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Article

Effects of Seasonal Rainfall Changes on N, P, and K Stoichiometric Characteristics in Leaves and Soil of Tropical Coastal Shelterbelt Forests

by
Shouqian Nong
1,2,
Haihui Chen
1,2,
Zongzhu Chen
1,2,
Zhipan Lin
1,2,
Shaofeng Su
1,2,
Xiangling Lei
1,2,
Junting Jia
1,2 and
Yiqing Chen
1,2,*
1
Hainan Academy of Forestry (Hainan Academy of Mangrove), Haikou 571100, China
2
Hainan Wenchang Forest Ecosystem Observation and Research Station, Wenchang 571300, China
*
Author to whom correspondence should be addressed.
Forests 2025, 16(4), 600; https://doi.org/10.3390/f16040600
Submission received: 15 February 2025 / Revised: 18 March 2025 / Accepted: 25 March 2025 / Published: 29 March 2025
(This article belongs to the Section Forest Soil)
Browse Figures
Versions Notes

Abstract

:
Plant nitrogen (N), phosphorus (P), and potassium (K) concentrations and ratios serve as critical indicators of nutrient constraints in coastal ecosystems. However, the response of leaf–soil N-P-K stoichiometry in tropical coastal shelterbelt forests to seasonal rainfall variations remains poorly understood. This study measured total N, P, and K contents in leaves and soils of three typical tropical coastal shelterbelt forests in Wenchang, China—Casuarina equisetifolia L., Cocos nucifera L., and Pinus elliottii × caribaea—during August 2022 (wet season) and February 2023 (dry season). Key findings are as follows: (1) All three forests exhibited low N-P-K contents in both leaves and soils, with significant stand-specific variations. Soil N:P ratios were consistently below 14, indicating chronic N limitation for plant growth. (2) Wet seasons significantly altered leaf–soil N-P-K contents and stoichiometric ratios. (3) Leaf and soil stoichiometric traits exhibited strong correlations, but these relationships diverged under seasonal transitions. (4) Shifts from wet to dry seasons increased the sensitivity of N-P-K stoichiometric homeostasis, reflecting weakened nutrient buffering capacity. This study reveals stand-specific nutrient cycling patterns in tropical coastal shelterbelts, with seasonal rainfall modulating soil–leaf nutrient coupling and stoichiometric stability. These findings provide a theoretical basis for optimizing nutrient management and species configuration in tropical coastal ecosystems under climate variability.

1. Introduction

Ecological stoichiometry is an important approach for exploring the balance of major elements and material cycling among the components of an ecosystem, reflecting the dynamic balance between energy and chemical elements [1,2,3]. It centers on the dynamic balance between elements, their interactions, and plant nutrient requirements [1,4]. Among essential nutrients, N, P, and K are particularly crucial for plant growth and development. Nitrogen is a basic component of proteins and nucleic acids and the fundamental element for chlorophyll molecules, plant hormones, and a number of coenzymes. Phosphorus plays structural and functional roles in the photosynthetic carbon cycle as nucleic acids, membrane lipids, energy metabolites and active intermediates, and is also involved in signal transduction. Potassium, as the most abundant cation in plant cells, is essential for a series of physiological processes, including osmoregulation, stomatal motility and protein biosynthesis [5]. Previous studies have shown that, in terrestrial ecosystems, aboveground plants are closely linked to belowground soil [6,7,8]. On one hand, soil is the direct source of plant nutrients, as plants utilize their roots to establish extensive contact surfaces for material exchange between plants and soil [9]. On the other hand, as an important ecological factor for plants, soil significantly affects plant growth through its physical and chemical properties [10]. Therefore, analyzing the ecological stoichiometric characteristics of nutrients between plants and soil in ecosystems is important and helps in elucidating the correlation between plants and nutrients in soil, as well as the relationship between constraints and equilibrium.
The ability of organisms to maintain a relatively stable chemical composition in response to changes in the external environment over a long period of time is known as eco-stoichiometric homeostasis, which also reflects the physiological and biochemical adaptations of plants to environmental changes [11]. Leaves are an important organ for photosynthesis in plants, and they have higher and more stable N, P, and K contents compared to other organs [12]. Elemental ratios characterizing plant homeostasis are mainly related to the contents and ratios of key nutrients in plants [13]. For instance, N:P ratios in leaves can reflect the growth rate, nutrient limitation status, and ecosystem productivity level of plants [14]. Under different conditions, plants adapt to environmental changes by optimizing their nutrient uptake and utilization strategies through N:P ratio adjustments [15].
Over the past two decades, ecological stoichiometry and stoichiometric homeostasis have been extensively researched. Many studies have focused on the relationship between plants and soil nutrients in temperate and subtropical regions, as well as plant adaptation to soil water and salt changes [16,17,18,19,20,21]. However, research in tropical regions has been limited, and most studies have been conducted on natural forests and grasslands rather than coastal shelterbelts. Additionally, most of these studies were conducted on single forests, with only a few studies considering different forests [8]. As a key factor influencing plant growth cycle and soil nutrient availability [22,23,24], seasonal variations in rainfall induce modifications of N, P, and K contents and the stoichiometric characteristics of plants [25]. Therefore, to achieve a deep understanding of the nutrient cycling mechanism of forest ecosystems, it is of great significance to study the effects of seasonal variations in rainfall on N, P, and K contents in plants and their stoichiometric characteristics. In this manner, the interrelationship between plants and soil nutrients can be determined and a basis for forest management and protection can be established.
The coastal shelterbelt in Wenchang, China is a typical tropical coastal ecosystem, with unique ecological functions and significant values. However, in recent years, the rates of soil and water erosion and vegetation degradation have increased due to severe climate events, such as typhoons, and human activities, leading to a significant decline in ecological function and stability [26]. The coastal shelterbelt in this region has a significant effect on biological diversity and ecosystem stabilization. However, the effects of seasonal changes in rainfall on the N, P, and K stoichiometric characteristics of leaves and soil of the coastal shelterbelt remain unclear. Therefore, this study investigated the N, P, and K stoichiometric characteristics of leaves and soil in three tropical coastal shelterbelt forests (Casuarina equisetifolia L. forest, Cocos nucifera L. forest, Pinus elliottii × caribaea forest) in Wenchang, China in wet and dry seasons. This study aims to address the following questions: (1) How do the N, P, and K contents and their ratios in leaves and soils of the three tropical coastal shelterbelt forests vary with seasonal rainfall changes? (2) What are the limiting nutrients for plant growth in these three tropical coastal shelterbelt forests? (3) Is there a leaf–soil coupling of nutrient elements in these three tropical coastal shelterbelt forests? (4) What are the homeostatic characteristics of leaf–soil nutrient systems in these forests, and how do they shift in response to seasonal rainfall variations? The findings will provide references for understanding nutrient cycling and ecological processes in tropical coastal shelterbelt ecosystems.

