Nutrient Stoichiometry and Tree Development: Insights from a 5-Year Study on Catalpa bungei Fertilization

Abstract

Short-term fertilization may provide limited improvements in tree growth and demonstrate suboptimal fertilizer efficiency; however, its benefits often fall short of expectations. Unfortunately, research addressing the sustained impacts of prolonged fertilization (e.g., beyond five years) on trees’ developmental dynamics and productivity remains relatively scarce. This study focused on a 7-year-old Catalpa bungei plantation located in Jinan City, Shandong Province, China. The study employed two fertilization techniques: hole fertilization (HF) and integrated water and fertilizer application (WF), with a no-fertilization treatment serving as the control (CK). The findings revealed that the WF significantly enhanced stand productivity. When comparing the different treatments, the productivity of WF stands demonstrated a remarkable increase of 39.7% compared to HF stands and 55.1% compared to CK stands. After five years of fertilization, the stands treated with WF exhibited a significant increase in volume accumulation, reaching 112.36 m³·hm⁻². Additionally, the productivity of these WF-fertilized stands achieved an impressive 41.75 m³·hm⁻²·a⁻¹. Fertilization notably enhanced the nitrogen content in the leaves and fine roots of C. bungei, as well as the potassium content in the coarse roots. These nutrients were found to be more concentrated in the corresponding organs within the WF stands. Over the entire growth cycle, there was a substantial consumption of key nutrients, with leaf nitrogen, phosphorus, and potassium contents decreasing by 30.5%, 18.8%, and 47.3%, respectively. Similarly, the coarse root potassium and fine root phosphorus content decreased by 24.7% and 24.4%, respectively. The enhancement in leaf nitrogen content following fertilization significantly contributed to increases in tree height, breast height diameter (DBH), and individual tree volume. Similarly, the enrichment of potassium in the branches and coarse roots was associated with improvements in DBH and tree volume. To maximize forest stand productivity, the WF fertilization method demonstrated superior results compared to HF. Therefore, WF should be prioritized in future fertilization experiments for C. bungei.

1. Introduction

Currently, the development of high-quality cultivation techniques for artificial forests remains a prominent area of interest among scholars. Catalpa bungei, a valuable broad-leaved species indigenous to China, has garnered significant attention within the forestry sector. The research on its cultivation technology continues to be a central focus, given its importance in enhancing the quality and sustainability of forest management practices. Fertilization plays a crucial role in accelerating the growth of C. bungei [1]. The choice of fertilizer, application method, and duration of fertilization are key factors that significantly influence the dynamic growth patterns of these trees. These variables determine the effectiveness of fertilization in enhancing tree development and optimizing forest management outcomes.

Ecological stoichiometry examines the elemental composition required by organisms—primarily carbon, nitrogen, and phosphorus—and the complex balance between these elements. In terrestrial ecosystems, nitrogen and phosphorus are key limiting nutrients, while carbon, assimilated through photosynthesis, serves as a critical substrate for plant growth [2,3,4]. Fertilization directly alters soil nutrient concentrations, which in turn influences the levels of nitrogen and phosphorus within plants and affects carbon fixation processes. These changes ultimately impact the stoichiometric ratios of nutrients in plant tissues [5,6]. Variations in the nitrogen-to-phosphorus ratios across different plant organs can serve as indicators of the nutrient availability and supply status within the ecosystem, providing valuable insights into the effectiveness of fertilization practices [6]. Moreover, the carbon-to-nitrogen (C:N) and carbon-to-phosphorus (C:P) ratios in various plant organs serve as valuable indicators of how plant growth rates respond to nutrient availability [7,8]. These ratios provide insights into the efficiency of nutrient utilization and the overall physiological responses of plants to different nutrient supply conditions. Analyzing the quantitative characteristics of plant nutrients within an ecological framework can significantly enhance our understanding of nutrient limitations in plant growth. This approach allows for the identification of key limiting elements, thereby improving the accuracy of nutritional diagnosis and informing more effective fertilization strategies in future forest management practices [9].

