Land-Use Impacts on Soil Nutrients, Particle Composition, and Ecological Functions in the Green Heart of the Chang-Zhu-Tan Urban Agglomeration, China

Abstract

Urban green hearts provide essential ecosystem services, including carbon sequestration, water purification, and hydrological regulation. The Green Heart Area of the Chang-Zhu-Tan Urban Agglomeration in Hunan Province, China, is the largest globally, and plays a critical role in regional water management. These functions are increasingly threatened by intensive land-use, while soil, as the foundational ecosystem component, mediates water retention, nutrient cycling, and erosion resistance. This study examined the effects of four land-use types—cropland, plantation, arbor woodland, and other woodland—on soil particle composition and key nutrients (organic carbon, total nitrogen, and total phosphorus). Statistical comparisons among land-use types were performed. Results indicated that silt was the dominant soil fraction across all land-uses (64–72%). Arbor woodland exhibited significantly higher sand content (29%) compared to cropland (19%; p < 0.05), suggesting improved water permeability and erosion resistance. Cropland showed elevated nutrient levels, with TN (1450.32 mg·kg⁻¹) and TP (718.86 mg·kg⁻¹) exceeding both national averages and those in arbor woodland. Coupled with acidic soil conditions (pH 5.23) and lower stoichiometric ratios (C/N: 10.82; C/P: 35.67; N/P: 3.29), these traits indicate an increased risk of nutrient leaching in croplands. In contrast, arbor woodland displayed more balanced C:N:P ratios (C/N: 12.21; C/P: 48.05; N/P: 3.84), supporting greater nutrient retention and aggregate stability. These findings underscore the significant influence of land-use type on soil ecological functions, including water infiltration, runoff reduction, and climate adaptability. The study highlights the importance of adopting conservation-oriented practices such as reduced tillage and targeted phosphorus management in croplands, alongside reforestation with native species, to improve soil structure and promote long-term ecological sustainability.

1. Introduction

Urban green heart areas play a vital role in enhancing residential environments, mitigating local climate extremes, preserving biodiversity, and strengthening ecological connectivity. More specifically, the concept of a “Green Heart” describes a large-scale ecological zone situated at the core of a polycentric urban agglomeration. Unlike conventional urban parks, scattered green spaces, or peripheral green belts, it serves as an integrated ecological core within metropolitan regions, combining farmlands, forests, wetlands, and water bodies into a cohesive and functional network [1,2]. Numerous studies have shown that urban green heart areas can regulate the urban microclimate, purify air and water bodies, and as a result, this sustainability solution has been promoted and applied to the spatial planning of many urban agglomerations in China [3,4,5]. Given its increasing prominence in China’s urban planning practices, the Chang-Zhu-Tan Green Heart provides a representative case for exploring both the ecological functions and the management challenges of such large-scale ecological zones.

The Chang-Zhu-Tan urban agglomeration Green Heart (528 km²) has grown 3.5 times larger than the Randstad Green Heart (150 km²), making it the world’s largest Green Heart [1]. It provides vital ecosystem services such as carbon sequestration, water purification, hydrological buffering, and microclimate regulation [5]. These services, however, are increasingly threatened by intensive land-use practices. Among the multiple environmental factors, soil plays a pivotal role as the foundational component of terrestrial ecosystems, directly influencing ecosystem stability through its physical structure and nutrient composition [6]. Its particle size distribution and elemental cycling are closely linked to land-use practices and external inputs, including both natural processes and human activities such as tillage, fertilization, and vegetation restoration [7,8]. Variations in land-use intensity lead to significant differences in soil physical and chemical properties, especially in aggregate stability and nutrient dynamics. Moreover, microbial activity, organic matter decomposition, and nutrient mineralization further regulate soil fertility and ecological functioning, thereby influencing soil water retention, infiltration capacity, and runoff generation—key processes of the terrestrial water cycle. Despite increasing research efforts in recent years, the role of land-use changes in shaping soil-mediated water cycling, particularly through alterations in structure and nutrient dynamics, remains insufficiently understood. Addressing this knowledge gap is essential for advancing the ecological sustainability of Green Heart areas. To contribute to this understanding, we conducted a case study in the Green Heart Area of the Chang-Zhu-Tan Urban Agglomeration.

