Soil Carbon, Nitrogen, and Phosphorus Stoichiometric Characteristics of Typical Artificial Vegetation in Qingshuihe County, Inner Mongolia

MIN Xue¹, WU Yeli¹, ZHANG Ying¹,², DING Guodong¹,²,³,⁴, YANG Zhiheng⁵

¹School of Soil and Water Conservation, Beijing Forestry University, Beijing 100083, China
²Key Laboratory of Soil and Water Conservation, State Forestry and Grassland Administration, Beijing Forestry University, Beijing 100083, China
³Engineering Research Center of Forestry Ecological Engineering, Ministry of Education, Beijing Forestry University, Beijing 100083, China
⁴National Positioning Observation and Research Station of Mu Us Sandy Land Ecosystem, Yanchi 751500, Ningxia, China
⁵Beijing Greensource Environment Planning & Design Institute Co., Ltd., Beijing 100083, China

Abstract: The stoichiometric relationship of carbon (C), nitrogen (N), and phosphorus (P) in soil is crucial for understanding the ecological function of different vegetation types. This study examined four typical artificial vegetation types (Populus L., Pinus sylvestris, Pinus tabulaeformis, and Caragana korshinskii) in Qingshuihe County, Inner Mongolia, China, with natural gramineous grassland as the control. The concentrations of soil C, N, and P along with their stoichiometric ratios within the 0–100 cm soil profile were investigated. The key results were as follows: (1) In the surface soil layer (0–20 cm), C, N, and P levels across the five vegetation types were 3.29–6.02 g·kg⁻¹, 0.52–0.69 g·kg⁻¹, and 0.37–0.62 g·kg⁻¹, respectively. Soil C and N levels were below the national surface soil average, indicating poor soil fertility. (2) In the 0–20 cm layer, soils under artificial vegetation had lower C content, N levels, and C/N ratios compared with those under grassland, with significantly lower C and C/N observed in P. sylvestris and C. korshinskii (P<0.05). At depths of 20–50 cm and 50–100 cm, C and N levels were highest under grassland and lower under artificial vegetation, with P. sylvestris showing the lowest C/N ratio. Soil P content was significantly higher under artificial vegetation than under grassland at all depths, whereas C/P and N/P ratios were significantly lower than those under grassland. (3) Correlation analysis revealed strong positive relationships between soil C and N and between C/P and N/P (P<0.001). Soil C, N, and P levels were significantly affected by vegetation type and soil depth (P<0.001) and all decreased with increasing depth, indicating strong surface accumulation. Given the high soil phosphorus availability in the study area, which supports the growth of herbaceous vegetation, and the superior soil nutrient conservation capacity of herbaceous plants relative to artificial woody vegetation, ecological restoration efforts in this region should primarily focus on conserving and restoring native herbaceous vegetation. This study contributes to understanding the soil nutrient cycle, assessing the ecological benefits of different vegetation types, and providing a scientific basis for regional ecological restoration and resource management.

Keywords: artificial vegetation; natural grassland; soil nutrients; ecological stoichiometry

Ecological stoichiometry examines the coupling relationships of soil carbon (C), nitrogen (N), and phosphorus (P) cycles, revealing the dynamic balance and interaction mechanisms of elements in ecosystems. Soil serves as a critical foundation for plant and microbial communities, and its nutrient supply ratios govern community structure and function. Vegetation restoration, as a core strategy for rehabilitating degraded ecosystems in ecologically fragile regions, plays a key role in enhancing ecosystem services. Research shows that vegetation restoration not only improves ecosystem service capacity through carbon sequestration, soil and water conservation, hydrological regulation, and soil improvement, but also profoundly influences the distribution patterns and storage characteristics of soil nutrients. Soil nutrients primarily originate from biological processes (such as plant litter decomposition and root exudates) and mechanical weathering of rocks, with relatively stable reserves. The stoichiometric characteristics of soil nutrients can directly reflect nutrient cycling, soil equilibrium features, and quality indicators. Different vegetation types create distinct soil nutrient cycling microenvironments, mainly regulated through litter input, root exudate release, and microbial communities. Litter decomposition directly affects soil organic matter accumulation and transformation, while root exudates regulate microbial activity and function. The biogeographical patterns of soil nutrient stoichiometry are significantly influenced by topography and vegetation type. Additionally, the spatial distribution characteristics of vegetation roots and their nutrient absorption strategies further promote spatial heterogeneity of soil nutrients, thereby profoundly affecting ecosystem function expression. The complexity of vegetation restoration strategies is highlighted by differences in water conservation, soil retention, ecological stability maintenance, and nutrient cycling performance among vegetation types.