Integrated water and fertilizer application is a crucial technology in agricultural production, known for its ability to enhance water and nutrient use efficiency. This method is widely adopted to boost the growth of various crops, including tomatoes, soybeans, and corn [10,11,12,13,14,15]. Despite its proven effectiveness in agriculture, the application of this technology in forestry is still in its early stages. As such, further research and development are needed to fully realize its potential in forest management. Research has shown that employing integrated water and fertilizer systems in Populus tomentosa plantations can lead to a reduction in fertilizer usage compared to traditional fertilization methods [16,17]. Additionally, this approach has been found to significantly enhance both the biomass and carbon storage capacity of poplar plantations [17]. These findings suggest that integrated water and fertilizer techniques offer a more efficient and sustainable alternative to conventional fertilization practices in forestry. Currently, there is limited research on the application of integrated water–fertilizer technology in monitoring the long-term dynamic growth, productivity, and nutrient stoichiometric characteristics of forest ecosystems. This gap highlights the need for more comprehensive studies to explore the potential benefits and challenges of this technology in forestry over extended periods.

Catalpa bungei, a valuable deciduous tree belonging to the Bignoniaceae family, is renowned for its ornamental appeal and economic significance. Known for its straight trunk and attractive wood grain, C. bungei commands a high market price, reaching up to 5000 RMB/m³ [18]. Its exceptional qualities make it highly sought after in both the timber and ornamental plant markets. Currently, research on C. bungei predominantly focuses on fertilization during the seedling stage. Key areas of study include formula fertilization [19], water–fertilizer coupling [20], and exponential fertilization techniques [21]. These studies aim to optimize early-stage growth and establish best practices for nurturing healthy seedlings. C. bungei plantations are increasingly favored for their dual benefits of timber and carbon sequestration. Recent studies on C. bungei plantations have increasingly focused on several key areas, including the optimization of planting density [22], strategies for effective fertilization [23], and the implementation of diverse cultivation practices [24]. The existing research primarily examines the growth status of C. bungei at specific time points under various silvicultural practices. However, there is a notable scarcity of studies that investigate long-term growth dynamics and the continuous changes that occur over time. Stand productivity is a critical indicator of forest growth and warrants a thorough investigation into its dynamic fluctuations over time.

Extensive plantations of C. bungei have been established in Jinan, Shandong Province, China. However, the region’s average annual precipitation, which falls below 650 mm, imposes limitations on the growth of C. bungei due to inadequate water supply. In practical applications, traditional management practices have resulted in inefficient use of water and fertilizer resources in C. bungei forests, leading to a decrease in their utilization rates. This inefficiency adversely impacts site quality and forest productivity. Adopting innovative fertilization methods is essential for enhancing the quality of C. bungei forests and optimizing the efficiency of water and fertilizer use. Exploring and implementing more effective strategies will address these challenges and contribute to improved forest management outcomes. Given the current trends in industry development, the advancement of integrated water–fertilizer technology represents a promising area of growth. This approach is poised to significantly enhance resource efficiency and sustainability, offering considerable potential for improving agricultural and forestry practices. In light of these considerations, this study focused on an artificial young forest of C. bungei in Zhangqiu District, Jinan City, Shandong Province. It employed integrated water–fertilizer technology, comparing its effects against traditional hole fertilization and a no-fertilization control. The objective was to investigate how different fertilization strategies impact the dynamic growth, productivity, and ecological stoichiometry of C. bungei forests. Following five years of fertilization research, this study proposed two key hypotheses. First, it is anticipated that C. bungei plantations under integrated water fertilization conditions will exhibit higher productivity compared to those under traditional hole fertilization and no-fertilization controls. Second, it is hypothesized that water fertilization trees receiving integrated water fertilization will have elevated nitrogen and phosphorus levels relative to those subjected to other fertilization methods, potentially enhancing tree growth to a significant extent.