This study examines the Green Heart Area of the Chang-Zhu-Tan Urban Agglomeration in Hunan Province, China—the world’s largest urban Green Heart. We focus on soil particle size composition (sand, silt, and clay) and key nutrient elements (carbon, nitrogen, and phosphorus) across four dominant land-use types: farmland, plantation, arbor woodland, and other woodland. These land-uses reflect a gradient from intensive cultivation to relatively stable natural forests. By analyzing soil structure and stoichiometry under different land-use systems, we aim to clarify how land-use change influences nutrient retention and hydrological functions, providing a scientific basis for sustainable land management and ecological restoration in urban green heart areas.

2. Materials and Methods

2.1. Study Area

The Green Heart Area of the Chang-Zhu-Tan Urban Agglomeration (27°43′30″–28°54′8″ N, 112°53′28″–113°17′51″ E) is located in Hunan Province, China, spanning parts of Changsha, Zhuzhou, and Xiangtan. As the first officially designated urban Green Heart in China, it plays a key role in regional ecological conservation and urban sustainability. The area covers 522.87 km² in total, with 305.73 km² in Changsha, 82.34 km² in Zhuzhou, and 134.80 km² in Xiangtan.

The region features a humid subtropical monsoon climate, with four distinct seasons, abundant rainfall, and mild temperatures. Annual precipitation ranges from 1200 to 1700 mm, with 270–310 frost-free days and average annual temperatures between 17.6 °C and 18.9 °C. Forest ecosystems dominate the area, accounting for approximately 71.4% of total land cover. Among these, arbor woodland occupies 50.9%, while water bodies, farmland, plantation, and other woodland types together comprise 15.37%. The main vegetation types include evergreen broadleaf forests, mixed coniferous–broadleaf forests, coniferous forests, and bamboo forests. The soil types are diverse and complex, including yellow soil, red soil, and paddy soil. Land-use planning divides the region into three functional zones: core ecological protection, limited development, and controlled utilization. With forest coverage exceeding 70%, the Green Heart Area plays a crucial role in climate regulation, biodiversity conservation, and ecological tourism development.

2.2. Soil Sample Collection and Preprocessing

From April to October 2022, a total of 138 soil sampling points were established across different land-use types—farmland, plantation, and arbor woodland—within the Green Heart Area. Specifically, 96 sampling points were located in woodland (including 80 in arbor woodland and 16 in other woodland), 28 in farmland, and 14 in plantation (

Table 1. Basic information on soil sampling points.

Land-Use Patterns	Number of Sampling Points	Predominant Soil Types	Dominant Disturbance Type	Dominant Vegetation
Farmland	28	Paddy soil, Yellow soil	Tillage, fertilization, irrigation	Rapeseed, Corn, Rice, Sweet potato
Other woodland	16	Red soil, Yellow soil	None or minimal	Osmanthus fragrans, Camellia, Cinnamomum camphora
Arbor woodland	80	Red soil, Yellow soil	None or minimal	Pinus massoniana, Cunninghamia lanceolata, Loropetalum chinense
Plantation	14	Red soil, Yellow soil, Paddy soil	Tillage, replanting, irrigation	Photinia serratifolia, Osmanthus fragrans, Camellia oleifera

, Figure 1).

The depth of soil sample collection is determined based on land-use types. For farmland, samples were taken from the plow layer at a depth of 0–30 cm. For woodland and plantation, samples were collected from the fine root zone at a depth of 0–60 cm. After collection, samples were air-dried, and plant roots, stones, and debris were removed. The soil was sieved through a 2 mm mesh and stored in sealed containers at room temperature prior to analysis.

