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Article

Stoichiometric Coupling of C, N, P, and K in the Litter and Soil of Rosa roxburghii Tratt Woodlands across Rocky Desertification Grades and Seasons

by
Mingjun Li
1,2,
Mingfeng Du
2,*,
Huajiang Chen
3,
Yan Shi
4,
Dan Yao
1 and
Qiusi Shi
1
1
Guizhou Forestry Survey and Planning Institute, Guiyang 550003, China
2
School of Karst Science, Guizhou Normal University, Guiyang 550025, China
3
Forestry Bureau of Xixiu District, Anshun 561000, China
4
Natural Resources Bureau of Shuicheng District, Liupanshui 553000, China
*
Author to whom correspondence should be addressed.
Forests 2024, 15(8), 1415; https://doi.org/10.3390/f15081415
Submission received: 27 July 2024 / Revised: 8 August 2024 / Accepted: 11 August 2024 / Published: 13 August 2024
(This article belongs to the Section Forest Soil)
Browse Figures
Versions Notes

Abstract

:
The purpose of this study was to explore the inherent links between elemental cycling in Rosa roxburghii Tratt litter and soil, as well as their coupled relationships, within barren soil environments typical of karst rocky desertification regions in Guizhou Province. Ecological stoichiometric methods were used to systematically analyze the nutrient concentrations of C, N, P, and K and their stoichiometry in the litter and soil of Rosa roxburghii, with a focus on the impacts of seasonal variations and rocky desertification regions. High C and K levels and low N and P levels are observed in the litter, whereas the soil has lower concentrations of C, N, P, and K, with nutrient replenishment priorities of N > P > K > C. Strong positive correlations are found among the C/N, C/P, and N/K stoichiometric ratios in both the litter and the soil. Furthermore, nutrient concentrations and stoichiometric ratios vary significantly by season. Seasonal variations influence nutrient concentrations, with notable increases in litter P and K levels and in soil N and P levels in September compared with March. Seasonal variations influence the stoichiometric ratios of C/N, C/P, and N/K in litter and soil, contributing to elemental balance and ecosystem stability. Moreover, significant variations in nutrient contents and stoichiometric ratios are observed across distinct rocky desertification grades. Nonrocky desertified areas present elevated P and K contents in litter, whereas light desertified areas present increased C and N concentrations. Moderately desertified areas presented increased soil P and K concentrations, whereas severely desertified areas presented the highest N levels. These discernible trends in nutrient profiles highlight the synergistic impacts of soil nutrient inadequacy and plant utilization strategies. These findings contribute to a better understanding of element cycling mechanisms in Rosa roxburghii woodland ecosystems, offering valuable information for sustainable forest management practices.

1. Introduction

Ecological stoichiometry is a crucial discipline for studying the dynamic balance between multiple elements and the mechanisms of energy cycling in ecosystems [1,2]. It encompasses various scales ranging from species, communities, and ecosystems to regions and the world [3], occupying a pivotal position in the development of ecological theory [4]. This field is profoundly theoretically important for revealing plant ecological and chemical processes [5], biogeochemical cycles [6], and the functional and stability mechanisms of ecosystems. Ecosystems are affected by various factors, including climate, site conditions, community structure, vegetation types, and soil types, resulting in complex nutrient cycling processes. These processes involve several steps, such as organic matter decomposition, mineralization, absorption, and excretion [7], ensuring the effective cycling and flow of nutrients between biotic and abiotic components to maintain the stability of their structure and function. Given the large differences among ecosystems or plant communities and the complexity and variability of element cycling pathways, exploring element dynamic balances and homeostatic mechanisms via ecological stoichiometry is practically relevant for guiding ecosystem nutrient management.
Litter and soil are pivotal for nutrient cycling and balance within ecosystems [8]. As litter is a primary source of organic matter, the elemental composition and C/N/P/K stoichiometric ratios of litter crucially impact soil physicochemical properties, nutrient pools, and microbial activities [9]. As litter decomposes, elements are released and may be fixed by the soil or reabsorbed by plants, maintaining the cycling and stoichiometric balance of the ecosystem. Numerous researchers have explored the stoichiometric relationships between litter and soil at temporal [10] and spatial scales [11]. Research has shown that changes in forest age [10], extreme climate events [12], and fertilization [13] can lead to alterations in plant phenological characteristics, affecting the quantity and quality of litter, soil nutrient concentration and distribution, and, thus, the nutrient cycling processes of ecosystems. Additionally, factors such as different vegetation types [14,15], community types [16], and soil properties [17,18] can lead to variations in the coupling relationships between litter and soil. Delving into the transfer and cycling of elements between litter and soil aids in understanding the mechanisms of nutrient flow and balance in ecosystems.
Rocky desertification refers to a form of land deterioration that manifests in the karst landscape areas of Southwest China and is characterized by large areas of exposed rock, infertile soil, and extremely low vegetation coverage [19]. With the intensification of worldwide climate variation and anthropogenic disturbances, the severity of rocky desertification is continuously increasing, leading to a large decrease in land productivity or complete loss, seriously threatening local ecological security and causing considerable economic losses to the regional economy [20]. Rosa roxburghii Tratt, a typical karst economic forest species, is edible; has medicinal, economic, and ecological value; and is widely planted in karst rocky desertified areas. Research on Rosa roxburghii has focused mostly on its edible and medicinal value [21], quality improvement [22], and other aspects. However, few studies have investigated the vegetation restoration value and ecological stoichiometric characteristics of Rosa roxburghii [23], and research on how seasonal changes and different grades of rocky desertification affect these characteristics is lacking. Investigating the influence of seasonal variations and varying grades of rocky desertification on the ecological stoichiometric characteristics of Rosa roxburghii will contribute to analyzing its nutrient utilization and limitation status at finer temporal and spatial scales. This will help reveal the balancing mechanisms of its dynamic productivity.
This study sought to assess the integrated impacts of seasonal variations and distinct rocky desertification regions on the C, N, P, and K stoichiometry of litter and soil within Rosa roxburghii systems. The hypotheses were as follows: (1) Is there a significant correlation between the stoichiometry of C, N, P, and K in the litter and soil within the Rosa roxburghii system? What are the critical factors? (2) Do seasonal variations and varying degrees of rocky desertification significantly affect the stoichiometry of C, N, P, and K in litter and soil? How can this be influenced? (3) In response to seasonal variations and rocky desertification regions, does the stoichiometry of C, N, P, and K in litter and soil exhibit regular changes? What are the coupling dynamics patterns between them? The objective of this study was to evaluate the ecological health and nutrient cycling processes in these ecosystems, providing essential theoretical support and practical guidance for ecological restoration and sustainable land management in rocky desertification regions.

