Seasonal Grazing Does Not Significantly Alter the Particle Structure and Pore Characteristics of Grassland Soil

Abstract

Seasonal grazing is a recognized and sustainable approach to livestock management, but there is still a lack of comprehensive research on its impact on soil structure. This study utilizes advanced scanning electron microscopy technology to quantitatively evaluate the long-term effects of seasonal grazing on grassland soil structure, focusing on soil pore distribution characteristics and particle size. The investigation offers a detailed visual representation of the arrangement of soil particles at a micro-level. In both grazed and ungrazed plots (NG), soil particles ranging from 0.005 to 0.05 mm and 0.075 to 0.25 mm in size were predominant, constituting 20% and 60%, respectively. In plots subjected to seasonal grazing (grazing in June and August, G68, and grazing in July and September, G79), micro-particles (0.002–0.005 mm) and particles sized 0.05–0.075 mm were significantly lower compared to NG. Scanning electron microscope (SEM) images demonstrate structural differences, with NG displaying a higher proportion of small to medium-sized particles, more small pores, and fewer large pores. Analysis of pore size and morphology reveals the prevalence of large pores in both grazed and ungrazed plots. Continuous grazing plots exhibit significantly higher proportions of large pores compared to NG, while seasonal grazing plots show no significant differences. Correlation analyses indicate associations between soil physicochemical properties, particle size, and pore structure. Total soil nitrogen (TN), total soil carbon (TC), and soil moisture positively correlate with 0.005–0.05 mm particle proportions, while EC is negatively correlated with 0.05–0.075 mm particles. This study enhances our understanding of the effects of grazing practices on soil structure and provides scientific evidence for sustainable land management.

1. Introduction

Grazing is a significant human activity in grassland ecosystems. Seasonal grazing is a sustainable and intelligent livestock management strategy that introduces livestock into pastures for a limited duration during specific periods, based on the growth season of the grass and the nutritional needs of the animals [1]. This approach is rooted in a deep understanding of ecosystems and the life cycles of flora and fauna. It maximizes the utilization of peak grass growth periods, ensuring that livestock have access to abundant forage resources under optimal conditions. The advantages of seasonal grazing include promoting pasture recovery, optimizing livestock resource utilization, alleviating soil compaction pressures, maintaining ecological balance, and fostering sustainable livestock development [2]. The aim of this management strategy is to bridge a balance between the needs of livestock and natural resource conservation, contributing to the sustainability and ecological well-being of agricultural systems.

Livestock grazing affects not only the above-ground vegetation biomass and community composition but also the soil structure and function. Previous studies have focused on the compaction effects of livestock trampling on the soil surface, which affects water infiltration and may reduce microbial diversity and activity [1,2]. As a result, this can influence nitrogen cycling and organic matter decomposition. However, the specific impact of seasonal grazing on soil structure, including soil pore distribution and particle size, is still not well understood.

Soil pore distribution characteristics and particle size are important factors for describing soil structure, which have a significant impact on the physicochemical properties and ecological functions of soil [3]. The distribution of soil pores reflects the permeability and moisture movement capacity within the soil, which are crucial for processes such as plant root growth, microbial activity, and organic matter decomposition [4]. Recent studies have utilized advanced tools such as scanning electron microscopy or high-resolution 3D micro-CT technology to accurately reveal the spatial structure and distribution of soil pore networks [5,6,7].

Soil particle size is an important indicator of soil structure, as it reflects the relative proportions of particles of different sizes in the soil. Recent research has shown that the distribution of soil particle sizes has a significant impact on soil water retention, permeability, and the storage and release of nutrients [8,9,10,11]. Advanced microscopy techniques, such as scanning electron microscopy, enable researchers to visually observe and analyze the morphology and arrangement of soil particles, providing detailed insights into the micro-scale changes in soil structure [12,13]. These meticulous observations offer a comprehensive understanding of how soil structure evolves under different grazing management conditions, which is valuable for sustainable grassland management.