2.3. Soil Particle Composition and Chemical Properties

The particle size distribution of the soil was measured using a Mastersizer 2000 laser particle size analyzer (Malvern Instruments, Malvern, UK). According to the USDA soil texture classification system, soil particles were classified into sand (0.050–2 mm), silt (0.002–0.050 mm), and clay (<0.002 mm), and the volume percentage of each particle-size fraction was calculated. These fractions were used to characterize soil texture and to derive coefficients for subsequent analysis of soil structure and hydrological behavior (

Table 2. Soil particle size grading.

Soil Particle Size	Clay	Silt	Sand
Particle size/mm	<0.002	0.002~<0.050	0.050~<2

).

Soil pH was measured in a 1:2.5 soil-to-water suspension using a glass electrode. OC was determined by the potassium dichromate oxidation method, TN by Kjeldahl digestion, and TP by sulfuric-perchloric acid digestion followed by molybdenum-antimony colorimetry. The combination of particle size distribution and chemical properties provides a comprehensive description of soil characteristics across different land-use types.

2.4. Data Analysis and Processing

The data for this study were initially processed using Excel 2016. The soil OC, TN, and TP contents for different land-use types were calculated to obtain the C/N, C/P, and N/P ratios. IBM SPSS Statistics 22 was then used to analyze the data, with one-way analysis of variance (ANOVA) conducted to compare the differences in soil OC, TN, and TP contents among different land-use types. Furthermore, Spearman correlation analysis was used to explore the relationship between the soil C/N, C/P, and N/P ratios. The data visualization was performed using OriginPro 2024 software.

3. Results

3.1. Soil Particle Composition Characteristics Under Different Land-Use Types

Soil particle composition in the Green Heart Area of the Chang-Zhu-Tan Urban Agglomeration was dominated by silt across all land-use types, followed by clay and sand. Forest areas (arbor woodland and other woodland) exhibited slight differences in particle composition compared to farmland and plantation, while minimal variation was observed between arbor woodland and other woodland (Figure 2). In farmland, silt, clay, and sand accounted for 47%, 34%, and 19%, respectively, with silt and clay contents significantly higher than sand (p < 0.05). A similar trend was observed in arbor woodland, where silt, clay, and sand comprised 37%, 34%, and 29%, respectively. For other woodland, the proportions were 42% silt, 31% clay, and 26% sand, while plantation soils contained 42% silt, 36% clay, and 22% sand. Silt content in other woodland and plantation soils was significantly higher than sand (p < 0.05).

3.2. Characteristics of Soil OC, TN, and TP Contents Under Different Land-Use Types

In the Green Heart Area of the Chang-Zhu-Tan Urban Agglomeration, OC, TN, and TP contents were highest in farmland compared to woodland and plantation areas, with significant variations observed in TN and TP across land-use types but not in OC (Figure 3). The average OC content followed the sequence: farmland (14,346.43 mg·kg⁻¹) > arbor woodland (14,280 mg·kg⁻¹) > plantation (12,992.86 mg·kg⁻¹) > other woodland (12,456.25 mg·kg⁻¹). For TN, farmland exhibited the highest mean value (1450.32 mg·kg⁻¹), significantly exceeding other woodland (1215.13 mg·kg⁻¹) and arbor woodland (1150.38 mg·kg⁻¹) (p < 0.05), while plantation and other woodland showed intermediate values (1239.57 and 1215.13 mg·kg⁻¹, respectively). TP content displayed a distinct gradient: farmland (718.86 mg·kg⁻¹) > other woodland (638.94 mg·kg⁻¹) > plantation (478.86 mg·kg⁻¹) > arbor woodland (319.28 mg·kg⁻¹), with woodland soils (arbor and other) having significantly lower TP than non-woodland types, and plantation TP being notably lower than farmland (p < 0.05). These results highlight land-use-specific patterns in soil nutrient dynamics, emphasizing the influence of anthropogenic practices on nutrient retention and depletion.