2. Materials and Methods

2.1. Overview of the Research Region

The research area is in Shuangbao Daba village, Xixiu District, Anshun city, Guizhou Province, China (Figure 1). It features a distinctive karst plateau gorge landform at an average elevation of 1350 m, with prevalent soil types, including yellow soil and yellow limestone soil. The area experiences a subtropical climate, characterized by an average annual temperature of 13.5 °C and an average yearly precipitation of 1150 mm. Rosa roxburghii, a high-value economic shrub species, has been under continuous cultivation in this region for numerous years. The vegetation in the study area mostly comprises subtropical evergreen and deciduous mixed forests. The wild vegetation includes vines, thorns, and shrubs, which predominantly feature thorny pears, narrow-leaved firethorns, Clematis, and jujubes. Additionally, there are scattered distributions of light-barked birch and Masson pine.

2.2. Plot Establishment

On the basis of the field reconnaissance of the study area and the classification standard of karst ecosystem degradation gradients proposed by Xiong Kangning et al. [24], the rocky desertification succession in the study area was categorized into four grades: nonrocky desertification, lightly rocky desertification, moderately rocky desertification, and severely rocky desertification. The soil properties of the experimental site are displayed in Table A1. For each grade, an 8-year-old monoculture woodland of Rosa roxburghii was selected as the research subject, and three randomly chosen 20 m × 20 m plots were established. Detailed information regarding the landform, elevation, woodland conditions, and rock bareness rates of all the plots is presented in Table A2.

2.3. Sampling and Indicator Measurement

Sampling was conducted in March and September of 2023, with average temperatures of 16 and 26 degrees Celsius, respectively. Owing to the shallow soil layer characteristic of the rocky desertified area, five plots (3 m × 3 m) were arranged in an S-shaped pattern at the center of the sampling site. Within each plot, one well-grown Rosa roxburghii plant was randomly selected, and litter leaves under the plant canopy (1 m × 1 m) were collected, mixed uniformly, and taken to the laboratory. Following initial drying in an oven at 105 °C for 30 min, the samples were further dried at a consistent temperature of 60 °C and pulverized for further use. For the soil under Rosa roxburghii woodland, 1 kg of surface soil (0~20 cm) was collected within a 1 m × 1 m range around the plant’s ground diameter via a diagonal method. The soil samples were mixed uniformly, air-dried, debris removed, quartered, ground, and sieved through a 100-mesh screen. The concentrations of C, N, P, and K in both the litter and the soil were determined, and each index was measured three times.
The total C and N contents were measured via a German vario MACRO cube elemental analyzer (Elementar, Hanau, Germany), the total K content was determined via the sodium hydroxide fusion-flame photometry method [25], the total P in the litter was assayed via the molybdenum–antimony anticolorimetry method, and the total P in the soil was determined via the sodium hydroxide fusion–molybdenum–antimony anticolorimetry method [26].

2.4. Statistical Analysis

Analysis of variance, multiple comparisons (Duncan test), Pearson correlation analysis, and cluster analysis were carried out via SPSS 23.0 (SPSS Inc., Chicago, IL, USA), redundancy analysis (RDA) was conducted via Canoco 5.0 (Microcomputer Power, Ithaca, NY, USA), and Origin 24.0 (Origin Lab, Hampton, MA, USA) was used to create charts and graphs.

3. Results

3.1. Overall Characteristics of Nutrient Contents and Stoichiometric Ratios in Both the Litter and Soil of Rosa roxburghii

Table 1 shows that the C, N, P, and K contents of the litter were 595 mg/g, 12 mg/g, 0.44 mg/g, and 6.5 mg/g, respectively, which were 14.2, 8, 1.3, and 1.4 times greater than those of the soil (42 mg/g, 1.5 mg/g, 0.35 mg/g, and 4.8 mg/g, respectively). This significant enrichment, particularly in carbon and nitrogen, indicated a more substantial nutrient contribution from the litter. Additionally, the stoichiometric ratios of C/P, C/K, N/P, N/K, C/N, and K/P in the litter were calculated as 1398, 97, 26, 1.8, 52, and 15, respectively, which were 10.8, 10.8, 6, 6, 1.7, and 1.1 times greater than those in the soil (129, 9, 4.3, 0.3, 30, and 14). These results indicated that the C, N, P, and K stoichiometric ratios in the litter were greater than those in the soil, revealing the diversity of litter sources and the impact of decomposition. Furthermore, the coefficients of variation for C and K in the litter were greater at 18.3 and 33.2, respectively, than at 16.8 and 26, respectively, in the soil. Conversely, the coefficients of variation for N and P in the litter were lower at 17.6 and 20.4, respectively, than those in the soil (23 and 25.2). Remarkably, the coefficient of variation for N/K reached its peak in both the litter (36.78) and the soil (30.64), indicating that the highest values were observed in these specific contexts. Conversely, the lowest coefficient of variation was consistently identified for K/P in the litter (22.74) and for N/P in the soil (21.44). The chemical composition of the litter varied more than that of the soil.