2. Materials and Methods

2.1. Study Area

The study area is located in Wenchang, China (108°21′ E–111°03′ E, 19°20′ N-20°10′ N), which lies in the northeastern part of Hainan. Located to the east of the South China Sea, it has a tropical oceanic monsoon climate, characterized by an annual average temperature of 23.9 °C and an annual average rainfall of 1850 mm. The wet season is mainly concentrated from May to October, which accounts for more than 80% of the annual rainfall, and the dry season is from November to the next April. The parent materials of soil are coastal sediments [27]. The primary forest types in the coastal shelterbelt of the study area are Casuarina equisetifolia, Cocos nucifera forest, and Pinus elliottii × caribaea [28].

2.2. Sample Site Selection and Setup

In July 2022, three tropical coastal shelterbelt forests (Casuarina equisetifolia forest, Cocos nucifera forest, Pinus elliottii × caribaea forest) around the observation location and research station of the forest ecosystem in Wenchang, China were selected as the study objects. Each forest had a sample area of more than 1 ha and was essentially under the same management method. The specific locations of the forests are shown in Figure 1. In each forest, six 20 × 20 m sample plots were randomly selected. Woody plants with a diameter at breast height greater than 5 cm were surveyed in the sample plots, and the diameter at breast height and the height of trees as well as the understory vegetation cover of the sample plots were recorded. The soil type of the sample plots was also recorded, and the soil physical properties, such as soil bulk weight and soil moisture content, were measured (Table 1).

2.3. Sample Collection and Testing

In August 2022 (wet season) and February 2023 (dry season), five healthy, disease-free standard trees were selected from each sample plot. Using high branch shears, fully matured, sun-exposed, and intact leaves were sampled from the east, south, west, and north orientations of standardized tree crowns. In this manner, a total fresh weight of 80 g per sampling unit was obtained. The collected leaves were cleaned of impurities and mixed, transported to the laboratory, dried at 70 °C to a constant weight, ground through a 100-mesh sieve, and then chemically analyzed. Soil samples were collected at the four corners and the center of each sample plot using a five-point sampling method. After removing the surface litter layer, a soil drill with a diameter of 5 cm was used to extract 0–20 cm of surface soil. The soil was mixed well after removing roots and stones, then placed in self-sealing bags, and labelled for transportation to the laboratory. For the determination of soil nutrients, the soil was filtered through a 100-mesh sieve after drying. Total nitrogen (N) in leaves and soil of plants was determined by the semimicro Kjeldahl method, total phosphorus (P) was determined by the acid dissolution–molybdenum–antimony colorimetric method, and total potassium (K) was determined by the alkaline dissolution–flame photometric method [29].

2.4. Data Analysis

Preliminary data organization and statistical analysis were conducted using EXCEL 2019. Subsequently, one-way ANOVA (α = 0.05) and LSD (Least Significant Difference) multiple comparison tests were performed using SPSS 26.0 to analyze the differences in N, P, and K concentrations and their stoichiometric ratios in leaves and soils among different forest stands and across different rainfall seasons. Additionally, Pearson correlation analysis was conducted using Origin 2021 to examine the relationships between these nutrient indicators within the context of different forest stands and rainfall seasons. The homeostasis index of leaves was calculated using an ecological stoichiometric homeostasis model [30,31]:
y = c x 1 H
Converting Equation (1) to the logarithmic linear form:
l g y = l g c + 1 H l g x
where y refers to the N, P, and K contents or stoichiometric ratio in leaves, x refers to the N, P, and K contents or stoichiometric ratio in soil, C is a constant, and H is an index of dynamic equilibrium. When H < 0, H = −H. H is positively correlated with plant homeostasis; larger H values suggest higher stability of the plant. If the regression relationship between lgy and lgx is not significant (p > 0.1), then the plant is considered to be in “absolute homeostasis”. Otherwise (p < 0.1), the data set with H > 0 is defined as follows: H < 0.75 indicates sensitivity, 0.75 < H < 2 indicates weak sensitivity, 2 < H < 4 indicates weak homeostasis, and H > 4 indicates homeostasis.