3.3. Characteristics of Soil OC, TN, and TP Ecological Stoichiometry

In the Green Heart Area of the Chang-Zhu-Tan Urban Agglomeration, soils under arbor woodland exhibited mean C/N, C/P, and N/P ratios of 12.21, 48.05, and 3.84, respectively, which were significantly higher than those in other land-use types (p < 0.05) (Figure 4). Plantation soils showed mean C/N, C/P, and N/P ratios of 9.94, 34.73, and 3.12, with C/P and N/P ratios significantly higher than those in farmland (p < 0.05). Farmland and other woodland soils displayed similar stoichiometric ratios: farmland had mean values of 9.78 (C/N), 21.89 (C/P), and 2.20 (N/P), while other woodland soils had values of 9.9 (C/N), 24.59 (C/P), and 2.30 (N/P). These patterns reflect distinct nutrient allocation mechanisms influenced by land-use practices, particularly the elevated C/P and N/P ratios in woodland systems.

3.4. Relationships Between Soil Particle Composition and OC, TN, TP Stocks with Stoichiometric Indices

Spearman correlation analysis was conducted to evaluate the relationships between soil particle composition and OC, TN, TP contents, as well as their stoichiometric ratios, under different land-use types in the Green Heart Area of the Chang-Zhu-Tan Urban Agglomeration. The correlations varied across land-use types (Figure 5). Across all four land-use types, OC showed significant positive correlations with TN, C/N, and C/P (p < 0.05). In farmland, TN was positively correlated with TP, while in arbor woodland, TN exhibited positive correlations with TP, C/N, C/P, and N/P (p < 0.05). In plantation, TN was positively correlated with C/P (p < 0.05). Soil pH in farmland and plantation showed positive correlations with silt content (p < 0.05). Soil particle composition correlated differently with OC, TN, and TP depending on land-use type: in farmland, sand content was positively correlated with OC, C/N, C/P, and N/P (p < 0.05); in arbor woodland, sand content was positively correlated with C/N, C/P, and N/P (p < 0.05), but negatively correlated with TP (p < 0.05).

Main effect tests were performed to further identify factors influencing soil particle composition and C, N, and P contents (

Table 3. Variance analysis of soil particles with OC, TN, and TP contents by land-use types and soil pH.

Factor		Type III Sum of Squares	Degree of Freedom 1	Mean Square	F Value	Significance
Land-use Patterns	OC	86,180,743	3	28,726,914	1.025	0.384
	TN	1,760,460	3	586,820	4.581	0.004
	TP	1,756,424	3	585,474	11.496	0.000
	C/N	59	3	19	5.116	0.002
	C/P	4306	3	1435	5.039	0.002
	N/P	17	3	5	4.875	0.003
	Clay	31,526	3	10,508	1.840	0.143
	Silt	24,067	3	8022	2.077	0.106
	Sand	103,721	3	34,573	3.330	0.022
pH	OC	102,302,553	1	102,302,553	3.651	0.058
	TN	187,507	1	187,507	1.464	0.228
	TP	63,571	1	63,571	1.248	0.266
	C/N	28	1	28	7.452	0.007
	C/P	3315	1	3315	11.638	0.001
	N/P	13	1	13	11.004	0.001
	Clay	48,587	1	48,587	8.508	0.004
	Silt	78,742	1	78,742	20.391	0.000
	Sand	3622	1	3622	0.349	0.556

¹ The land-use factor was classified into four categories (k = 4), resulting in 3 degrees of freedom (df = k − 1 = 3). Soil pH was included in the model as a single predictor variable, either in continuous form or as a dichotomous variable, and thus contributed 1 degree of freedom.

). Land-use type significantly affected TN, TP, C/N, C/P, N/P, and sand content (p < 0.05), but had no significant impact on clay, silt, or OC content (p > 0.05) (Figure 2 and Figure 3a). Soil pH significantly influenced C/N, C/P, N/P, clay, and silt (p < 0.05).