2. Materials and Methods

2.1. Information on Sample Plots

The plot is situated at Zaoyuan Nursery Reservoir in Zhangqiu District, Jinan City, Shandong Province, China, within the coordinates 36°25′–37°09′ N and 117°10′–117°35′ E. The plot is located at an altitude of 140 m. The area experiences a temperate monsoon climate, characterized by an average annual temperature of 12.8 °C. The hottest month is July, with an average temperature of 27.2 °C, while the coldest month, January, sees an average low of −3.2 °C. The region has a frost-free period of 192 days, an average annual precipitation of 600.8 mm, and receives approximately 2647.6 h of sunshine each year. Zhangqiu District features a variety of soil types, including drab soil, fluvo-aquic soil, and brown soil, with the research plot specifically situated on brown soil. For a detailed overview of the average monthly temperature and precipitation patterns from 2017 to 2023, refer to Figure S1.

2.2. Experimental Materials

In March 2017, a 2-year-old clone of C. bungei, designated as “9−1”, was selected for afforestation. The plantation was established as a pure stand with a planting density of 833 trees per hectare, utilizing a wide-row, narrow-spacing configuration of 3 × 4 m. Prior to establishing the new plantation, the site was previously cultivated with a 15-year-old stand of Liriodendron tulipifera. During the site preparation for the new afforestation, the land was thoroughly prepared by digging planting holes and incorporating organic fertilizer as a foundational nutrient source. Three months following afforestation, the trees exhibited an average height of 4.2 m and an average diameter at breast height (DBH) of 4.0 cm. Following afforestation, the soil properties (0–20 cm depth) of the C. bungei plantation were analyzed. The results revealed a pH of 7.67, an organic matter content of 19.64 g·kg⁻¹, total nitrogen at 0.91 g·kg⁻¹, total phosphorus at 0.53 g·kg⁻¹, and total potassium at 16.7 g·kg⁻¹. Additionally, the soil contained 81.88 mg·kg⁻¹ of alkaline-hydrolyzable nitrogen, 32.10 mg·kg⁻¹ of available phosphorus, and 176.82 mg·kg⁻¹ of available potassium. The primary growing season for C. bungei spans from May to September each year, covering approximately 130 to 150 days. In the first year following afforestation, the trees exhibited a survival rate exceeding 95%. By the conclusion of the study, the tree preservation rate remained robust at 90%.

2.3. Fertilization Schedule

In early May 2018, a randomized block design was implemented to establish nine experimental plots, each covering an area of 384 m², for the purpose of studying fertilization effects. The design drawings of the sample plots referred to the literature of [25]. Each plot contained 45 trees, arranged in a grid of 5 rows by 9 columns. The study employed two fertilization treatments: integrated water–fertilizer application and hole fertilization, with a third set of plots left unfertilized to serve as a control. To ensure robust and reliable results, each fertilization treatment was randomly assigned across the nine experimental plots, with each treatment replicated three times. To minimize potential cross-contamination between plots, isolation belts measuring approximately 6 to 8 m in width were established between the plots. This layout was designed to maintain the integrity of the experiment by preventing interference among different treatment areas. The fertilization process commenced in May 2018 and continued until May 2023, spanning a total of five years. Based on prior fertilization trials, the initial application per plant consisted of 24 g of nitrogen (N), 8 g of phosphorus (P₂O₅), and 16 g of potassium (K₂O). To optimize growth and nutrient absorption, the fertilizer dosage was systematically increased by 20% annually throughout the study period. This progressive adjustment aimed to enhance the long-term effectiveness of the fertilization strategy.

The annual fertilizer dosage for both hole fertilization and integrated water–fertilizer treatments was consistent. In the hole fertilization method, a hole approximately 20 to 30 cm deep was dug at a distance of 50 cm on the north and south sides of the tree trunk (Figure S2). The fertilizer was evenly distributed around the tree roots in a single application, followed by a light watering to facilitate nutrient absorption. The hole was then covered with soil to retain moisture and nutrients, ensuring optimal conditions for root uptake. The integrated water–fertilizer approach involves dissolving the fertilizer in water and delivering it directly to the root system through an advanced drip irrigation system (HN-BXE, Huinong Automation Company, Beijing, China). This method divides the annual fertilizer requirement into 12 equal portions. Starting in early May, fertilization is conducted every 10 days, amounting to three applications per month. This schedule continues until early September, ensuring a consistent and efficient nutrient supply throughout the growing season. The control plots received no fertilization treatment or any other interventions, serving as a baseline for comparison against the fertilized plots. In the fertilization treatments, urea and potassium sulfate were utilized as sources of nitrogen and potassium, respectively. For phosphorus supplementation, superphosphate was applied in the hole fertilization method, while diammonium phosphate was used in the integrated water fertilization approach. The fertilizers purchased were all from the Taobao website (merchant name: Feiwocunshe). Following the establishment of the sample plots, five standard trees were selected within each plot based on tree growth survey data. These standard trees were chosen for their symmetrical crowns, straight trunks, and robust growth, ensuring they accurately represented the overall growth conditions of the entire plot.