3.2. Nutrient Concentrations and Stoichiometric Characteristics of Rosa roxburghii Litter in Different Seasons and under Different Degrees of Rocky Desertification

Compared with those in March, the concentrations of litter P and K were notably greater in all grades of rocky desertification in September. The litter C concentration was notably lower in moderate and severely rocky desertified areas, whereas the litter N concentration was significantly lower in severely rocky desertified areas but significantly greater in other rocky desertified areas. Furthermore, the C/P and C/K stoichiometric ratios of litter were significantly lower in all the rocky desertified areas, whereas the litter C/N ratio was significantly greater in the nonrocky desertified areas but significantly lower in the other rocky desertified areas. Additionally, the litter N/P ratios in nonrocky, moderately, and severely rocky desertified areas; the N/K ratios in non- and severely rocky desertified areas; and the litter K/P ratios in nonrocky desertified areas presented significantly lower values (Figure 2, Table A3).
In nonrocky desertified areas, the highest concentrations of litter P and K were detected, whereas the highest levels of litter C and N were detected in lightly rocky desertified areas. Furthermore, the stoichiometric ratios of litter C/N and K/P were notably greater in the nonrocky desertified areas than in the other grades, the ratio of litter C/K was significantly greater in the moderately rocky desertified areas than in the other grades, and the stoichiometric ratios of litter N/K and N/P were notably greater in the severely rocky desertified areas than in the other grades. Additionally, there were no significant differences in the litter C/P ratios among the different grades of rocky desertification. With the increasing grade of rocky desertification, the litter C concentration began to increase and later decreased, whereas the litter N concentration and litter N/P ratio first increased but then decreased before increasing again. Furthermore, the levels of litter K and P as well as the stoichiometric ratios of litter C/N and K/P initially decreased but then increased, whereas the stoichiometric ratios of litter N/K and C/K gradually increased (Figure 2, Table A3).

3.3. Nutrient Concentrations and Stoichiometric Characteristics of Soil in Rosa roxburghii Woodland during Different Seasons and under Different Grades of Rocky Desertification

Compared with those in March, the concentrations of soil N and P were significantly greater in all grades of rocky desertification in September, whereas the soil K concentration was notably greater in the lightly, moderately, and severely rocky desertified areas. Conversely, the soil C concentration was notably lower in moderately rocky desertified areas. The soil K/P ratio was significantly greater in severely rocky desertified areas but notably lower in other grades, and the soil N/K ratio was significantly greater in nonrocky desertified areas but notably lower in severely rocky desertified areas. Furthermore, the soil C/N and C/P stoichiometric ratios were significantly lower in all the rocky desertified areas, and the soil C/K ratio was notably lower in the lightly, moderately, and severely rocky desertified areas. No significant difference in the soil N/P ratio was detected between March and September (Figure 3, Table A3).
The highest C concentration in the soil was detected in the lightly rocky desertified area, whereas the highest P and K concentrations in the soil were detected in the moderately rocky desertified area. Additionally, the soil N concentration was highest in the severely rocky desertified area. Moreover, the stoichiometric ratios of soil C/P, C/N, and K/P were markedly greater in the lightly rocky desertified area than in the other desertified regions, whereas the stoichiometric ratios of soil N/K and N/P were notably greater in the severely rocky desertified area than in the other desertified regions. No significant differences were observed in the soil C/K ratios among the rocky desertification grades. With increasing grades of rocky desertification, the C, P, and K concentrations in the soil, as well as the soil C/N, C/P, and K/P ratios, initially increased and then decreased, whereas the soil N/K ratio decreased, followed by an increase. The soil N concentration and soil N/P ratio gradually increased (Figure 3, Table A3).

3.4. Interactive Effects of Rosa roxburghii Litter on Soil Nutrient Concentrations and Chemical Stoichiometry

3.4.1. Correlations between Rosa roxburghii Litter and Soil Nutrient Concentrations and Chemical Stoichiometry

As illustrated in Figure 4, the Pearson correlations between litter and soil were classified into two major branches. In the first branch of clustering, highly significant positive correlations were found between C, C/N, C/P, C/K and N/P in the litter and C/N, C/P, and C/K in the soil (p < 0.01). There were also highly significant positive correlations between C/P, C/K, and N/P in the litter and C in the soil (p < 0.01) and between C/K in the litter and N/P in the soil (p < 0.01). Additionally, there were significant positive correlations between C in the litter and C and C/K in the soil (p < 0.05), between C/N in the litter and K/P in the soil (p < 0.05), and between N/P in the litter and N/P in the soil (p < 0.05). Moreover, there were highly significant negative correlations between C/P and C/N in the litter and N, P, and K in the soil (p < 0.01) and between C/K in the litter and P in the soil (p < 0.01). Furthermore, there was a highly significant negative correlation between litter C and soil K (p < 0.01) and a significant negative correlation with soil N (p < 0.05).
The second branch of clustering revealed highly significant positive correlations between N and P in the litter and N, P, and N/K in the soil (p < 0.01). Additionally, there were highly significant positive correlations between litter N/K and soil C, N, C/P, N/P, and N/K (p < 0.01); between litter K and soil P (p < 0.01); between litter N and soil N/P (p < 0.01); and between litter K/P and the soil C/N (p < 0.01). Moreover, there were significant positive correlations between P in the litter and K in the soil (p < 0.05) and between N/K in the litter and C/K in the soil (p < 0.05). Furthermore, there were highly significant negative correlations between P in litter and soil C/N and C/P (p < 0.01), between K in litter and C/P in soil (p < 0.01), and between K/P in litter and N in soil (p < 0.01). Finally, significant negative correlations were revealed between N in the litter and C/N in the soil (p < 0.05), between K in the litter and N/P in the soil (p < 0.05), and between the litter K/P and the soil N/P and N/K ratios (p < 0.05).

3.4.2. RDA of Litter—Soil Nutrient Concentrations and Stoichiometric Ratios in Rosa roxburghii Woodland

The RDA results (Figure 5 and Figure 6) revealed that the explanatory power of the nutrient concentrations and stoichiometric ratios of the litter–soil mixtures on the first and second axes were 42.67% and 28.34% and 46.43% and 29.43%, respectively. The cumulative contribution rates were 71.01% (F = 23.1, p = 0.001) and 75.86% (F = 31.05, p = 0.001), indicating robust sorting outcomes. These findings suggest that nutrient cycling and stoichiometric changes effectively explain the stoichiometric relationship between litter and soil.
The angles of the litter C/N and C/P ratios to the first axis were all less than 30°, indicating a significant positive correlation. The angle between the litter K and the first axis was close to 180°, indicating a significant negative correlation. Additionally, the angle between the litter K/P ratio and the second axis was less than 30°, indicating a significant positive correlation. The angle between the litter N/K ratio and the second axis was close to 180°, indicating a significant negative correlation. On the basis of the RDA sorting and the length of the connecting lines, the litter C/N, C/P, and N/K ratios had greater impacts on the soil, whereas the litter K and K/P ratios had smaller impacts on the soil.
The angles between the soil N and K and the first axis were less than 30°, indicating a significant positive correlation. The angles between the soil C, C/K, C/N, and C/P ratios and the first axis were close to 180°, indicating a significant negative correlation. On the basis of the RDA sorting and the length of the connecting lines, the soil C/N and C/P ratios had greater impacts on the litter, whereas the soil C, N, K, and C/K ratios had smaller impacts on the litter.

