
	20–40	0.026	0.232	−0.013	−0.372
NO3−-N (mg·kg−1)	0–20	0.180	−0.126	−0.129	−0.044
	20–40	0.038	0.400	0.356	0.033
TP (g·kg−1)	0–20	0.881 **	0.896 **	0.766 *	0.838 **
	20–40	0.356	0.570	0.268	0.383
AP (mg·kg−1)	0–20	−0.074	0.115	0.120	−0.222
	20–40	0.680 *	0.508	−0.033	0.238
TK (g·kg−1)	0–20	−0.076	0.061	0.074	0.035
	20–40	−0.208	−0.066	0.212	−0.202
AK (mg·kg−1)	0–20	−0.450	−0.416	−0.224	−0.570
	20–40	−0.248	−0.153	0.084	−0.435

** Significant correlation at 0.01 (bilateral), * correlation at 0.05 (bilateral).

, in the 0–20 cm soil layer, NH₄⁺-N had a significant negative correlation (−0.726 < r < −0.948) with Chao1 and Shannon indices for both bacterial and fungal community diversity (p < 0.01). TP had a significant positive correlation (0.766 < r < 0.896) with Chao1 and Shannon indices for both bacterial and fungal community diversity (p < 0.01). AP showed significant positive correlation (p < 0.05) with bacterial Chao1 index in the 20–40 cm soil layer.

The RDA results show that, in the 0–20 cm soil layer (Figure 5a), the bacterial community structure was mainly driven by pH, TN, TP and NH₄⁺-N factors. The first axis (RDA1) explains 56.59% of the total variation of bacterial phyla, the second axis (RDA2) explains 19.11% of the total variation of bacterial phyla, and both axes explain 75.70% of the total bacterial variation. The bacterial dominant phylum Acidobacteriota showed a strong positive correlation with TP and a strong negative correlation with TN, NO₃⁻-N, and pH. The bacterial dominant phylum Proteobacteria showed positive correlations with OM and AP. The bacterial dominant phylum Chloroflexi showed a positive correlation with NO₃⁻-N. Gemmatimonadota, Methylomirabilota, Planctomycetota, Nitrospirota, and Bacteroidota showed strong correlations with pH, TN, NO₃⁻-N, and AK.

In the 20–40 cm soil layer (Figure 5b), bacterial community structure was mainly driven by pH, OM, AP and AK factors. The first axis (RDA1) explains 69.27% of the variation and has the strongest positive correlation with pH. The second axis (RDA2) explains 14.64% of the variation and has the strongest positive correlation with AP. Both axes explain 83.91% of the variation in environmental factors and bacterial community structure. The bacterial dominant phylum Acidobacteriota was positively correlated with TP and negatively correlated with pH, AK, and TN. The bacterial dominant phylum Chloroflexi was strongly positively correlated with NO₃⁻-N.

The results of the RDA between the fungal community structure and soil physicochemical properties in the 0–20 cm soil layer are shown in Figure 5c. The first axis (RDA1) explains 59.42% of the variation, the second axis (RDA2) explains 17.54% of the variation, with the strongest positive correlations with TP. Both axes explain 76.96% of the variation in environmental factors and bacterial community structure. TN, NH₄⁺-N, TP and TK are the main drivers of fungal community structure, mainly manifested in the strong positive correlation between NH₄⁺-N and Basidiomycota and Calcarisporiellomycota, and a strong negative correlation with Mortierellomycota, Mucoromycota, and TP phosphorus. TK was positively correlated with Olpidiomycota and Chytridiomycota.

In the 20–40 cm soil layer (Figure 5d), the fungal community structure was mainly driven by OM, TP, TK, and AK factors. The first axis (RDA1) explains 73.17% of the variation, with positive contributions from most physical and chemical property factors, the second axis (RDA2) explains 14.66% of the variation, with a positive correlation with TP. Both axes together explain 87.83% of the variation in environmental factors and bacterial community structure. Mortierellomycota, Chytridiomycota, and Olpidiomycota were positively correlated with AK, NO₃⁻-N, and pH, while they were negatively correlated with TP, AP, and TK. Ascomycota, Mucoromycota, Calcarisporiellomycota, and Rozellomycota were negatively correlated with OM, NH₄⁺-N, and TN.

4. Discussion

4.1. Variation in Growth of Successive Plantations of Eucalyptus

Eucalyptus plantations with an eight-year cultivation period showed a significant increase in growth index with an increase of successive planting generations (p < 0.05). The third generation of plantation had the highest average tree height, diameter at breast height, and wood volume per tree. Compared with the first generation of plantation, there was an increase of 15.87%, 10.94%, and 42.58% in these parameters, respectively. This indicates that, with a cultivation period of eight years, the successive plantation increased growth of Eucalyptus plantations.