3. Results and Analysis

3.1. N, P, and K Stoichiometric Characteristics in Leaves of the Three Shelterbelt Forests

3.1.1. N, P, and K Contents

Among various forests, the N content in leaves during both wet and dry seasons followed a consistent order: Casuarina equisetifolia > Pinus elliottii × caribaea > Cocos nucifera. During the dry season, measurements showed that Casuarina equisetifolia contained 72.74% and 37.82% more leaf N than Cocos nucifera and Pinus elliottii × caribaea, respectively (p < 0.05). This superiority persisted during the wet seasons, with Casuarina equisetifolia exhibiting 29.79% higher N content than Cocos nucifera (p < 0.05), although its 21.03% lead over Pinus elliottii × caribaea did not achieve statistical significance. In contrast, wet season data revealed that Cocos nucifera accumulated 27.13% more foliar P than Pinus elliottii × caribaea and 50.71% more than Casuarina equisetifolia (p < 0.05), while the latter two species maintained comparable P levels (p > 0.05). During the dry seasons, Casuarina equisetifolia exhibited the highest leaf P content, significantly outperforming Cocos nucifera by 24.09% (p < 0.05), and showing a non-significant 7.78% increase compared to Pinus elliottii × caribaea (p > 0.05). Regarding K content, Pinus elliottii × caribaea exhibited the highest leaf K levels during the wet seasons, surpassing Casuarina equisetifolia by 90.35% (p < 0.05) and Cocos nucifera by 35.86% (although this difference was not statistically significant, p > 0.05). However, this pattern reversed in the dry seasons, with Casuarina equisetifolia exhibiting the highest K content—27.79% greater than Pinus elliottii × caribaea (p < 0.05)—while maintaining comparable levels to Cocos nucifera (p > 0.05) (Figure 2). Overall, the leaf N, P, and K contents varied among different forests. The leaf N content of the three shelterbelt forests exhibited a consistent pattern across different wet seasons, with Casuarina equisetifolia consistently showing the highest value.
The leaf N, P, and K contents of the three shelterbelt forests exhibited varied responses to seasonal changes. Specifically, during the dry season, Casuarina equisetifolia showed increases in leaf N, P, and K contents by 15.05%, 35.41%, and 16.15%, respectively, although these changes were not statistically significant (p > 0.05). In contrast, Cocos nucifera experienced decreases in leaf N and P contents by 13.55% and 27.59%, respectively, during the dry season (p < 0.05), with a slight, albeit insignificant, decrease in leaf K content (p > 0.05). Notably, Pinus elliottii × caribaea exhibited a significant decrease in leaf K content by 52.25% during the dry season (p < 0.05), while its leaf N and P contents remained relatively stable across the wet and dry seasons (p > 0.05) (Figure 2).

3.1.2. Stoichiometric Ratios of N, P, and K

Across both the wet and dry seasons, the foliar N:P ratios of the three shelterbelt forests consistently adhered to the order: Casuarina equisetifolia > Pinus elliottii × caribaea > Cocos nucifera. Specifically, during the wet seasons, Casuarina equisetifolia exhibited significantly higher N:P ratios by 45.19% and 96.12%, compared to Pinus elliottii × caribaea and Cocos nucifera, respectively (p < 0.05). Conversely, during the dry season, no significant differences were observed among the three forests (p > 0.05). Regarding the N:K ratios, Casuarina equisetifolia dominated during the wet seasons, surpassing Cocos nucifera by 128.79% and Pinus elliottii × caribaea by 77.42% (p < 0.05). No significant difference was noted between Pinus elliottii × caribaea and Cocos nucifera (p > 0.05). In the dry seasons, Casuarina equisetifolia retained its superiority over Cocos nucifera (68.23%, p < 0.05) but its N:K ratio was comparable to that of Pinus elliottii × caribaea (p > 0.05). During the wet seasons, K:P ratios peaked in Pinus elliottii × caribaea, surpassing Casuarina equisetifolia by 60.33% and Cocos nucifera by 75.52% (p < 0.05). No significant difference was found between Casuarina equisetifolia and Cocos nucifera (p > 0.05). In contrast, during the dry seasons, no interspecific variations were observed in K:P ratios (p > 0.05) (Figure 2). Overall, except for the K:P ratio during the dry seasons, the stoichiometric ratios of leaf N, P, and K in the three shelterbelt forests significantly differed during both the wet and dry seasons. Notably, the leaf N:P ratio of the three shelterbelt forests showed the same size pattern in both seasons, with Casuarina equisetifolia exhibiting the maximum leaf N:P and N:K ratios during both seasons.
The leaf N:P, N:K, and K:P ratios of the three shelterbelt forests differed in response to changes in the wet season. In the Cocos nucifera forest, no statistically significant seasonal differences were observed in the leaf N:P, N:K, or K:P ratios (p > 0.05). Casuarina equisetifolia displayed a significant reduction in leaf K:P ratios during the dry seasons, decreasing by 13.32% (p < 0.05). However, its N:P and N:K ratios showed minimal declines, with non-significant differences (p > 0.05). Conversely, Pinus elliottii × caribaea exhibited contrasting patterns. During the dry seasons, the leaf N:K ratios increased sharply by 110.21% (p < 0.05), while the K:P ratios decreased significantly by 44.11% (p < 0.05). The N:P ratios showed a marginal, non-significant increase (p > 0.05).

3.2. N, P, and K Stoichiometric Characteristics in Soil of the Three Shelterbelt Forests

3.2.1. N, P, and K Contents

Across the three shelterbelt forests, the soil N content of the Casuarina equisetifolia forest consistently ranked the lowest in both the wet and dry seasons. Specifically, during the wet seasons, the soil N content of the Casuarina equisetifolia forest was 41.81% and 39.83% lower than that of the Pinus elliottii × caribaea and Cocos nucifera forests, respectively (p < 0.05). These differences widened to 50.63% and 56.67% during the dry seasons (p < 0.05), while the Pinus elliottii × caribaea and Cocos nucifera forests showed no significant interspecific differences in soil N content in either season (p > 0.05). Regarding P, the Pinus elliottii × caribaea forest maintained the highest soil concentrations throughout the year. Wet season measurements revealed that the Pinus elliottii × caribaea forest had 38.17% and 77.49% higher P levels compared to the Casuarina equisetifolia and Cocos nucifera forests, respectively (p < 0.05), with no significant difference observed between the Casuarina equisetifolia and Cocos nucifera forests (p > 0.05). In terms of soil P content during the dry seasons, the Pinus elliottii × caribaea forest surpassed the Casuarina equisetifolia forest by 25.04% and the Cocos nucifera forest by 53.87% (p < 0.05), while the Casuarina equisetifolia forest exhibited 23.05% higher P levels than the Cocos nucifera forest (p < 0.05). Soil K displayed seasonal reversals in its dynamics. During the wet seasons, the Cocos nucifera forest ranked highest in soil K content, followed by the Pinus elliottii × caribaea and Casuarina equisetifolia forests, although these differences were not statistically significant (p > 0.05). However, during the dry seasons, the Pinus elliottii × caribaea forest emerged as the dominant species, with more than two-fold higher soil K content compared to both the Casuarina equisetifolia and Cocos nucifera forests (p < 0.05). Meanwhile, the Casuarina equisetifolia and Cocos nucifera forests maintained statistically comparable levels of soil K (p > 0.05) (Figure 3). Overall, there were differences in the three shelterbelt forests except for soil K content during the wet season. The Pinus elliottii × caribaea forest exhibited the highest soil N and P contents during different wet seasons and the highest soil K content during the dry seasons. The Casuarina equisetifolia forest showed the lowest soil N contents and the Cocos nucifera forest showed the lowest soil K contents during different wet seasons.
The Casuarina equisetifolia forest exhibited no statistically significant seasonal variations in soil N and P contents (p > 0.05) but experienced a notable 54.30% decrease in soil K during the dry seasons (p < 0.05). In contrast, the Cocos nucifera forest showed non-significant increases in soil N and P contents during the dry seasons (p > 0.05), whereas soil K experienced a significant decline by 59.42% (p < 0.05). The Pinus elliottii × caribaea forest demonstrated marginal, non-significant increases in soil N and K during the dry seasons (p > 0.05), accompanied by a slight, non-significant decrease in soil P (p > 0.05) (Figure 3). Overall, the soil N and P contents in soil of the three shelterbelt forests were stable during different wet seasons, while the soil K contents of the Casuarina equisetifolia and Cocos nucifera forests decreased significantly during the dry season.