Qingshuihe County is a typical ecologically fragile region located at the transition zone between the Loess Plateau and Inner Mongolia Plateau, severely affected by water erosion. Since the 1980s, a series of afforestation activities have been conducted to reduce soil erosion, improve soil nutrient conditions, and enhance ecosystem services, making it an ideal area for studying soil C, N, and P content and stoichiometry in artificial vegetation. This study selected four typical artificial vegetation types—Populus L., Pinus tabulaeformis, Pinus sylvestris, and Caragana korshinskii—with gramineous grassland as a control, aiming to reveal the vertical distribution characteristics of soil C, N, and P content and stoichiometry within the 0–100 cm profile. The results provide baseline data for estimating soil nutrient storage and assessing nutrient limitations under different vegetation types, offering a scientific basis for regional vegetation restoration type selection and strategy optimization.

1.1 Study Area Overview

The study area is located in Qingshuihe County, Hohhot City, Inner Mongolia Autonomous Region (111°18′–112°07′E, 39°35′–40°12′N), situated in the transitional zone between the Inner Mongolia Plateau and the Shanxi-Shaanxi Loess Plateau. This typical semi-arid region experiences severe river erosion, with terrain gradually decreasing from southeast to west and an average elevation of 1373.6 m. The area has a temperate continental climate with an average annual temperature of 7.1°C and average annual precipitation of 413.8 mm, concentrated in summer. The region features diverse vegetation, with dominant tree species including Populus, Pinus tabulaeformis, Pinus sylvestris, and Caragana korshinskii. Animal husbandry is well-developed, with natural pasture grasses dominated by Leymus chinensis and Stipa capillata. The main soil types are chestnut soil, cinnamon soil, and gray-cinnamon soil.

1.2 Field Sampling

Field surveys and sampling were conducted in August 2023. Based on the representativeness and typicality of regional vegetation species, four artificial vegetation types were selected: Populus, Pinus tabulaeformis, Pinus sylvestris, and Caragana korshinskii, along with gramineous grassland (Stipa capillata and Leymus chinensis) as the control. The number of sample plots for each vegetation type was determined based on their proportional area in the study region. Sample plots with similar disturbance levels were selected: three plots each for Populus, C. korshinskii, and grassland; two plots for P. tabulaeformis; and one plot for P. sylvestris, totaling ten sample plots. Each plot measured 20 m × 20 m, with basic characteristics including elevation, slope, and vegetation coverage recorded (Table 1). Within each plot, a 1 m × 1 m quadrat was established for herbaceous aboveground biomass and litter collection, and a soil profile was excavated. Soil samples were collected from three layers (0–20 cm, 20–50 cm, and 50–100 cm) using a ring knife for bulk density and additional soil samples. Three soil profiles were sampled per plot, and soil from the same layer within a plot was thoroughly mixed and sealed in bags. All samples were transported to the laboratory for processing.

1.3 Sample Processing and Measurement

Ring knife samples were used to determine soil bulk density by the ring knife method. Mixed soil samples were air-dried indoors, impurities were removed, and the samples were ground and passed through a 0.25-mm sieve. Soil organic C content was determined using the potassium dichromate oxidation-sulfuric acid copper titration method. Total N and P contents were measured after H₂SO₄ digestion using an automatic chemical analyzer. Litter and herbaceous aboveground biomass were oven-dried at 65°C to constant weight for dry weight determination.

1.4 Data Processing and Analysis

SPSS 27 software was used for statistical analysis and Origin 2022 for graphing. Data are presented as "mean ± standard error." One-way ANOVA was used to analyze differences in soil C, N, P contents and stoichiometric ratios among vegetation types and soil layers, with Duncan's multiple range test for post-hoc comparisons. Two-way ANOVA was used to analyze the effects of vegetation type and soil depth on soil C, N, P and their stoichiometric ratios. Pearson correlation analysis, linear regression, and principal component analysis were used to examine relationships between soil C, N, P stoichiometric characteristics and influencing factors.

2 Results and Analysis

2.1 Distribution Characteristics of Soil C, N, and P Contents Under Different Vegetation Types

Soil C, N, and P contents across the five vegetation types in the 0–100 cm profile ranged from 1.34–6.02 g·kg⁻¹, 0.25–0.62 g·kg⁻¹, and 0.27–0.62 g·kg⁻¹, respectively. In the surface layer (0–20 cm), soil C, N, and P contents were 3.29–6.02 g·kg⁻¹, 0.52–0.69 g·kg⁻¹, and 0.37–0.62 g·kg⁻¹, respectively. All artificial vegetation types had lower C and N contents than grassland in all layers, with C. korshinskii and P. sylvestris showing significantly lower values (P<0.05). Soil P content was lowest in the 20–50 cm layer across all vegetation types. Artificial vegetation had lower P content than grassland in all layers, with P. sylvestris and C. korshinskii showing significantly lower values (P<0.05). In the 0–20 cm layer, P. tabulaeformis had the highest P content, significantly higher than P. sylvestris and C. korshinskii across all layers (P<0.05). In the 20–50 cm and 50–100 cm layers, P. tabulaeformis had the highest soil P content, with this difference being significant in the 20–50 cm layer (P<0.05). All vegetation types showed significantly different C, N, and P contents among soil layers (P<0.05), with contents generally decreasing with depth, except for P. tabulaeformis P content. The surface accumulation effect was evident.