4. Discussion

4.1. Nutrient Concentrations of C, N, P, and K in the Litter and Soil of Rosa roxburghii Woodland

Litter decomposition bridges nutrient recycling between plants and soil [27]. This study revealed higher concentrations of C and K (595 mg/g and 6.5 mg/g, respectively) in litter than in economic forests in Guizhou [14] and shrubland in Guangxi [28] but lower levels of N and P (12 mg/g and 0.44 mg/g). Lower N and P levels in the litter indicate reabsorption by leaves when these nutrients drop below 7 mg/g and 0.5 mg/g, respectively [29]. The low P concentration (0.44 mg/g) suggests complete absorption driven by P deficiency [30], whereas the N concentration (11.6 mg/g) implies incomplete absorption.
Soil nutrients are essential for plant growth and play a crucial role in the material cycles of ecosystems [8]. In the 0–20 cm soil layer, the average concentrations of C, N, P, and K were 42, 1.5, 0.35, and 4.8 mg/g, respectively. The C content (42 mg/g) was higher than that in local shrublands [14] (31.8–38.17 mg/g) but lower than that in the karst regions of northwestern Guangxi [28] and southern Yunnan [31] (86.76–92 mg/g). Compared with the N (0.58–7.52 mg/g), P (0.54–2.06 mg/g), and K (4.14–13.68 mg/g) concentrations in the karst regions of Guizhou, Yunnan, and Guangxi [28,32], the identified lower N, P, and K concentrations indicate poor nutrient conditions. These deficiencies are due to unique karst features and seasonal heavy rainfall, which cause significant soil leaching. In rocky desertification areas, inadequate vegetation cover, shallow soil layers, and unstable SOC and available N lead to substantial C and N losses through leaching [33]. P and K levels in the soil are largely influenced by parent materials. The abundant water and heat resources in karst regions accelerate rock weathering and decomposition, releasing P and K [34]. However, intense leaching may cause significant P loss, while the migration of active K leads to its depletion in the soil [35]. This interplay of factors underscores the critical role of geological and climatic conditions in shaping soil nutrient dynamics in karst ecosystems.
The present study revealed elevated concentrations of C, N, and P in the litter relative to those in the soil, as corroborated by significant positive correlations (p < 0.05), indicating that these elements were transferred from the litter to the soil. Both litter and soil presented low N and P levels, suggesting a decelerated decomposition rate [36] that impedes nutrient release and causes soil nutrient deficiencies. The disparity in K levels, with higher concentrations in the litter and lower concentrations in the soil, was evidenced by the absence of correlation between the litter and soil potassium levels (p > 0.05). The possible reasons included the soil type, plant type, and growth stage. In barren soil, the K content is low and susceptible to leaching loss through run-off or soil surface erosion [37]; however, plants have a high demand for K during growth, increasing K uptake from the soil through microbial or root exudates [38], resulting in a relatively high K content in the litter. This implied an imbalance in potassium exchange between the soil and plants. These findings are consistent with Hu Qijuan’s research on southwestern karst rocky desertification [39] but diverge from Yu Yuefeng’s observations in the Guangxi karst region [40]. These differences in nutrient dynamics within barren soil ecosystems can be attributed to the collective impact of intricate interactions among diverse vegetation communities, species characteristics, and environmental conditions.

4.2. Stoichiometric Characteristics of C, N, P, and K in the Litter and Soil of Rosa roxburghii Woodland

The stoichiometric ratios of C/N, C/P, and N/P in litter are critical for nutrient retention and release and are often inversely correlated with decomposition rates [41,42]. Lower C/N and C/P ratios indicate higher N and P concentrations, stimulating microbial activity and accelerating litter decomposition, thereby increasing nutrient return efficiency [41]. In this study, the elevated C/N (52) and C/P (1398) ratios in the litter exceeded the critical thresholds for nutrient release (C/N < 40 or C/P < 480) [43] and those observed for terrestrial plants in China (C/N: 31.4–50.57, C/P: 440–842.51) [28], indicating a sluggish decomposition rate. The N/P ratio of 26 suggests that P deficiency might be a key constraint on litter decomposition [44], diverging from findings in karst regions by Yu Yuefeng [16] and Yue Xiangfei [45], potentially due to regional differences in climate, vegetation, and soil properties. Additionally, the C/K, N/K, and K/P ratios in the litter were 98, 1.8, and 15, respectively. Compared with the economic forests in the karst region of Guizhou Province (C/K: 117.27, N/K: 4.65, and K/P: 1.75) [14], those in this study presented lower C/K and N/K ratios, indicating higher K concentrations in the litter. This suggests strategic nutrient absorption and utilization by plants, where K uptake is restricted during leaf senescence, leading to its accumulation in the litter for subsequent soil enrichment [29], thus sustaining plant growth and facilitating nutrient recycling.
The stoichiometric ratios of soil C, N, P, and K are crucial indicators of soil nutrient status and fertility [17]. This study revealed a higher soil C/N ratio (30) than that in karst forest regions (3.47–18.85) [17,46], various regions in China (10–12) [47], and the global average (13.33) [48]. This high ratio is associated with the slow decomposition rate of soil organic matter, which contributes to the preservation of soil fertility [49]. The higher soil C/P ratio (129), although within the range observed in karst forest regions (4.89–157.02) [17,46], exceeds the national average in China (61.00) [28], indicating a reduced nutrient release capacity and available P concentration. The lower soil N/P ratio (4.3) than the national average (5.2) [50] suggests nitrogen limitation due to an imbalance between nitrogen and phosphorus availability [44]. Additionally, the observed soil C/K, N/K, and K/P ratios were 9.4, 0.3, and 14, respectively. Compared with forestland soils in the Yunnan–Guizhou Plateau (C/K = 0.24–13.72, N/K = 0.07–1.14, K/P = 1.69–20.00) [39,51], these findings highlight a unique nutrient balance characterized by high potassium availability and nitrogen limitation in the study area [35].
The stoichiometric ratios of C, N, and P in litter and soil are crucial for ecosystem nutrient cycling [52], helping identify limiting elements and their ecological impacts. This study revealed significant interactions (RDAs) among the C/P, C/N, and N/K ratios in both the litter and the soil (p < 0.01). The elevated C/P and C/N ratios, alongside low N/K ratios, suggest N and P deficiencies in the rocky desertification area, potentially impeding nutrient cycling. These deficiencies may result from plant strategies for N and K absorption [38], leading to low N/K ratios in litter and, subsequently, in soil. The nutrient demand hierarchy was identified as N > P > K > C, emphasizing the urgent need for N and P fertilizer supplementation to promote forestland growth and maintain the nutrient balance of the ecosystem.