The depiction of the integrated water and fertilizer intelligent drip irrigation system was referred to Figure S3. The primary role of water in this system was to dissolve the fertilizer, creating a liquid solution. For each fertilization cycle, the fertilizer was placed into a 500 L pressure barrel, where it was thoroughly dissolved. Using a pressurization device, the nutrient-rich solution was then delivered directly to the root zone of the trees. The process continued until the entire contents of the barrel were distributed, with each fertilization session lasting approximately 30 min. The drip irrigation system was designed with specific parameters to optimize water and nutrient delivery. The system had a flow rate of 40 m³/h, utilizing drip irrigation pipes with a diameter of 16 mm and a wall thickness of 1.0 mm. The drippers were spaced 50 cm apart, each delivering water at a rate of 2.2 L/h. The irrigation pipelines were laid flat on the ground and were constructed from 1.0 MPa UPVC material, ensuring durability and efficiency. The main pipe had a diameter of 110 mm, while the branch pipes measured 90 mm and 63 mm in diameter, respectively. The pipes were connected using sockets and spigot joints for secure assembly. The drippers were strategically positioned near the trees to ensure optimal water distribution (refer to Figure S4 for the layout).

2.4. Growth Investigation

In each experimental plot, 20 trees exhibiting uniform growth and robust health were selected for consistent long-term monitoring. Tree height and diameter at breast height were systematically measured at multiple intervals: May, August, and November in 2018, May–November in 2019, November in 2020, and May and November in 2021, 2022, and 2023. These measurements were conducted using precision tools, specifically a height meter and a diameter ruler, to ensure accurate tracking of growth dynamics over time.

2.5. Measurement of Organ Nutrients

In May (early growing season), mid-July (mid-growing season), and late September (late growing season) of 2021, branches from the southern side of the upper and middle canopy were carefully pruned. From these branches, 5 to 6 functional leaves were selectively harvested to serve as representative branch and leaf samples for analysis. Simultaneously, roots were excavated approximately 50 cm south of the trunk at a soil depth of 0–30 cm. These roots were then categorized based on their diameter into coarse roots (>2 mm) and fine roots (≤2 mm). After collection, the samples were promptly stored in a refrigerated incubator to preserve their integrity. Once transported to the laboratory, they were rinsed with purified water and left to air dry naturally. Subsequently, the samples were placed in an oven at 75 °C until they reached a constant weight. After drying, the samples were ground into a fine powder, sieved through a 2 mm mesh, and then sealed in storage bags for further analysis. The contents of nitrogen (N), phosphorus (P), and potassium (K) in the leaves, branches, coarse roots, and fine roots were analyzed following the protocol described by [26]. Based on these measurements, the ratios of N:P, N:K, and P:K were subsequently calculated to assess the nutrient balance within the different plant organs.

2.6. Data Analysis

The volume of an individual tree was calculated using the volume formula for broad-leaved trees commonly applied in North China, as outlined by [27]. The formula used is as follows:

$$\mathrm{V} =0.000057468552\times {\mathrm{D}}^{1.915559}\times {\mathrm{H}}^{0.9265972}$$

where V represents the volume of a single tree in cubic meters (m³), D is the diameter at breast height in centimeters (cm), and H denotes the tree height in meters (m).