2.2 Stoichiometric Ratio Characteristics of Soil C, N, and P Under Different Vegetation Types

Soil C/N, C/P, and N/P ratios across the five vegetation types in the 0–100 cm profile ranged from 4.36–16.67, 3.80–8.79, and 0.70–2.59, respectively. In the 0–20 cm layer, all artificial vegetation types had lower C/N, C/P, and N/P ratios than grassland, with P. sylvestris and C. korshinskii showing significantly lower C/N (P<0.05). At 20–50 cm and 50–100 cm depths, grassland maintained the highest ratios while artificial vegetation had lower values, with P. sylvestris showing the lowest C/N ratio. Soil P content was significantly higher under artificial vegetation than under grassland at all depths, while C/P and N/P ratios were significantly lower.

2.3 Relationships Between Soil C, N, P Stoichiometry and Influencing Factors

Two-way ANOVA results (Table 2) showed that vegetation type significantly affected soil C, N, P contents and their stoichiometric ratios (P<0.01), while soil depth had significant effects (P<0.001). The interaction between vegetation type and soil depth was not significant for any variable. Pearson correlation analysis (Table 3) revealed that soil C, N, and P contents were significantly negatively correlated with soil bulk density (P<0.01) and positively correlated with elevation (P<0.01). Bulk density was significantly negatively correlated with elevation (P<0.01). Soil C, N, and P contents showed extremely significant positive correlations with each other (P<0.001). Linear regression showed that soil C and N contents had a strong positive correlation (R²=0.556, P<0.001), while C/P and N/P ratios were significantly positively correlated (R²=0.073, P<0.004). Principal component analysis (Figure 3) showed that the first two principal components explained 30.9% and 23.6% of total variance, respectively, with a cumulative explanation of 54.5%. PC1 primarily reflected soil nutrient content and stoichiometric characteristics, being positively correlated with soil C, N, P, C/N, C/P, and N/P, and negatively correlated with bulk density and slope. PC2 reflected the regulatory effects of topographic factors and vegetation productivity on soil nutrient distribution, being positively correlated with elevation, aboveground biomass, and litter quantity, and negatively correlated with slope and bulk density. Populus soil nutrients were significantly influenced by soil C, N, P, aboveground biomass, and litter quantity.

3 Discussion

3.1 Distribution of Soil C, N, and P Nutrient Contents Under Different Vegetation Types

Soil nutrient characteristics exhibit spatial heterogeneity, with substantial variation across scales and regions. In the surface layer (0–20 cm), soil C, N, and P contents under Populus, P. tabulaeformis, P. sylvestris, C. korshinskii, and grassland were 4.05, 6.02, 6.01, 3.29, and 5.71 g·kg⁻¹ for C; 0.60, 0.59, 0.69, 0.52, and 0.62 g·kg⁻¹ for N; and 0.57, 0.62, 0.48, 0.37, and 0.53 g·kg⁻¹ for P, respectively. Soil C and N contents were significantly lower than the national surface soil averages (24.56 g·kg⁻¹ for C and 1.88 g·kg⁻¹ for N), indicating low soil fertility due to fragmented terrain, arid climate, severe water erosion, and consequently low soil productivity and water/nutrient retention capacity, consistent with previous studies in arid regions. Soil C/N, C/P, and N/P ratios across the five vegetation types in the 0–100 cm profile ranged from 4.36–16.67, 3.80–8.79, and 0.70–2.59, respectively—lower than those in Inner Mongolia grasslands but higher than in the Loess Plateau. This reflects the study area's location in the northern Loess Plateau, where drought and soil conditions limit nutrient levels below national averages.

Soil nutrient content is significantly influenced by biological factors, including soil organic matter accumulation, litter decomposition, root networks, and root exudates. In this study, surface soil C and N contents under all artificial vegetation types were lower than under grassland because herbaceous plants have denser, more abundant shallow root systems that promote soil organic matter and nutrient accumulation through root exudates and complex root networks, extending this nutrient conservation effect to deeper layers. The consistently low C and N contents under C. korshinskii may result from limited, woody litter that is difficult to decompose. In water-limited semi-arid regions, Populus growth is constrained by insufficient precipitation, hindering nutrient accumulation. Differences between P. tabulaeformis and P. sylvestris may relate to variations in understory litter quantity.