3.2.2. Stoichiometric Ratios of N, P, and K

During both the rainy and dry seasons, the soil N:P ratios of the three shelter forests exhibited a consistent pattern: Cocos nucifera forest > Pinus elliottii × caribaea forest > Casuarina equisetifolia forest. Specifically, in the rainy season, the Cocos nucifera forest exhibited 73.25% and 115.51% higher N:P ratios compared to the Pinus elliottii × caribaea and Casuarina equisetifolia forests, respectively (p < 0.05), whereas the Pinus elliottii × caribaea forest showed a 24.39% higher N:P ratio than the Cocos nucifera forest (p < 0.05). During the dry season, the N:P ratios of the Cocos nucifera forest surpassed those of the Pinus elliottii × caribaea and Casuarina equisetifolia forests by 35.61% and 150.84%, respectively (p < 0.05), with the Pinus elliottii × caribaea forest exhibiting an 84.98% higher N:P ratio than the Cocos nucifera forest (p < 0.05). Regarding soil N:K ratios, during the rainy season, the Pinus elliottii × caribaea forest showed the highest values, followed by the Cocos nucifera forest and then the Casuarina equisetifolia forest, although the differences among these three forests were not statistically significant (p > 0.05). In contrast, during the dry season, the Cocos nucifera forest exhibited the highest N:K ratios, exceeding the Casuarina equisetifolia and Pinus elliottii × caribaea forests by 192.17% and 117.59%, respectively (p < 0.05), with no significant difference between the latter two forests (p > 0.05). Regarding soil K:P ratios, during the rainy season, the Cocos nucifera forest further exhibited the highest values, followed by the Casuarina equisetifolia forest and then the Pinus elliottii × caribaea forest, with no statistically significant differences among the three forests (p > 0.05). However, during the dry season, the Pinus elliottii × caribaea forest displayed the highest K:P ratios, surpassing the Cocos nucifera and Casuarina equisetifolia forests by over two-fold (p < 0.05), with no significant difference observed between the latter two forests (p > 0.05) (Figure 3).
The Casuarina equisetifolia forest showed no notable seasonal variations in soil N:P and N:K ratios (p > 0.05); nevertheless, they exhibited a substantial 58.49% decrease in soil K:P ratios during the dry seasons (p < 0.05). Conversely, in the Cocos nucifera forest, soil N:P ratios (p > 0.05) presented a slight, insignificant decline during the dry season. Notably, soil N:K ratios increased significantly by 161.18% (p < 0.05), while K:P ratios decreased markedly by 63.28% (p < 0.05). In the Pinus elliottii × caribaea forest, the dry seasons were associated with minor increases in soil N:P and K:P ratios, accompanied by a slight decrease in N:K ratios, although these changes were not statistically significant (p > 0.05) (Figure 3). Overall, the soil N:P, N:K, and K:P ratios of different forests exhibited different responses to changes in rainfall seasons, while the soil N:P ratio of different forests remained stable during different rainfall seasons.