4.3. Coupling Interactions of C, N, P, and K Chemistry in the Litter–Soil System of Rosa roxburghii under Seasonal Change

Seasonal changes are crucial for regulating plant growth, litter decomposition, and soil microbial activities by influencing water and heat conditions, thus shaping nutrient cycling and patterns at various trophic levels of the ecosystem [53]. In this study, litter P and K and soil N and P increased across all grades in September compared with March, demonstrating a clear seasonal pattern with a consistent increase in P content in the litter and soil. Increased plant nutrient uptake in September increases the P content in leaf litter, whereas P-rich litter inputs drive an increase in the soil P level. Seasonal variations synchronize environmental factors with the growth rhythm of plants, promoting phosphorus cycling between plants, fallen leaves, and soils in Rosa roxburghii woodland ecosystems. This indirectly reflects the plant’s growth strategy in selecting phosphorus in response to the nitrogen- and phosphorus-poor soil environment [54].
In September, significant decreases in the C/N and C/P ratios were observed in the litter and soil across the four rocky desertified areas compared with those in March, except for a notable increase in the litter C/N ratio in the nonrocky areas. Slower plant growth in September led to soil N and P accumulation due to reduced demand, with plants allocating more N and P to litter, promoting decomposition and aiding in nutrient cycles [29]. In nonrocky areas, favorable soil conditions facilitate plant growth and organic matter accumulation [55], leading to an accelerated rate of litter decomposition and relatively high litter carbon concentrations. The consistent trends in the litter and soil C/P and C/N ratios are linked to shifts in plant nutrient absorption and distribution strategies influenced by seasonal variations. These changes reflect a dynamic response to seasonal fluctuations and interconnected nutrient cycling patterns within the ecosystem. Moreover, the observed decrease in N/K ratios in litter and soil in severely rocky desertified areas in September, along with the contrasting trends in nonrocky areas where the N/K ratio decreased in litter but increased in soil, can be attributed to nutrient stress and distinct soil characteristics influenced by rocky desertification. In severely rocky desertified areas, extreme rockiness limits nutrient availability due to poor soil structure and reduced organic matter content [38], which is speculated to induce plants to selectively absorb and distribute N and K, disrupting the elemental balance of these nutrients. This leads to decreased N/K ratios in both litter and soil as plants adapt to nutrient stress [56]. Conversely, in nonrocky areas, well-structured soils with efficient nutrient retention support relatively high soil nutrient contents, leading to an increased soil N/K ratio [38]. During the peak growing season, plants increase nutrient uptake for rapid growth, resulting in a greater allocation of nutrients to biomass production and a reduction in the N/K ratio in the litter as available nitrogen and potassium are depleted from the litter layer [57].
In summary, seasonal changes triggered elevated P contents in litter and soil in September, stimulating P cycling in Rosa roxburghii woodland ecosystems. This seasonal shift influenced the C/N, C/P, and N/K ratios by altering nutrient absorption and allocation in plants, impacting litter decomposition and nutrient release. These adaptive mechanisms optimize nutrient utilization efficiency and maintain ecosystem stability in nutrient-poor soils.

4.4. Coupling Interactions of C, N, P, and K Chemistry in the Litter–Soil System of Rosa roxburghii under Different Rocky Desertification Grades

The exacerbation of rocky desertification severely threatens ecosystem stability and function [17], leading to ecological issues such as increased soil erosion, the disruption of nutrient cycling, and imbalances in stoichiometric ratios. In this study, nonrocky desertified areas presented relatively high P and K levels in litter, whereas lightly desertified areas presented increased C and N contents. Notably, lightly desertified soils were enriched in C, whereas P and K levels increased in moderate areas, with the highest N concentration observed in severe areas. This finding indicated that a nutrient gradient was present: leaf litter nutrients were more abundant in less desertified areas, whereas soil nutrients were enriched in highly desertified areas. Ecosystems respond to desertification by adjusting soil nutrient availability through enhancing the nutrient quality of litter. However, soil nutrient enrichment does not consistently increase litter decomposition; it can, at times, impede decomposition rates and nutrient release [58]. These intricate dynamics highlight the adaptability and resilience of ecosystems in mitigating the impacts of rocky desertification. Surprisingly, contrary to our initial hypothesis of hindered soil nutrient accumulation with worsening rocky desertification, significant N, P, and K accumulation was observed in soils of moderately to severely rocky desertified areas. This finding suggests that rocky desertification may not inevitably have a negative impact on soil nutrient accumulation, with potential influences from factors such as plant species, soil characteristics, microbial activity, and moisture levels [59].
This study revealed significant variations in the C/P, C/N, and N/K stoichiometric ratios in litter and soil across different grades of rocky desertification. In lightly rocky desertified areas, favorable soil conditions promote organic matter decomposition and nutrient release [55], leading to C accumulation and increased soil C/N and C/P ratios. However, as rocky desertification intensifies, soil quality deteriorates, resulting in decreased soil C/N and C/P ratios due to the accumulation of N and P, as well as reduced inputs of organic matter [60]. Conversely, nonrocky desertified areas presented relatively high C and N concentrations in litter, increasing the litter C/N ratios. With increasing rocky desertification, reduced litter quantity led to lower C and N levels, causing a decrease in the litter C/N ratios. However, under extreme rocky desertification, plants may adapt by altering nutrient uptake strategies [61], resulting in increased litter C/N ratios. The variability in distinct frequencies of the C/N ratio between soil and litter as rock desertification progresses suggests ecosystem adaptations in terms of soil properties and plant nutrient strategies to maintain elemental equilibrium. This enhances their resilience in barren environments and regulates nutrient cycling efficiency [18]. Furthermore, in severely rocky desertified areas, the highest soil and litter N/K values are likely due to nutrient scarcity and plant adaptations. Fragile soil structure and severe nutrient loss lead to elevated soil N/K values. Plants in nutrient-deficient soils have increased N uptake efficiency, resulting in increased litter N concentrations and elevated N/K ratios [38]. These interactions indicate that soil physicochemical properties evolve uniquely under various rocky desertification grades, complicating plant-mediated element cycling and metabolism and leading to inconsistent soil and litter N/K dynamics.
In summary, rocky desertification impacts ecosystem stoichiometry, leading to complex nutrients across different desertification grades. Nutrient enrichment in highly rocky desertified soils defies conventional expectations, underscoring ecosystem adaptability. Variations in the C/N, C/P, and N/K ratios reflect soil quality changes and plant nutrient strategies in response to rocky desertification, demonstrating ecosystem resilience and nutrient cycling efficiency adjustments.