Soil P content was higher under artificial vegetation than grassland at all depths, particularly in the 0–20 cm layer where P. tabulaeformis and P. sylvestris values were significantly higher (P<0.05). This likely results from P enrichment from conifer litter decomposition. Overall, artificial vegetation showed weaker nutrient conservation capacity than grassland. The low C/P and N/P ratios, combined with low soil C content, indicate P limitation in the region, which negatively impacts primary productivity and ecosystem processes. Therefore, improving soil fertility is critical for ecological restoration. Grassland soil C/P and N/P ratios were significantly higher than under artificial vegetation, reflecting adequate and highly available P that creates favorable conditions for herbaceous growth. The dense vegetation cover and shallow root systems of grassland effectively retain soil moisture, resist erosion, and provide better nutrient sequestration and ecological benefits. Thus, conserving and restoring native herbaceous vegetation should be prioritized in ecological restoration efforts in Inner Mongolia, with artificial woody vegetation as a secondary consideration.

3.2 Distribution of Soil C, N, and P Stoichiometric Ratios Under Different Vegetation Types

Ecological stoichiometry serves as a key indicator of ecosystem nutrient cycling, profoundly influencing ecosystem function and services. Soil C, N, and P exist in tightly coupled relationships mediated primarily by microbial decomposition processes, with C/N ratio being a critical factor regulating organic matter decomposition rates. This explains why soil C/N, C/P, and N/P ratios show similar trends across vegetation types. The strong correlation between C/P and N/P ratios primarily reflects P regulation, as the study area's developed grassland industry and herbaceous microenvironments promote P cycling.

Vegetation and environmental factors significantly influence soil C, N, P and their stoichiometric ratios. Two-way ANOVA showed vegetation type significantly affected these parameters (P<0.01), reflecting differences in nutrient absorption, allocation, and return strategies among vegetation types. These differences are influenced not only by inherent vegetation characteristics but also by management practices such as thinning and organic fertilizer application. Soil depth also had significant effects (P<0.001) because surface and deep soils accumulate nutrients differently, with deep soil nutrients primarily derived from leaching and root penetration.

Correlation analysis showed soil C, N, and P contents were highest in surface layers and decreased with depth due to high microbial activity and abundant surface litter, consistent with previous research. Soil C, N, and P contents were significantly negatively correlated with bulk density (P<0.01) because higher bulk density indicates greater soil compaction, reducing water content and pore space, thereby affecting nutrient infiltration and vegetation growth. Elevation was significantly negatively correlated with bulk density, indicating that at small scales, elevation indirectly affects soil nutrients by influencing bulk density. Higher elevations improve soil porosity and nutrient retention capacity, benefiting nutrient balance.

Principal component analysis showed soil C, N, and P contents were positively correlated with elevation, aboveground biomass, and litter quantity because soil nutrients originate from both parent material weathering and vegetation nutrient return. Higher elevations facilitate rock weathering and inhibit organic matter decomposition. Populus soil nutrients were more strongly influenced by vegetation biomass and litter than other vegetation types, likely because as a broadleaf forest it has greater litter input and nutrient consumption. Microtopography affects water storage and heat distribution, creating nutrient differences. Slope aspect influences soil enzyme activity and microbial nutrient limitation, with shady aspects promoting moisture retention and nutrient accumulation. Slope gradient affects soil nutrients by influencing runoff and soil particle transport, with steeper slopes facilitating nutrient loss. The weak correlation between soil stoichiometry and microtopography in this study may reflect the relatively uniform terrain, indicating that at small scales, vegetation and soil factors have more significant effects on nutrient distribution.

Soil stoichiometric ratios are influenced by soil properties, vegetation type, and geographic environment. Future studies should incorporate additional factors such as stand age, vegetation density, and microbial communities, which may significantly influence soil nutrient distribution. This would enable comprehensive analysis of nutrient distribution mechanisms and help establish criteria for evaluating soil nutrient limitations under different vegetation types in the same natural ecological region.

4 Conclusion

This study examined several typical artificial vegetation types in Qingshuihe County, Inner Mongolia, measuring soil C, N, and P contents within the 0–100 cm profile to assess vegetation effects on soil nutrients and stoichiometric ratios. The main conclusions are:

1) Soil C and N contents under both artificial vegetation and grassland were below national averages, indicating poor soil fertility with N limitation.

2) The capacity of artificial vegetation to improve soil fertility and accumulate C and N was lower than that of gramineous grassland.

3) Artificial vegetation had lower demand for soil N and P than grassland. The high P availability in the study area favors herbaceous vegetation survival and development.

Therefore, ecological restoration in this region should prioritize conservation and restoration of native herbaceous vegetation.

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