4. Discussion

4.1. Effects of Land-Use on Soil Particle Composition

Variations in land-use types alter soil microecological environments, triggering changes in physical, chemical, and biological processes that influence soil particle composition [5,9,10]. A higher proportion of large aggregates generally enhances soil stability and resistance to erosion [11,12]. In this study, arbor woodland soils had 10% and 7% higher sand content than farmland and plantation soils, respectively. This is attributed to the susceptibility of large soil particles to disruption from tillage. Frequent plowing and disturbance in farmland and plantation areas fragment soil aggregates, reducing coarse sand particles and promoting smaller particle formation. This textural shift has direct hydrological consequences: low sand content in farmland (19%) diminishes infiltration capacity by ~24% compared to arbor woodland (29%), increasing surface runoff during monsoon rains. Previous studies have shown that long-term fertilizer application also inhibits the formation of large aggregates [13]. Prolonged chemical fertilization induces soil acidification, lowering pH and altering colloidal surface charge properties and interparticle adhesion, thereby weakening aggregate cohesion [14,15]. This aligns with our findings, where farmland and plantation soils exhibited pH values of 5.23 and 4.54, respectively. In contrast, soil texture in arbor woodlands is shaped primarily by natural factors, such as litter inputs, root activity, and exudates. Although previous research has reported higher clay content in farmland compared to woodland [16], this study found higher silt content in farmland soils, likely reflecting differences in tillage intensity between farmland and plantation.

These findings highlight the significant impact of land-use on soil particle composition. To reduce anthropogenic disturbance, practices like straw return, no-tillage farming, and organic fertilization should be promoted, as they enhance aggregate formation, improve soil structure, and mitigate surface runoff.

4.2. Land-Use Driven Nutrient Cycling and Its Ecological–Hydrological Implications

Soil organic carbon (OC) and total nitrogen (TN) contents under all four land-use types exceeded the national average values in China (OC: 11,130.27 mg·kg⁻¹; TN: 1065.76 mg·kg⁻¹) [17]. However, only farmland soil TP content surpassed the national average (647.27 mg·kg⁻¹) [17], while arbor woodland, other woodland, and plantation soils fell below this threshold. According to Chinese soil nutrient classification, farmland, other woodland, and plantation soils were categorized as low in OC (10–15 g·kg⁻¹), very high in TN (≥1.20 g·kg⁻¹), and medium in TP (0.4–0.8 g·kg⁻¹). Arbor woodland soils showed lower TN (classified as high: 1.00–1.20 g·kg⁻¹) and TP (<0.4 g·kg⁻¹), indicating nutrient limitations despite relatively favorable organic matter levels. Farmland soils had significantly higher TN and TP than arbor woodland (p < 0.05), likely due to long-term fertilization practices [18]. Elevated TP in farmland represents a potential source for phosphorus mobilization under high rainfall events, particularly given acidic soil conditions (pH = 5.23) that reduce phosphorus adsorption capacity. Additionally, the low TP levels in arbor woodland may reflect the influence of vegetation succession, as phosphorus often peaks and then declines with stand age [19]. Higher TP in farmland versus plantation correlated with greater phosphorus demand during annual cropping cycles. To address P deficiency in non-farmland soils, targeted phosphorus supplementation is recommended to enhance nutrient balance and vegetation productivity.

A significant positive correlation was observed between OC and TN across all land-use types (p < 0.05), indicating that soil organic matter plays a central role in nitrogen accumulation and cycling. In contrast, TP exhibited weaker associations with OC and TN, suggesting that phosphorus availability remains relatively constrained and less responsive to organic matter dynamics. Previous studies have also highlighted close linkages among OC, TN, TP, and soil pH [20,21,22]. In arbor woodland, for instance, the strongly acidic pH (4.05) was negatively correlated with OC and TN (p < 0.05), implying that low pH conditions may suppress organic matter mineralization and reduce the flux of soluble nitrogen [23]. These results underscore that land-use type not only influences nutrient inputs but also regulates soil nutrient interactions through shifts in texture and acidity. Consequently, targeted phosphorus supplementation and pH regulation should be considered as essential strategies for maintaining nutrient balance and enhancing soil fertility under contrasting land-use scenarios.