To calculate the volume per hectare, the average volume of 20 trees in each plot was first determined. This average volume was then multiplied by the number of trees per hectare (1170 trees) to obtain the total volume per hectare. The forest stand productivity for that year was represented by this calculated volume per hectare.

Before performing variance analysis using IBM SPSS 25.0, it was essential to verify that the data meets the assumptions of independence, normality, and homogeneity of variances. Once these prerequisites were confirmed, a one-way analysis of variance (ANOVA) was conducted to assess the impact of different fertilization treatments on various growth parameters, including tree height, diameter at breast height, single tree volume, hectare volume, forest stand productivity, organ nutrient content, and stoichiometric ratios. Turkey’s HSD test was then used for post hoc comparisons to determine the significance of differences between treatments (p < 0.05). All charts were created using Origin Pro 2022.

3. Results

3.1. Growth Performance

Over the five-year period, growth indicators between the CK and HF treatments showed only slight variations. Specifically, tree height increments were 5.83 m for CK and 5.71 m for HF (Figure 1A). The DBH was recorded at 11.63 cm for CK and 11.67 cm for HF (Figure 1B). Similarly, the volume per individual tree was 0.090 m³ for CK compared to 0.094 m³ for HF (Figure 1C). Stand volume was measured at 83.95 m³·ha⁻¹ for CK and 86.88 m³·ha⁻¹ for HF (Figure 1D). These differences suggested a minimal impact of hole fertilization compared to the control over the studied period. Over a five-year study, the growth increments observed under the WF treatment for tree height, DBH, individual tree volume, and stand volume were 6.18 m, 13.35 cm, 0.117 m³, and 108.68 m³·ha⁻¹, respectively. These values represented increases of 5.9%, 14.8%, 29.3%, and 29.5% compared to the corresponding measurements under the CK treatment. The integration of water and fertilizer led to a marked improvement in forest stand growth. After five years, the stand volume achieved under this treatment reached 112.36 m³·ha⁻¹ (Figure 1D), highlighting the effectiveness of this approach in enhancing forest productivity.

3.2. Productivity Performance

In the early growth stages (2018–2019), the trees exhibited slow growth across all fertilization treatments (Figure 2). Between 2019 and 2021, the trees under HF and CK treatments experienced a growth peak lasting two years, followed by a gradual decline in growth rate from 2021 to 2022. In contrast, the trees subjected to WF treatment maintained a rapid growth trajectory from 2019 to 2022, characterized by a sustained three-year growth peak. The analysis of forest stand productivity in 2022 revealed notable differences across the three fertilization treatments. The productivity levels were recorded as 41.75 m³·ha⁻¹·yr⁻¹ under the WF treatment, 29.88 m³·ha⁻¹·yr⁻¹ under the HF treatment, and 26.91 m³·ha⁻¹·yr⁻¹ under the CK treatment. These results indicated that the stand productivity under the WF treatment was significantly higher, with increases of 39.7% compared to the HF treatment and 55.1% compared to the CK treatment.

3.3. Stoichiometric Characteristics

3.3.1. Element Content in the Leaf

Different fertilization strategies led to notable changes in leaf N content and the N:P ratio (Figure 3A,D). Specifically, the integrated water and fertilizer treatment resulted in a significantly higher leaf N content and N:P ratio compared to the hole fertilization and control treatments, with the differences statistically significant at p < 0.05. The leaf N content in the integrated water–fertilizer treatment (31.54 g·kg⁻¹) was 8.3% higher than that in the hole-fertilized treatment (29.12 g·kg⁻¹) and 6.5% higher than in the control treatment (29.62 g·kg⁻¹). However, the other indicators, including leaf P content (Figure 3B), K content (Figure 3C), N:K ratio (Figure 3E), and P:K ratio (Figure 3F), did not show significant differences across the fertilization treatments (p > 0.05).