3.3. N:P:K Stoichiometric Ratio in Leaves and Soil of the Three Shelterbelt Forests

Pearson correlation analysis was applied for N, P, and K contents in leaves and soil of the three shelterbelt forests and their stoichiometric ratios during the wet and dry seasons. The results showed that the N contents in the leaves and soil of the Cocos nucifera and Casuarina equisetifolia forests had significant positive and negligible correlations during the wet (p < 0.05, same below) and dry seasons (p > 0.05, same below), respectively. The N contents of leaves and soil of the Pinus elliottii × caribaea forest had negligible correlations during both the wet and dry seasons. The P contents in the leaves and soil of the Casuarina equisetifolia forest had an extremely significant negative correlation during the dry seasons (p < 0.01, same below), but the correlation was negligible during the wet seasons. The P contents in the leaves and soil of Cocos nucifera and Pinus elliottii × caribaea had negligible correlations during both the wet and dry seasons. The K contents in the leaves and soil of Cocos nucifera and Pinus elliottii × caribaea had significant positive correlations during both the wet and dry seasons; the K contents in the leaves and soil of the Casuarina equisetifolia forest had negligible correlations during both the wet and dry seasons. During the dry seasons, the N:P ratio in the leaves and soil of the Casuarina equisetifolia, Cocos nucifera, and Pinus elliottii × caribaea forests had an extremely significant positive correlation, significant negative correlation, and a negligible correlation, respectively. During the wet seasons, the N:P ratio in the leaves and soil of Pinus elliottii × caribaea had an extremely significant positive correlation, and those of the Casuarina equisetifolia and Cocos nucifera forests had negligible correlations. The N:K ratio of the leaves and soil in the Cocos nucifera forest had an extremely significant positive correlation during the wet seasons; in contrast, the N:K ratio of the leaves and soil in the Casuarina equisetifolia, Cocos nucifera, and Pinus elliottii × caribaea forests had extremely significant positive correlations during the dry seasons. The correlations between the N:K ratio of the leaves and soil in the Casuarina equisetifolia and Pinus elliottii × caribaea forests were negligible during the wet seasons. The correlation between the K:P ratio of the leaves and soil of the Casuarina equisetifolia forest was extremely significantly negative during the dry seasons; the correlation was extremely significantly positive in the Cocos nucifera forest during the dry seasons; the correlations were negligible in the Casuarina equisetifolia and Cocos nucifera forests during the wet seasons; it was also negligible in the Pinus elliottii × caribaea forest during both the dry and wet seasons (Figure 4).
Overall, the N, P, and K contents and their stoichiometric ratios in the leaves and soil of the three shelterbelt forests were strongly correlated, but the correlation between them also varied with the change of the wet season. Additionally, the N contents of the leaves and soil in the Pinus elliottii × caribaea forest were not significantly correlated during both seasons. The K contents of the leaves and soil in the Casuarina equisetifolia forest were not significantly correlated during both seasons. The P contents of the leaves and soil in the Cocos nucifera and Pinus elliottii × caribaea forests were not significantly correlated during both seasons, and leaves and soil in the Casuarina equisetifolia forest were not significantly correlated during the wet seasons.

3.4. N:P:K Stoichiometric Homeostasis of Leaves of the Three Shelterbelt Forests

In the wet seasons, the leaf N and K contents of Casuarina equisetifolia were weakly sensitive and weakly homeostatic, respectively, while the remaining indicators corresponded to absolute homeostasis; in the dry seasons, the leaf N and K contents of Casuarina equisetifolia corresponded to absolute homeostasis, while its P content and N:P ratio changed from absolute homeostasis to sensitive, and its N:K and K:P ratios changed from absolute homeostasis to homeostasis and weak homeostasis, respectively. In the wet seasons, the leaf N, P, and K contents and their stoichiometric ratios of Cocos nucifera corresponded to absolute homeostasis; in the dry seasons, its N and P contents and N:P ratio were sensitive, while its K content and N:K ratio corresponded to homeostasis and weak homeostasis, respectively. In the wet seasons, all parameters (except for leaf N content, which was sensitive) of Pinus elliottii × caribaea corresponded to absolute homeostasis; in the dry seasons, its N content corresponded to absolute homeostasis, while its K content changed from absolute homeostasis to weak homeostasis, and its P content and N:P ratio changed from absolute homeostasis to sensitive (Table 2). Overall, the leaf N, P, and K contents and their stoichiometric homeostasis of the three shelterbelt forests were different in different seasons: the leaf N, P, and K contents and their stoichiometric homeostasis were higher during the wet seasons than during the dry seasons; the leaf N:P, N:K, and K:P ratios corresponded to absolute homeostasis during the wet seasons.

4. Discussion

4.1. N, P, and K Stoichiometric Characteristics of Leaves in the Three Shelterbelt Forests

The leaf N, P, and K contents of the three shelterbelt forests are lower than those of plants at the global and national scale during both the rainy and dry seasons [3,32,33]. This is mainly related to the soil parent material and climatic conditions of the study area, and its soil being coastal sedimentary soil, which is usually infertile. Moreover, the nutrients in the soil are more prone to washout and loss because of the high temperature and heavy rainfall, which results in the lower content of soil nutrients in this area and directly limits the ability of plants to absorb N, P, and K from the soil [34]. The leaf N content of the Casuarina equisetifolia forest was the highest among the three shelterbelt forests in both the rainy and dry seasons, which is related to the physiological characteristics of Casuarina equisetifolia. Casuarina equisetifolia can deeply absorb nutrients from the soil through its well-developed roots, convert atmospheric nitrogen into plant-available nitrogen as a nitrogen-fixing tree in symbiosis with rhizobia, and reabsorb nutrients into other organs before leaf senescence. These unique physiological features enable the tree to maintain high N content in leaves in the nutrient-poor coastal sand [35]. The leaf N and P contents of Cocos nucifera forest were significantly higher in the rainy season than in the dry season, and the leaf K content of Pinus elliottii × caribaea was significantly higher in the rainy season than in the dry season. This is because plants grow more vigorously in the rainy season and have a higher demand for nutrients, and absorb more N, P, and K from the soil to meet the demand for growth and development [36]. At the same time, the soil N, P, and K contents are more easily dissolved and released in the rainy season, which also enables the plants to absorb more nutrients, thus increasing the N, P, and K contents in leaves [37]. Nevertheless, compared with the Cocos nucifera and Pinus elliottii × caribaea forests, the Casuarina equisetifolia forest did not show significant changes in its leaf N, P, and K contents during different wet seasons. This is related to the adaptability of Casuarina equisetifolia to environmental changes. Casuarina equisetifolia has a high-water utilization efficiency, maintains normal physiological functions under drought conditions, and is able to adjust its nutrient uptake and utilization strategies in response to changes in environmental conditions. In nutrient-poor environments, it responds by increasing the area of root uptake and improving nutrient uptake efficiency, while in nutrient-rich environments, it maintains nutrient balance by reducing nutrient uptake and increasing nutrient storage [38].
Nutrient utilization efficiency and limiting factors can be further understood by measuring the ratio of N, P, and K in plant leaves [39]. The N:P ratio is an indicator of the extent to which forests are limited by the availability of N and P. N:P < 14 suggests that the plant is limited by N availability, while N:P > 16 suggests that the plant is limited by P availability, and 14 < N:P < 16 suggests that the plant is simultaneously limited by both N and P availability [40]. In this study, the Cocos nucifera forest was N-limited in both the rainy and dry seasons, the Casuarina equisetifolia forest was P-limited in both the rainy and dry seasons, and the Pinus elliottii × caribaea forest was N- and P-limited in the rainy season and P-limited in the dry season. Olde Venterink et al. [41] reported that plant growth is K-limited when N:K > 2.1 and K:P < 3.4. In this study, the Cocos nucifera forest during the wet seasons and the Casuarina equisetifolia and Pinus elliottii × caribaea forests during the dry seasons were K-limited. This difference is primarily attributable to the nutrient utilization strategies, growth characteristics, and spatial heterogeneity of different tree species in different forests [42].