5. Conclusions

This study examined the stoichiometry of C, N, P, and K in litter and soil within the Rosa roxburghii ecosystem, accounting for seasonal fluctuations and the impact of rocky desertification. The findings revealed that Rosa roxburghii litter had high C and K contents but low N and P contents, whereas woodland soil presented low levels of all nutrients, prioritizing N and P replenishment. Nutrient concentrations and stoichiometric ratios in the litter and the soil varied significantly with season and rocky desertification grade. Furthermore, strong positive correlations were observed among the C/N, C/P, and N/K stoichiometric ratios in both the litter and the soil. The seasonal variations in the C/N, C/P, and N/K ratios of the litter and soil exhibited consistent trends, contributing to the elemental balance and ecosystem stability. As rocky desertification has advanced, inconsistencies in the C/N, C/P, and N/K ratios have emerged in both litter and soil, likely due to soil nutrient deficiency and plant nutrient strategies. These findings provide valuable information regarding the complex interactions among litter decomposition, soil nutrients, and ecosystem functions in Rosa roxburghii woodlands. Nevertheless, the presence of both decomposed and undecomposed materials in the collected litter samples limits a comprehensive understanding of litter decomposition dynamics and their interactions with soil nutrients. Future research should focus on accurately delineating the stages of litter decomposition and investigating potential relationships with bioavailable soil nutrients. This detailed examination is crucial for unraveling the complex mechanisms governing the interplay between litter decomposition and soil nutrient cycling in forest ecosystems.

Author Contributions

Formal analysis, M.L., D.Y. and Q.S.; funding acquisition, M.D.; investigation, M.L., H.C. and Y.S.; methodology, M.D.; resources, H.C. and Y.S.; writing—original draft, M.L.; writing—review and editing, M.D. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by grants from the Science and Technology Support Project of Guizhou Province (Qian-ke-he Zhicheng (2020) 1Y120).

Data Availability Statement

The original contributions presented in the study are included in the article.

Acknowledgments

We would like to thank all the technicians of the Forestry Bureau of Xixiu District involved in the field trial as well as the academic technician and the student assistants of Guizhou Academy of Agricultural Sciences for their excellent work and support in the labs.

Conflicts of Interest

The authors have no competing interests to disclose.

Abbreviations

Corganic carbon
Nnitrogen
Pphosphorus
Kpotassium
C/Ncarbon-to-nitrogen ratio
C/Pcarbon-to-phosphorus ratio
N/Pnitrogen-to-phosphorus ratio
C/Kcarbon-to-potassium ratio
N/Knitrogen-to-potassium ratio
K/Ppotassium-to-phosphorus ratio