3.3.2. Element Content in the Branch

The various fertilization treatments did not produce significant differences in the N, P, and K contents of branches, nor in the ratios of N:P, N:K, and P:K (p > 0.05). Specifically, the branch N content ranged from 11.14 to 11.47 g·kg⁻¹ (Figure 4A), P content ranged from 1.29 to 1.35 g·kg⁻¹ (Figure 4B), and K content ranged from 6.08 to 7.32 g·kg⁻¹ (Figure 4C). Additionally, the N:P ratio varied between 8.90 and 9.01 (Figure 4D), the N:K ratio ranged from 1.77 to 1.97 (Figure 4E), and the P:K ratio fell between 0.20 and 0.23 (Figure 4F).

3.3.3. Element Content in the Coarse Root

The K content in the coarse root varied significantly across different fertilization treatments (p < 0.05), with fertilization notably enhancing the K levels. Specifically, the integrated water and fertilizer treatment resulted in a K content of 6.06 g·kg⁻¹ in the coarse roots, representing a 12.0% and 20.0% increase compared to the hole fertilization (5.41 g·kg⁻¹) and control (5.05 g·kg⁻¹) treatments, respectively (Figure 5C). In contrast, there were no significant differences in the N content (Figure 5A), P content (Figure 5B), N:P ratio (Figure 5D), N:K ratio (Figure 5E), or P:K ratio (Figure 5F) among the various fertilization treatments (p > 0.05).

3.3.4. Element Content in the Fine Root

The N content, K content, N:K ratio, and P:K ratio in the fine roots exhibited significant variation across different fertilization treatments (p < 0.05). Fertilization notably enhanced the N levels in the fine roots. Specifically, compared to the unfertilized stand (16.23 g·kg⁻¹), the N content increased by 7.6% in the hole-fertilized stand (17.46 g·kg⁻¹) and by 4.6% in the stand with integrated water and fertilizer (16.98 g·kg⁻¹) (Figure 6A). Interestingly, the K content was highest in the hole-fertilized stand compared to both the unfertilized and water–fertilizer-integrated stands (Figure 6C). However, this trend was reversed for the N:K ratio (Figure 6E) and the P:K ratio (Figure 6F), where the hole-fertilized stands exhibited lower ratios. No significant differences were observed in the P content (Figure 6B) and the N:P ratio (Figure 6D) across the different fertilization treatments (p > 0.05).

3.4. Temporal Dynamics of Organ Nutrients

3.4.1. Nitrogen Changes

Between the early growth stage (May) and the middle stage (July), the N content in the leaves was substantially depleted, decreasing by 44.7%. However, from the middle to the final stage (September), there was a notable recovery, with leaf N content increasing by 25.6% (Figure 7A). Throughout the growth cycle, the N content in the branches, coarse roots, and fine roots followed a pattern of ‘initial decline followed by recovery’. By the final stage, compared to the early stage, there was a significant reduction in the N content of leaves, coarse roots, and fine roots, with decreases of 30.5%, 11.1%, and 20.0%, respectively (p < 0.05).

3.4.2. Phosphorus Changes

Throughout the growth stages, the P content in the leaves consistently decreased, while the branches exhibited an initial increase followed by a decline. In contrast, the P levels in both coarse and fine roots displayed a pattern of ‘initial decline followed by recovery’ (Figure 7B). By the final stage, compared to the early stage, there was a significant reduction in P content in the leaves and fine roots, with decreases of 18.8% and 24.4%, respectively (p < 0.05).

3.4.3. Potassium Changes

During the growth cycle, the K content in the leaves and branches initially increased and then decreased, while the coarse roots exhibited a steady decline in K levels. In contrast, the K content in the fine roots followed a pattern of ‘initial decrease followed by a subsequent increase’ (Figure 7C). By the final growth stage, significant reductions in K content were observed across all tissues compared to the initial stage, with decreases of 47.3% in the leaves, 21.4% in the branches, 24.7% in the coarse roots, and 15.1% in the fine roots (p < 0.05).