4.2. N, P, and K Stoichiometric Characteristics of Soil in the Three Shelterbelt Forests

According to the national soil nutrient content grading standard [43], during both the rainy and dry seasons, the soil N content of the three shelterbelt forests was classified as level 6 (very low), soil P content as level 5 (low), and soil K content as level 6 (very low), except for the Pinus elliottii × caribaea forest in the dry season (level 5; low), which is consistent with the results of Xue et al. [34]. This indicates the relative scarcity of soil nutrients in the study area. Among the three shelterbelt forests, the Casuarina equisetifolia forest had a significantly lower soil N content compared to the other two forests. Soil N content has been reported to be often related to the decomposition rate of litter and the nutrient release rate [44]. The Casuarina equisetifolia forest has a slower decomposition rate of litter, and the N released into the soil is reduced. The soil in the Casuarina equisetifolia forest has a relatively loose texture, with poor water and fertilizer holding capacities; consequently, the N content in soil is relatively low [45]. Among the three shelterbelt forests, the Pinus elliottii × caribaea forest exhibited the highest soil N and P contents during both the rainy and dry seasons, and the highest soil K content during the dry season, which might be related to the spatial heterogeneity of the forests. Compared with other forests, the Pinus elliottii × caribaea forest is farther away from the coastline, and the degree of weathering erosion of the soil parent material is lower; thus, the nutrient content of the soil is higher [46]. Additionally, the results of the present study revealed that the Pinus elliottii × caribaea forest has the most abundant understory shrub plants, which is conducive to soil microbial activity and nutrient transformation, and more conducive to nutrient accumulation in soil [47]. The soil N and P contents of the three shelterbelt forests did not differ significantly between the rainy and dry seasons, indicating that the soil N and P contents of the three shelterbelt forests were relatively stable during different rainfall seasons. However, the soil K contents of the Casuarina equisetifolia and Cocos nucifera forests were significantly lower in the dry season than in the rainy season, which might be related to the lower compaction and lower water retention of the soils in these two forests, as well as the form of K in soil.
The stoichiometric ratio of N, P and K in soil can indicate the state of nutrient balance, coordination of nutrient supply, and ecosystem function and stability. In this study, the soil N:P ratio of the three shelterbelt forests ranged from 0.73 to 2.04, which is slightly lower than that of a coastal shelterbelt in Changle, China (2.04–3.11) [48], and significantly lower than the national average (9.3) [49] and the global land average (13.1) [50]. The soil N:P ratio can be used as an indicator to measure nitrogen saturation and has been widely used to determine the critical threshold for nutrient limitation. An N:P ratio less than 14 indicates insufficient nitrogen supply. In this study, the soil N:P ratios of the three shelterbelt forests were all much lower than 14, suggesting insufficient N supply for plant growth. This may be related to the slow decomposition rate of litter, soil nutrient deficiency, and low nutrient availability in the area [48].

4.3. Correlation Analysis of N:P:K Stoichiometric Ratio of Leaves and Soil in the Three Shelterbelt Forests

N, P, and K cycling processes between plants and soil are fundamental components of the ecosystem, and their correlation strongly affects plant growth and development [51]. A strong correlation has been reported between plants and N, P, and K in soil [52,53]. In this study, the P contents of leaves and soil were found to be uncorrelated in the Casuarina equisetifolia forest during the wet seasons and in the Cocos nucifera and Pinus elliottii × caribaea forests during both the dry and wet seasons, which is consistent with previous studies [31,54]. In this study, the correlations between N, P, and K and their stoichiometric ratios of leaves and soil in the three shelterbelt forests were not completely consistent, which agrees with the results of numerous studies [25,31]. Furthermore, the N contents of leaves and soil in the Pinus elliottii × caribaea forest were not significantly correlated, and the K contents of leaves and soil in the Casuarina equisetifolia forest were not significantly correlated. This is related to different tree species having developed different nutrient uptake and utilization mechanisms during the long-term evolutionary process [55]. The N, P, and K contents and their stoichiometric ratios of leaves and soil in different forests during the wet and dry seasons showed different correlations, which is primarily attributable to the combination of environmental factors and plant physiological mechanisms. During the rainy season, abundant rainfall and sufficient soil moisture favor the uptake and transport of soil N, P, and K by plants. Meanwhile, rainfall and runoff may promote the release and solubilization of soil nutrients, and the rainy season is usually accompanied by higher temperatures, which is conducive to photosynthesis and the metabolism of plants. N, P, and K in plant leaves may be more active and participate in more biochemical reactions. During the dry season, rainfall decreases and soil moisture is insufficient, resulting in limited uptake of nutrients by plants. The release of soil nutrients is also slowed down, the photosynthesis and metabolism of plants are inhibited by the lower temperature, and the activity of N, P, and K in leaves is also reduced [36,56].