Appendix A

Table A1. Soil background values of Rosa roxburghii woodlands at the experimental site.
Table A1. Soil background values of Rosa roxburghii woodlands at the experimental site.
Soil TypeSoil TightnesspHC (mg/g)N (mg/g)P (mg/g)K (mg/g)
Yellow soilCompactness6.0323.800.690.5413.98
Table A2. Basic characteristics of the sample plots.
Table A2. Basic characteristics of the sample plots.
SiteDegree of Rocky DesertificationWoodland
Type		Age
(a)		Altitude
(m)		Soil Moisture Content (%)Canopy Density (%)Stand Density
(Plant/HECTARE)		Rock Bareness Rate (%)Vegetation + Soil Coverage (%)
1Noneplantation8129018.8(Mar.)~24.1(Sep.)65825<20>80
2Noneplantation8129218.8(Mar.)~24.1(Sep.)68795<20>80
3Noneplantation8129118.8(Mar.)~24.1(Sep.)70855<20>80
4Lightplantation8128820.5(Mar.)~24.8(Sep.)6584020–4580–55
5Lightplantation8128920.5(Mar.)~24.8(Sep.)6681020–4580–55
6Lightplantation8129120.5(Mar.)~24.8(Sep.)6882520–4580–55
7Moderateplantation8127015.9(Mar.)~29.8(Sep.)6582546–7054–30
8Moderateplantation8127315.9(Mar.)~29.8(Sep.)7285546–7054–30
9Moderateplantation8127115.9(Mar.)~29.8(Sep.)6884046–7054–30
10Severeplantation8149516.3(Mar.)~27.9(Sep.)60675>70<30
11Severeplantation8150016.3(Mar.)~27.9(Sep.)62720>70<30
12Severeplantation8150016.3(Mar.)~27.9(Sep.)60690>70<30
Table A3. C, N, P, and K concentrations and stoichiometric ratios of litter and soil in Rosa roxburghii woodland under different levels of rocky desertification in different seasons (means ± SEss). Letters in the same row represent a significant difference at a significance level of p ≤ 0.05 according to the least significant difference (LSD) test.
Table A3. C, N, P, and K concentrations and stoichiometric ratios of litter and soil in Rosa roxburghii woodland under different levels of rocky desertification in different seasons (means ± SEss). Letters in the same row represent a significant difference at a significance level of p ≤ 0.05 according to the least significant difference (LSD) test.
ItemNoneLightModerateSevere
Mar.Sep.Mar.Sep.Mar.Sep.Mar.Sep.
Litter C (mg/g)576 ± 47 b636 ± 168 ab667 ± 55 a646 ± 171 ab670 ± 72 a442 ± 51 c665 ± 56 ab459 ± 40 c
Litter N (mg/g)9.6 ± 0.7 d12 ± 1.3 b11 ± 0.9 b15 ± 1.8 a9.8 ± 0.8 d11 ± 1.6 bc14 ± 1.0 a11 ± 1.6 c
Litter P (mg/g)0.45 ± 0.02 c0.60 ± 0.06 a0.37 ± 0.04 d0.52 ± 0.04 b0.36 ± 0.03 d0.45 ± 0.04 c0.37 ± 0.03 d0.47 ± 0.05 c
Litter K (mg/g)6.6 ± 0.5 b9.8 ± 1.7 a6.3 ± 0.4 bc9.4 ± 1.7 a4.3 ± 0.3 e5.1 ± 1.0 de4.9 ± 0.3 de5.6 ± 1.2 cd
Litter C/N58 ± 5.5 b68 ± 12 a62 ± 5.3 ab37 ± 13 d64 ± 6.2 ab35 ± 4.1 d52 ± 3.9 c35 ± 6.9 d
Litter C/P1781 ± 137 a990 ± 262 c1858 ± 185 a1080 ± 322 b1913 ± 234 a989 ± 91 c1789 ± 144 a976 ± 112 c
Litter C/K87 ± 8.8 d62 ± 22 e105 ± 8.2 c68 ± 25 e162 ± 19 a91 ± 13 c140 ± 11 b88 ± 20 d
Litter N/P27 ± 2.0 b20 ± 1.5 d28 ± 3.2 b29 ± 3.4 b27 ± 2.4 b22 ± 2.6 c37 ± 2.6 a25 ± 2.3 b
Litter N/K1.5 ± 0.1 c1.2 ± 0.2 d1.7 ± 0.1 c1.6 ± 0.2 c2.3 ± 0.2 b2.2 ± 0.2 b2.9 ± 0.2 a2.1 ± 0.5 b
Litter K/P19 ± 1.6 a16 ± 2.0 b17 ± 1.7 b18 ± 3.2 b12 ± 0.9 cd11 ± 1.7 d13 ± 0.9 c12 ± 2.2 cd
Soil C (mg/g)33 ± 4.6 c33 ± 2.8 c51 ± 6.5 a47 ± 3.8 a48 ± 3.8 a42 ± 6.4 b43 ± 3.1 b43 ± 3.0 b
Soil N (mg/g)0.9 ± 0.2 g1.4 ± 0.2 de1.2 ± 0.3 f1.7 ± 0.2 bc1.3 ± 0.1 ef1.9 ± 0.2 a1.6 ± 0.1 cd1.8 ± 0.1 ab
Soil P (mg/g)0.28 ± 0.02 c0.41 ± 0.08 ab0.27 ± 0.02 c0.45 ± 0.12 a0.37 ± 0.02 b0.44 ± 0.04 a0.31 ± 0.02 c0.36 ± 0.05 b
Soil K (mg/g)3.7 ± 0.4 e3.6 ± 0.6 e5.1 ± 0.7 cd5.9 ± 0.5 ab4.6 ± 1.0 d6.3 ± 0.9 a3.3 ± 0.7 e5.6 ± 0.5 bc
Soil C/N37 ± 5.7 b24 ± 3.0 cd43 ± 10 a28 ± 2.3 c37 ± 3.0 b22 ± 2.5 d27 ± 2.5 c24 ± 1.8 cd
Soil C/P118 ± 17 bc84 ± 18 e187 ± 24 a112 ± 30 c176 ± 14 a95 ± 13 de139 ± 10 b118 ± 14 c
Soil C/K8.8 ± 1.2 cde9.6 ± 2.1 bcd10 ± 1.7 bc8 ± 0.7 def11 ± 2.0 b6.7 ± 0.7 f13 ± 3.0 a7.6 ± 0.8 ef
Soil N/P3.2 ± 0.7 e3.6 ± 0.9 de4.6 ± 1.0 abc4 ± 1.2 cd4.8 ± 0.3 ab4.3 ± 0.4 bcd5.1 ± 0.5 a5 ± 0.7 a
Soil N/K0.33 ± 0.01 c0.41 ± 0.1 b0.25 ± 0.1 de0.28 ± 0.02 d0.30 ± 0.1 d0.30 ± 0.01 cd0.49 ± 0.1 a0.32 ± 0.02 c
Soil K/P13 ± 1.5 c8.8 ± 1.0 d19 ± 2.3 a14 ± 3.5 c17 ± 3.6 b14 ± 2.0 c11 ± 2.3 d16 ± 2.3 bc