3.5. Distribution of Nutrient Content in Organs

In the distribution of N content among the tree organs, the leaves had the highest N concentration, ranging from 42.1% to 44.4%, followed by the fine roots at 23.7%–25.3%, the branches at 15.8%–16.7%, and the coarse roots at 15.8%–16.5% (Figure 8A). The integrated water and fertilizer application (44.4%) notably increased the proportion of N in the leaves, compared to 43.1% in the unfertilized stands and 42.1% in those with hole fertilization. P distribution also showed leaves with the highest proportion, exceeding 35%, with other organs containing P levels between 19.5% and 23.7% (Figure 8B). Similarly, the leaves exhibited the highest K content, ranging from 35.7% to 37.5%, while the K levels in other organs remained below 25.4% (Figure 8C). Notably, fertilization with integrated water and fertilizer (20.4%) significantly increased the K content in the coarse roots, compared to 16.9% with no fertilization and 16.4% with hole fertilization.

3.6. Relationship between Tree Growth and Organ Nutrients

A correlation analysis revealed that the height growth of the tree was primarily influenced by an increase in leaf N and P contents (Figure 9A). Additionally, as leaf N levels and K contents in the branches and coarse roots rose, there was a significant positive impact on the DBH (Figure 9B). Furthermore, the volume of individual trees showed a strong positive correlation with leaf N content and the K levels in the branches and coarse roots (Figure 9C).

4. Discussion

4.1. Growth Performance in Different Fertilization Regimes

During the early growth period of C. bungei (2018–2019), the newly planted saplings underwent a phase of environmental and climatic adaptation, resulting in relatively slow initial growth. However, as the trees matured, with increased diameter at breast height (DBH), expansion of crown width, and enhanced nutrient availability in the soil, both the volume of individual trees and the overall biomass accumulation in the forests accelerated significantly. This study demonstrated that fertilization played a crucial role in boosting this accumulation. Specifically, the integrated water and fertilizer method led to a substantial increase in forest volume, with an accumulation of 112.36 m³·hm⁻², representing a 23.6% increase compared to the hole-fertilized forests, which accumulated 90.88 m³·hm⁻². Some studies indicated that water-soluble fertilizers were highly efficient when applied through irrigation systems. They were rapidly absorbed by plant roots, leading to quick and noticeable effects on plant growth and health. This method of fertilization not only ensured precise nutrient delivery but also optimized the overall efficiency of the fertilization process [28,29,30,31,32].

Furthermore, ref. [33] discovered that drip irrigation was more effective than flood irrigation in preserving soil structure. It maintained the soil in a loose state, leading to increased porosity and improved aeration. Unlike flood irrigation, drip irrigation did not disrupt the soil’s aggregate structure, thus supporting better soil health and plant growth. Ref. [34] demonstrated that the integration of water and fertilizer markedly enhanced soil nutrient content and enzyme activity. Their study found that this integrated approach significantly increased the levels of total N, total P, alkaline N, soil pH, urease content, and phosphatase activity compared to conventional fertilization methods. Additionally, the integrated treatment resulted in higher concentrations of total K, available P, and available K than those achieved through ordinary fertilization techniques. The integration of water and fertilizer appeared to significantly enhance the nutrient absorption efficiency of C. bungei roots, largely due to the improved soil structure and nutrient availability [25]. This optimized soil environment, characterized by a loose structure and an abundant supply of nutrients, played a crucial role in promoting the robust growth of C. bungei. However, it was important to note that the detailed soil data supporting these findings were not fully explored in this article, indicating a need for further research to comprehensively understand these interactions.

4.2. Nutrient Changes in Different Fertilization Regimes

This study found that integrated water and fertilizer application notably enhanced the N content in the leaves and coarse roots of C. bungei. Additionally, a positive correlation was observed between the overall biomass of forests and the N content in the leaves as well as the K content in the coarse roots. The increased stand volume appeared to be driven by the higher N levels in the leaves and K levels in the coarse roots following fertilization, suggesting that these nutrients played a critical role in the growth and development of C. bungei. Moreover, an increase in leaf N content boosted net photosynthesis, leading to enhanced organic matter accumulation, which in turn supported tree growth and overall productivity [35]. An increase in K content in the coarse roots played a crucial role in activating plant enzymes, thereby enhancing photosynthesis and promoting sugar metabolism. This, in turn, supported protein synthesis and bolstered the plant’s resilience to environmental stresses such as drought, cold, salinity, and pest infestations. Additionally, the elevated K levels contributed to overall tree growth and vitality, making it a key factor in the health and development of the plants [36,37].