4.3. Stoichiometric Ratios Indicate Nutrient Constraints and Soil Water Resilience

Soil C/N, C/P, and N/P ratios are widely recognized indicators of nutrient balance and ecosystem function [17]. The C/N ratio serves as a sensitive metric for soil environmental or quality changes, reflecting the mineralization capacity of C and N, organic matter decomposition rates, and nutrient equilibrium. Generally, soil C/N is inversely related to organic matter decomposition rates. In this study, all four land-use types exhibited C/N ratios below China’s national average (11.9) [17]. Lower C/N ratios indicate faster microbial decomposition of organic matter and higher available nitrogen content, making C/N a widely recognized measure of relative nitrogen availability. The low C/N ratios in the study area suggest rapid organic matter decomposition and mineralization, which may reduce nutrient retention capacity and hinder soil fertility maintenance, potentially reducing aggregate stability and erosion resistance [24]. The C/P ratio reflects carbon availability, while N/P identifies nutrient limitation types [25,26,27,28]. Compared to national averages (C/P: 61; N/P: 5.2), the study area’s soils showed lower C/P and N/P values. A C/P ratio exceeding 200 indicates strong iron–aluminum binding capacity, which suppresses the conversion efficiency of available phosphorus [29]. However, the observed C/P range (21.89–48.05) was far below 200, suggesting rapid phosphorus turnover and high phosphorus availability in the study area. Limited phosphorus availability intensifies competition between plants and microorganisms for phosphorus, reducing microbial phosphorus accumulation and further lowering C/P. An N/P ratio below 14 typically signals nitrogen limitation [17,30], aligning with the study area’s N/P values. Compared to natural arbor woodland, C/N, C/P, and N/P ratios in farmland decreased by 3, 26, and 2, respectively. This decline is attributed to tillage practices in farmland enhancing soil aeration, promoting aerobic microbial respiration, and accelerating mineralization and decomposition, thereby increasing the availability of N and P, consistent with previous findings [27]. Arbor woodland soils, by contrast, had higher C:N:P ratios, reflecting slower organic matter turnover and greater potential for nutrient accumulation. These traits enhance long-term soil fertility and hydrological buffering capacity. Reforestation efforts should prioritize native tree species that promote such stoichiometric advantages, thereby improving soil resilience under future climate variability. Future studies should integrate crop yield and socioeconomic data to enable a more comprehensive assessment of the trade-offs between ecological functions (e.g., nutrient retention, hydrological resilience) and socioeconomic benefits across different land-use practices in the Green Heart Area.

5. Conclusions

This study examined how different land-use types affect soil particle composition, nutrient cycling, and hydrological functions specifically in the Chang-Zhu-Tan Green Heart, aiming to identify land-use-driven differences that influence soil quality and water regulation within this ecological zone. We found that land-use significantly shaped soil nutrient stoichiometry and water-related functions, although variations in particle composition may also partly reflect differences in microrelief, parent materials, and soil types. Across all land-uses, soils were dominated by silt (64–72%), a texture that restricts infiltration and water storage. Farmland exhibited significantly lower sand content (19%) compared to arbor woodland (29%; p < 0.05), potentially amplifying surface runoff during monsoon rainfall. While soil organic carbon (OC) and total nitrogen (TN) exceeded national averages, farmland soils showed elevated total phosphorus (TP; 718.86 mg·kg⁻¹) under acidic conditions (pH = 5.23), raising the risk of phosphorus leaching. In contrast, arbor woodland exhibited higher C:N:P ratios (C/N: 12.21; C/P: 48.05), which favored nutrient retention, aggregate stability, and infiltration capacity. These findings underscore the need for land-use-specific management in the Green Heart: in farmland, conservation tillage and precision phosphorus application are recommended to mitigate nutrient loss and hydrological disruption, whereas reforestation with native arbor species can promote soil stabilization through litter inputs and root activity.

Taken together, this study provides new insights into how land-use conversions influence soil nutrient dynamics and hydrological processes within the Chang-Zhu-Tan Green Heart. Future research that incorporates bulk density, soil moisture reserves, and soil carbon stocks will allow for a more comprehensive evaluation of soil ecological functions and strengthen the scientific basis for local land-use management.