The significant increase in stand growth at the study site can be attributed to several factors. Firstly, the Guangxi state-owned Dongmen Forest Farm has excellent maintenance and management practices for Eucalyptus plantations and uses high-quality strains for planting. Secondly, the second and third generation plantations have well-developed root systems due to budding from the previous generation, which reduces their demand for soil nutrients. Eucalyptus is also known for its fast growth rate, and the growth indexes of the third generation plantation are exceptional under adequate nutrient supply. Thirdly, as successive planting generations are added, the content of OM, TP, and AP increases. OM is a significant source of plant nutrients that retains water and absorbs cations [23], and it can also serve as food for soil microorganisms, which enhances soil activity and physical properties. This further promotes forest tree growth [24]. TP and AP are essential macronutrients required for plant growth and metabolism, and they are also critical components of the photosynthesis process [25,26]. Most activities, such as plant growth, respiration and reproduction, depend on the phosphorus level in the soil [27], so the rise of TP and AP content also promotes the growth of trees.

4.2. Changes in Soil Nutrients in Successive Plantations of Eucalyptus

The study found that, after an eight-year period of cultivating Eucalyptus, the negative impact of successive planting on soil nutrients decreased and some nutrients were returned. In the 0–20 cm soil layer, N content decreased significantly (p < 0.05), indicating that N supplementation is necessary during successive planting. However, OM and TP content significant increased with the increase of successive planting generations (p < 0.05). In contrast, in the 20–40 cm soil layer, most indicators showed insignificant changes, with only the soil OM, TP, and AP content increased significantly. This could be because the surface soil layer (0–20 cm) is more affected by external environmental conditions compared with the deeper layer (20–40 cm), which has a more stable soil environment [28]. The increase in some nutrient content in both soil layers can be attributed to the eight-year cultivation period allowing for some repair of the damage caused by full reclamation methods. Additionally, as the age of the plantation trees increased, the biodiversity of the understory also increased [29,30]. As a result, the forest’s soil nutrient content was replenished [29,31].

4.3. Diversity and Structure Change of Bacterial and Fungal Communities in Continuous Eucalyptus Plantation

The diversity of soil bacteria and fungi in Eucalyptus plantations differed significantly among successive planting generations (p < 0.05). The Chao1 index for bacteria was highest in both soil layers of the second generation plantation, increasing by 28.00% and 22.91% compared with the first generation plantation. The bacterial Shannon index was also highest in the second generation plantation. Additionally, fungal community richness and diversity were best in the second generation plantation and lowest in the first generation plantation. PH, NH₄⁺-N, TP and AP factors were found to mainly drive bacterial community structure, while NH₄⁺-N, TP, TK, and OM factors drove fungal community structure, as determined by RDA results. Correlation analysis showed that changes in soil bacterial and fungal community diversity were significantly related to NH₄⁺-N, TP, and AP content (p < 0.05), with NH₄⁺-N showing mainly negative correlation, indicating a lesser diversity index in higher NH₄⁺-N soils. The diversity and richness of soil bacterial and fungal communities in different successive planting generations of Eucalyptus plantations may be affected by microclimate changes, litter production, secretions, and root symbionts, as well as root-associated microorganisms [32,33]. The reasonable cultivation period may also indirectly contribute to an increase in understory vegetation diversity and soil faunal diversity [34].

The study found significant changes in the structure of soil bacterial and fungal communities in Eucalyptus plantations across different succession generations (p < 0.05). Acidobacteriota, Proteobacteria, and Chloroflexi showed a significant increase in relative abundance with successive planting generations. Moreover, Acidobacteriota was positively correlated with TP, Proteobacteria with OM and AP, and Chloroflexi with NO₃⁻-N, which are the primary drivers of changes in bacterial community structure. Basidiomycota and Ascomycota are the two main fungal groups in forest soils [35,36,37], with Basidiomycota being capable of completely decomposing lignin in soil litter into water and carbon dioxide [38]. The relative abundance of Ascomycota increased slightly and then decreased significantly with increasing generations in the 20–40 cm soil layer. The relative abundance of Ascomycota was significantly lower in the third generation plantation than in the second generation plantation. The relative abundance of the dominant phylum Basidiomycota decreased significantly with the addition of successive planting generations, driven mainly by NH₄⁺-N, TN, and pH.

5. Conclusions

In conclusion, there were significant differences in tree growth, soil physical and chemical properties, and soil fungal bacterial structure between successive plantation generations of Eucalyptus plantations with an eight-year cultivation period. The results of the study show that the growth of the stands became better with the addition of successive generations. The diversity and richness of soil bacterial and fungal communities were significantly enhanced. The content of some nutrients, such as OM, TP, and AP, also increased with the addition of successive planting generations. Consequently, successive plantings of Eucalyptus plantations during the eight years of cultivation period is reasonable, and it improves the productivity of Eucalyptus plantations. It brings efficient economic benefits and alleviates soil quality problems caused by short-cycle successive planting of Eucalyptus and further solves social and environmental problems. This demonstrates that the use of more scientific and quantitative plantation management measures can maintain the trend of increasing productivity and soil fertility of eucalyptus plantations even after multiple generations of successive plantings. This study provides an empirical basis for the multi-generational succession management of Eucalyptus in Guangxi and promotes the sustainable development of the Guangxi Eucalyptus plantation forest industry.