4.4. N:P:K Stoichiometric Homeostasis in Leaves and Soil of the Three Shelterbelt Forests

Plants are able to maintain a constant balance of nutrients by regulating their internal stability, which helps them to maintain desired growth levels and adapt to changes in external conditions, such as changes in soil nutrient availability [4]. The homeostasis of N content in the leaves of Pinus elliottii × caribaea was higher than those of Casuarina equisetifolia and Cocos nucifera during both the rainy and dry seasons, suggesting that Pinus elliottii × caribaea tended to utilize N more conservatively and maintained slower growth even under nutrient-poor conditions [31]. In this study, the N:P ratio in the leaves of Casuarina equisetifolia, Cocos nucifera and Pinus elliottii × caribaea showed absolute homeostasis and sensitivity during the wet and dry seasons, respectively. This may be attributable to the abundant soil moisture during the rainy season and the high solubility of nutrients and their absorption by plant roots. At the same time, the input of nutrients from rainfall may also increase the effectiveness of soil nutrients [57]. Under such environmental conditions, Casuarina equisetifolia, Cocos nucifera, and Pinus elliottii × caribaea could maintain high nutrient uptake and utilization efficiencies, thus maintaining absolute homeostasis of their leaf N:P ratio. Nevertheless, the case changed during the dry seasons; the homeostasis of their leaf N:P ratio degraded, suggesting that the N:P ratios in leaves were significantly affected by changes in soil nutrients, and their internal stability was reduced. This may be due to the decrease in soil moisture and nutrient availability during the dry season [57], resulting in limited nutrient uptake and utilization by Casuarina equisetifolia, Cocos nucifera, and Pinus elliottii × caribaea. In this case, these trees may not be able to effectively regulate their leaf N:P ratios in order to adapt to the fluctuation of soil nutrients. Overall, the leaf N, P, and K contents and their stoichiometric homeostasis of the three shelterbelt forests were higher in the rainy season than in the dry season. This is primarily attributable to a combination of factors such as sufficient soil moisture, increased microbial activity and adjustment of plant physiological mechanisms during the rainy season [58,59]. These factors jointly promote the uptake and utilization of N, P, and K by plants, thereby increasing the N, P, and K contents in plant leaves and their stoichiometric homeostasis.

5. Conclusions

This study reveals notably low N, P, and K contents in both leaves and soils in three shelterbelt forests, with soil N:P ratios being significantly below 14, indicating insufficient nitrogen supply for plant growth. To address this, future coastal shelterbelt afforestation attempts should incorporate nitrogen-fixing tree species and the application of nitrogen fertilizers. The stoichiometric characteristics of plant leaves in these shelterbelts are strongly influenced by soil properties, and the three forest types exhibit distinct nutrient uptake/utilization mechanisms. Consequently, species-specific management protocols are essential for coastal shelterbelt maintenance. Notably, leaf N, P, and K stoichiometric homeostasis (e.g., N:P, N:K, K:P ratios) is markedly higher during wet seasons, reaching absolute homeostasis. To enhance ecosystem stability, we recommend targeted supplementation of limiting nutrients during the wet season, which is the peak period of nutrient absorption by plants.

Author Contributions

Conceptualization, S.N. and Y.C.; methodology, S.N. and Z.C.; investigation, S.N., H.C., Z.L., S.S., X.L. and J.J.; data curation, H.C. and Z.L.; writing—original draft preparation, S.N.; writing—review and editing, Y.C. and Z.C.; project administration, S.N. and Y.C.; funding acquisition, Y.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Hainan Province Department of Science and Technology (Technological Innovation Special Grant for Provincial Research Institutes, No. KYYSLK2023-015).