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Figure 1. Location and terrain of the study area. The red area in the left image indicates the location of the study region, whereas the blue area in the middle image represents the location of the sample plots. The black area in the right image outlines the four distinct karst desertification gradient ranges, and the red numbers indicate the locations of the sampling points.
Figure 1. Location and terrain of the study area. The red area in the left image indicates the location of the study region, whereas the blue area in the middle image represents the location of the sample plots. The black area in the right image outlines the four distinct karst desertification gradient ranges, and the red numbers indicate the locations of the sampling points.
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Figure 2. C, N, P, and K concentrations and stoichiometric ratios of litter in Rosa roxburghii Tratt woodland under different levels of rocky desertification in different seasons. Eight distinct colors were employed in the image to represent the measured values across seasons and karst desertification gradients. On the X-axis, odd numbers (1, 3, 5, and 7) corresponded to measurements taken in March, whereas even numbers (2, 4, 6, and 8) corresponded to measurements taken in September. The pairs of numbers (1, 2), (3, 4), (5, 6), and (7, 8) represent non-, lightly, moderately, and severely rocky desertification, respectively. On the Y-axis, the first row shows the elemental concentrations of C, N, P, and K, whereas the second and third rows show the stoichiometric ratios between these elements. Letters in the same row indicate a significant difference at p ≤ 0.05 based on the LSD test.
Figure 2. C, N, P, and K concentrations and stoichiometric ratios of litter in Rosa roxburghii Tratt woodland under different levels of rocky desertification in different seasons. Eight distinct colors were employed in the image to represent the measured values across seasons and karst desertification gradients. On the X-axis, odd numbers (1, 3, 5, and 7) corresponded to measurements taken in March, whereas even numbers (2, 4, 6, and 8) corresponded to measurements taken in September. The pairs of numbers (1, 2), (3, 4), (5, 6), and (7, 8) represent non-, lightly, moderately, and severely rocky desertification, respectively. On the Y-axis, the first row shows the elemental concentrations of C, N, P, and K, whereas the second and third rows show the stoichiometric ratios between these elements. Letters in the same row indicate a significant difference at p ≤ 0.05 based on the LSD test.
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Figure 3. C, N, P, and K concentrations and stoichiometric ratios of soil in Rosa roxburghii woodland across varying degrees of rocky desertification during different seasons. Please refer to Figure 2 for the meanings of the various parameters in the figures.
Figure 3. C, N, P, and K concentrations and stoichiometric ratios of soil in Rosa roxburghii woodland across varying degrees of rocky desertification during different seasons. Please refer to Figure 2 for the meanings of the various parameters in the figures.
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Figure 4. Correlation analysis of C, N, P, and K concentrations and stoichiometric ratios in the litter and soil of Rosa roxburghii woodland. Note: The red and blue areas indicate positive and negative correlations, respectively. *, (p ≤ 0.05); **, (p ≤ 0.01); ***, (p ≤ 0.001).
Figure 4. Correlation analysis of C, N, P, and K concentrations and stoichiometric ratios in the litter and soil of Rosa roxburghii woodland. Note: The red and blue areas indicate positive and negative correlations, respectively. *, (p ≤ 0.05); **, (p ≤ 0.01); ***, (p ≤ 0.001).
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Figure 5. Redundancy analysis of litter and soil C, N, P, and K concentrations and stoichiometric ratios in Rosa roxburghii woodland. Litter is depicted as environmental factors indicated by red arrows, and the red dots indicate the distributions of soil stoichiometric indicators on the first and second axes of the RDA ordination. The arrow length represents the strength of a relationship, and the circle size represents the magnitude of a particular variable. The longer the arrow, the stronger the correlation. The larger the red dot, the larger the numerical value.
Figure 5. Redundancy analysis of litter and soil C, N, P, and K concentrations and stoichiometric ratios in Rosa roxburghii woodland. Litter is depicted as environmental factors indicated by red arrows, and the red dots indicate the distributions of soil stoichiometric indicators on the first and second axes of the RDA ordination. The arrow length represents the strength of a relationship, and the circle size represents the magnitude of a particular variable. The longer the arrow, the stronger the correlation. The larger the red dot, the larger the numerical value.
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Figure 6. Redundancy analysis of litter and soil C, N, P, and K concentrations and stoichiometric ratios in Rosa roxburghii woodland. Soil is depicted as environmental factors indicated by red arrows, and the red dots indicate the distributions of litter stoichiometric indicators on the first and second axes of the RDA ordination. Please refer to Figure 5 for the meanings of the various parameters in the figures.
Figure 6. Redundancy analysis of litter and soil C, N, P, and K concentrations and stoichiometric ratios in Rosa roxburghii woodland. Soil is depicted as environmental factors indicated by red arrows, and the red dots indicate the distributions of litter stoichiometric indicators on the first and second axes of the RDA ordination. Please refer to Figure 5 for the meanings of the various parameters in the figures.
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Table 1. General characteristics of the C, N, P, and K contents and stoichiometric ratios in the litter and soil of Rosa roxburghii Tratt woodland. SD: standard deviation, CV: coefficient of variation, df: degrees of freedom, MS: mean square, F: f value, **: p < 0.01.
Table 1. General characteristics of the C, N, P, and K contents and stoichiometric ratios in the litter and soil of Rosa roxburghii Tratt woodland. SD: standard deviation, CV: coefficient of variation, df: degrees of freedom, MS: mean square, F: f value, **: p < 0.01.
ItemLitterSoil
Mean
(mg/g)		SD
(mg/g)		CV (%)dfMS
(mg2/g2)		FMean
(mg/g)		SD
(mg/g)		CV (%)dfMS
(mg2/g2)		F
C59510918.3780,2878.6 **427.116.8739719.8 **
N12217.673320.5 **1.50.32370.929.8 **
P0.440.120.470.149.4 **0.350.125.270.115.8 **
K6.52.233.273835.5 **4.81.226711.824.5 **
ItemMeanSDCV (%)dfMSFMeanSDCV (%)dfMSF
C/N521935.77117119.3 **308.327.6752724.2 **
C/P139838127.371,590,47439.6 **1293829.7712,07635.5 **
C/K973536.3710,74237.7 **92.526.974013.9 **
N/P267.127723736.1 **4.30.921.474.17.2 **
N/K1.80.736.872.648 **0.30.130.670.115.5 **
K/P153.422.778924.4 **143.726.579014.8 **
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MDPI and ACS Style

Li, M.; Du, M.; Chen, H.; Shi, Y.; Yao, D.; Shi, Q. Stoichiometric Coupling of C, N, P, and K in the Litter and Soil of Rosa roxburghii Tratt Woodlands across Rocky Desertification Grades and Seasons. Forests 2024, 15, 1415. https://doi.org/10.3390/f15081415

AMA Style

Li M, Du M, Chen H, Shi Y, Yao D, Shi Q. Stoichiometric Coupling of C, N, P, and K in the Litter and Soil of Rosa roxburghii Tratt Woodlands across Rocky Desertification Grades and Seasons. Forests. 2024; 15(8):1415. https://doi.org/10.3390/f15081415

Chicago/Turabian Style

Li, Mingjun, Mingfeng Du, Huajiang Chen, Yan Shi, Dan Yao, and Qiusi Shi. 2024. "Stoichiometric Coupling of C, N, P, and K in the Litter and Soil of Rosa roxburghii Tratt Woodlands across Rocky Desertification Grades and Seasons" Forests 15, no. 8: 1415. https://doi.org/10.3390/f15081415

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