Furthermore, during the early growth stage, the N, P, and K contents in the leaves were notably higher across various fertilization treatments. This increase can be attributed to the plant’s active absorption of mobile nutrients from the soil, where N, P, and K were translocated from other soil compartments to the leaves, meeting the high nutrient demands of the plants during this critical growth phase [38]. As leaf growth accelerated and the biomass continued to increase, the nutrient absorption rate of plant roots may not keep pace with the expanding needs of leaf cells. Consequently, the concentrations of N, P, and K in the leaves become increasingly diluted, leading to a gradual decline in their contents [39,40]. As the growth period concluded, older leaves underwent senescence and abscission, leading to the redistribution of nutrients. This process facilitated the transfer of N and other essential elements back to the remaining, younger leaves. Consequently, there was an observed increase in N content in the leaves toward the end of the growth period, as these nutrients were reallocated to support ongoing physiological processes and leaf maintenance [41].

By the end of the growing season, the nutrient contents in the various organs of C. bungei had been depleted. This depletion occurred due to several factors: the ongoing growth and development of the tree, including tissue differentiation and respiration, required significant nutrient expenditure. Additionally, the synthesis of new leaves, flowers, and pollen demanded considerable amounts of water and nutrients to support these metabolic processes. As a result, both the physiological growth activities and the reproductive processes contributed to the overall reduction in nutrient levels across the plant’s organs [42]. The shedding of flowers and fruits resulted in significant nutrient loss, which can impact several critical aspects of plant development. This loss disrupted flower bud differentiation and hampered the accumulation and storage of nutrients within the current growing season. Consequently, these nutrient deficiencies may adversely affect the plant’s growth and development in the subsequent year, potentially leading to reduced productivity and vitality [43]. In summary, the growth of C. bungei necessitated the utilization of essential nutrients—such as N, P, and K—to support trunk development and overall biomass accumulation. This nutrient consumption was integral for regulating vital processes like respiration and photosynthesis. Such nutrient dynamics likely represented an adaptive strategy of the plant, ensuring its survival and continued growth by balancing the demands of structural and functional development.

4.3. Research Limitations and Prospects

The enhancement in wood volume and biomass accumulation observed in young C. bungei forests following fertilization are significantly linked to the increased nutrient contents in plant organs. Nevertheless, the effects of fertilization extend beyond nutrient uptake; alterations in soil physical and chemical properties, as well as changes in soil enzyme activity and microbial communities, including bacteria and fungi, also play a crucial role in influencing plant growth [44,45,46]. These factors collectively shape the growth dynamics and overall health of the forest ecosystem. Alterations in understory vegetation diversity and the nutrient release from leaf litter can significantly impact the soil nutrient cycle, thereby influencing tree development. These factors play a crucial role in the broader ecosystem and warrant careful examination in future research to understand their full implications for forest health and productivity. It is important to highlight that this study focused exclusively on a single year (2021) for the analysis of nutrient stoichiometry in tree organs. This limitation indicates a need for more extensive long-term dynamic research, which should be a priority in future investigations. By incorporating multi-year data, we can better understand the temporal variations and underlying mechanisms influencing nutrient dynamics in tree ecosystems.

5. Conclusions

Following five years of research on fertilization, the findings indicate that fertilization can significantly alter the growth trajectory of young C. bungei plantations. A comparison of two fertilization approaches reveals that an integrated strategy combining water and fertilizer markedly enhances stand growth, particularly in terms of stand volume. This integrated approach not only boosts forest stand productivity but also contributes to more efficient nutrient uptake (e.g., increment of nitrogen contents). Specifically, fertilization has been shown to increase nitrogen concentrations in the leaves and potassium levels in the branches and coarse roots, thereby supporting overall tree health and growth (e.g., increment of volume). These results underscore the importance of tailored fertilization strategies in optimizing the productivity of C. bungei plantations. To improve forest productivity, it is essential that future studies on the fertilization of C. bungei emphasize the integration of water management and fertilizer applications.