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Forests 16 00600 g001
Figure 1. Schematic diagram of the study area and sample plot locations. Note: Ca, Co, and Pi are forests of Casuarina equisetifolia, Cocos nucifera, and Pinus elliottii × caribaea, respectively.
Figure 1. Schematic diagram of the study area and sample plot locations. Note: Ca, Co, and Pi are forests of Casuarina equisetifolia, Cocos nucifera, and Pinus elliottii × caribaea, respectively.
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Forests 16 00600 g002
Figure 2. N, P, and K stoichiometric characteristics in leaves of the three shelterbelt forests. Note: Ca represents Casuarina equisetifolia forest; Co represents Cocos nucifera forest; Pi represents Pinus elliottii × caribaea forest. Different uppercase letters indicate significant differences in different seasons (p < 0.05); different lowercase letters indicate significant differences for different forests (p < 0.05). LN, LP, and LK are N, P, and K contents in leaves, respectively. (A–C) represent N, P, and K contents in the leaves of the three forests; (D–F) represent N:P, N:K, and K:P ratios in the leaves of the three forests.
Figure 2. N, P, and K stoichiometric characteristics in leaves of the three shelterbelt forests. Note: Ca represents Casuarina equisetifolia forest; Co represents Cocos nucifera forest; Pi represents Pinus elliottii × caribaea forest. Different uppercase letters indicate significant differences in different seasons (p < 0.05); different lowercase letters indicate significant differences for different forests (p < 0.05). LN, LP, and LK are N, P, and K contents in leaves, respectively. (A–C) represent N, P, and K contents in the leaves of the three forests; (D–F) represent N:P, N:K, and K:P ratios in the leaves of the three forests.
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Forests 16 00600 g003
Figure 3. N, P, and K stoichiometric characteristics in soil of the three shelterbelt forests. Note: Ca represents Casuarina equisetifolia forests; Co represents Cocos nucifera forests; Pi represents Pinus elliottii × caribaea forests. Different uppercase letters indicate significant differences during different rainfall seasons (p < 0.05); different lowercase letters indicate significant differences in different forests (p < 0.05). SN, SP, and SK are N, P, and K contents in soil, respectively. (A–C) represent N, P, and K contents in soil of the three forests; (D–F) represent N:P, N:K, and K:P ratios of the three forests.
Figure 3. N, P, and K stoichiometric characteristics in soil of the three shelterbelt forests. Note: Ca represents Casuarina equisetifolia forests; Co represents Cocos nucifera forests; Pi represents Pinus elliottii × caribaea forests. Different uppercase letters indicate significant differences during different rainfall seasons (p < 0.05); different lowercase letters indicate significant differences in different forests (p < 0.05). SN, SP, and SK are N, P, and K contents in soil, respectively. (A–C) represent N, P, and K contents in soil of the three forests; (D–F) represent N:P, N:K, and K:P ratios of the three forests.
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Forests 16 00600 g004
Figure 4. Correlation analysis of N, P, and K contents and their stoichiometric ratios in the leaf–soil continuum of the three shelterbelt forests. Note: * p < 0.05; ** p < 0.01. (A): Casuarina equisetifolia forest during the wet seasons; (B): Casuarina equisetifolia forest during the dry seasons; (C): Cocos nucifera forest during the wet seasons; (D): Cocos nucifera forest during the dry seasons; (E): Pinus elliottii × caribaea forest during the wet seasons; (F): Pinus elliottii × caribaea forest during the dry seasons.
Figure 4. Correlation analysis of N, P, and K contents and their stoichiometric ratios in the leaf–soil continuum of the three shelterbelt forests. Note: * p < 0.05; ** p < 0.01. (A): Casuarina equisetifolia forest during the wet seasons; (B): Casuarina equisetifolia forest during the dry seasons; (C): Cocos nucifera forest during the wet seasons; (D): Cocos nucifera forest during the dry seasons; (E): Pinus elliottii × caribaea forest during the wet seasons; (F): Pinus elliottii × caribaea forest during the dry seasons.
Forests 16 00600 g004
Table 1. Overview of the three coastal shelterbelt forests.
Table 1. Overview of the three coastal shelterbelt forests.
IndexForest
CaCoPi
Longitude110°58′53″ E110°51′32″ E110°54′52″ E
Latitude19°43′59″ N19°34′51″ N19°41′32″ N
Soil typeSandy loamSandy loamSandy loam
Tree height (m)9.51 ± 0.4711.54 ± 2.1310.67 ± 0.96
Diameter at breast height (cm)17.84 ± 1.4423.72 ± 2.7618.65 ± 1.61
Soil capacity (g/cm3)1.46 ± 0.031.47 ± 0.021.62 ± 0.04
Soil moisture content (%)5.90 ± 0.066.03 ± 0.088.56 ± 0.06
Understory vegetation cover (%)10.23 ± 3.5612.1 ± 4.4721.32 ± 3.81
Ca represents Casuarina equisetifolia L. forests; Co represents Cocos nucifera L. forests; Pi represents Pinus elliottii × caribaea forests.
Table 2. N, P, and K contents and stoichiometric homeostasis indexes in leaves of the three shelterbelt forests.
Table 2. N, P, and K contents and stoichiometric homeostasis indexes in leaves of the three shelterbelt forests.
ForestSeasonIndex1/HHR2PLevel
CaWet seasonN0.631.590.7830.019Weakly sensitive
P0.0812.710.0070.872Absolute homeostasis
K−0.352.900.5480.093Weak homeostasis
N:P−0.0335.560.0020.953Absolute homeostasis
N:K0.0249.360.0070.879Absolute homeostasis
K:P−0.0260.500.0620.634Absolute homeostasis
Dry seasonN0.234.400.2180.351Absolute homeostasis
P−2.280.440.9540.001Sensitive
K−0.0247.370.0050.890Absolute homeostasis
N:P1.990.500.9640.002Sensitive
N:K0.195.260.8560.008Homeostasis
K:P−0.372.730.9750.003Weak homeostasis
CoWet seasonN−0.0521.200.0690.615Absolute homeostasis
P−0.561.800.3680.202Absolute homeostasis
K0.661.510.4820.126Absolute homeostasis
N:P−0.0335.560.0020.935Absolute homeostasis
N:K0.195.240.1910.386Absolute homeostasis
K:P0.109.850.0910.561Absolute homeostasis
Dry seasonN1.140.870.9820.001Sensitive
P−1.680.600.9790.002Sensitive
K0.0714.110.9590.001Homeostasis
N:P−8.260.120.8000.016Sensitive
N:K0.263.870.8410.010Weak homeostasis
K:P−0.214.790.2930.268Absolute homeostasis
PiWet seasonN−1.780.560.9080.003Sensitive
P−0.382.650.2900.270Absolute homeostasis
K0.214.750.0760.597Absolute homeostasis
N:P−0.175.980.4010.177Absolute homeostasis
N:K0.119.470.0880.568Absolute homeostasis
K:P0.156.720.2700.290Absolute homeostasis
Dry seasonN−0.0911.700.0180.801Absolute homeostasis
P−0.961.040.5680.083Sensitive
K0.452.240.9710.004Weak homeostasis
N:P1.190.840.9660.002Sensitive
N:K0.195.200.4850.124Absolute homeostasis
K:P0.0262.420.0280.753Absolute homeostasis
Ca represents the Casuarina equisetifolia forest; Co represents the Cocos nucifera forest; Pi represents the Pinus elliottii × caribaea forest.
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Nong, S.; Chen, H.; Chen, Z.; Lin, Z.; Su, S.; Lei, X.; Jia, J.; Chen, Y. Effects of Seasonal Rainfall Changes on N, P, and K Stoichiometric Characteristics in Leaves and Soil of Tropical Coastal Shelterbelt Forests. Forests 2025, 16, 600. https://doi.org/10.3390/f16040600

AMA Style

Nong S, Chen H, Chen Z, Lin Z, Su S, Lei X, Jia J, Chen Y. Effects of Seasonal Rainfall Changes on N, P, and K Stoichiometric Characteristics in Leaves and Soil of Tropical Coastal Shelterbelt Forests. Forests. 2025; 16(4):600. https://doi.org/10.3390/f16040600

Chicago/Turabian Style

Nong, Shouqian, Haihui Chen, Zongzhu Chen, Zhipan Lin, Shaofeng Su, Xiangling Lei, Junting Jia, and Yiqing Chen. 2025. "Effects of Seasonal Rainfall Changes on N, P, and K Stoichiometric Characteristics in Leaves and Soil of Tropical Coastal Shelterbelt Forests" Forests 16, no. 4: 600. https://doi.org/10.3390/f16040600

APA Style

Nong, S., Chen, H., Chen, Z., Lin, Z., Su, S., Lei, X., Jia, J., & Chen, Y. (2025). Effects of Seasonal Rainfall Changes on N, P, and K Stoichiometric Characteristics in Leaves and Soil of Tropical Coastal Shelterbelt Forests. Forests, 16(4), 600. https://doi.org/10.3390/f16040600

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