Considering the characteristics of soil pore distribution and particle size, along with the use of advanced scanning electron microscopy technology, this study was designed to quantify the long-term impact of various grazing management measures on the structure of grassland soil. The objective is to visually represent the arrangement of soil particles at a micro-level. Our hypothesis is that seasonal grazing, as opposed to continuous grazing, may alleviate soil compaction pressures, resulting in less significant effects on soil structure. This study will enhance our understanding of the effects of different grazing management practices on soil structure, nutrient cycling, and overall soil system health. Furthermore, it will provide scientific evidence in support of sustainable grazing and grassland management.

2. Methods

2.1. Study Area

The long-term grazing experiment for this study was conducted at the Inner Mongolia University Grassland Ecology Research Base, located in Maodeng Pasture, Xilinhot City, Inner Mongolia (44°10′ N, 116°29′ E). The study site is situated in the central part of the Inner Mongolia Autonomous Region, which experiences a typical semi-arid continental climate characterized by arid conditions and low precipitation. The region has an annual mean temperature of −0.4 °C and an average annual rainfall of 365.6 mm, with most of the precipitation occurring during the summer season.

The vegetation in the study area predominantly comprised perennial plants, including Stipa krylovii Roshev, Leymus chinensis (Trin.) Tzvel., Cleistogenes squarrosa (Trin.) Keng, and others. These plants are characteristic of the typical grassland type [14].

The soil type in the study area was classified as calcicorthic aridisol, characterized by a calcic horizon and nutrient-poor sandy loam. The soil texture was predominantly sandy loam, exhibiting high aeration and drainage but a poor water retention capacity [1].

2.2. Grazing Experiment Design

The grazing experiment commenced in 2011, and the soil survey sampling for this study was conducted in August 2021. The experiment consisted of five grazing treatments, and each grazing treatment was replicated with three plots: no grazing (NG), continuous grazing from May to September (CG), grazing in May and July (G57), grazing in June and August (G68), and grazing in July and September (G79). The grazing experiment commenced on the 20th of each grazing month and concluded when the plant stubble reached approximately 6cm. The subsequent grazing month starts on the 20th and is referred to as adaptive grazing utilization. Each treatment had three replicated plots in the size of 33.3 m × 33.3 m, and the grazing intensity was set at 6 sheep per plot. The experimental sheep used were Ujimqin lambs born in the same year.

2.3. Soil Sampling and Physicochemical Analysis

Three soil samples were collected from each plot, specifically from the 0–20 cm surface layer, and then mixed for physicochemical analysis. Here were the soil physicochemical indicators tested according to standard methods: The dry combustion method was used to determine the total soil carbon (TC); the conventional method for determining total soil nitrogen (TN) was the Kjeldahl nitrogen determination method; the molybdenum antimony anti-colorimetric method was employed to determine the total phosphorus (TP); soil moisture content (Water) was determined using the weight method, and soil electrical conductivity (EC) was measured using the conductivity meter method [15].

The composition characteristics of soil particles with different particle sizes were investigated using the laser particle size analyzer [16]. The Zeiss SUPRA55 scanning electron microscope (SEM, Carl Zeiss AG, located in Oberkochen, Germany) was used to observe the microstructure and pore characteristics of the soil particles. Soil pore data were extracted using Image-Pro Plus software 6.0, and calculations were conducted to determine the pore area ratio, average pore diameter, and maximum pore diameter. Each treatment underwent triplicate Scanning Electron Microscopy (SEM) analyses. Pore structure analysis was performed as an integral part of these SEM examinations. Figure 1 was generated by randomly selecting one representative image to construct a composite figure.

2.4. Data Analysis

The data were analyzed using SPSS software version 23.0 (SPSS Inc., Chicago, IL, USA). Differences in soil physicochemical characteristics among the five treatments were analyzed using one-way analysis of variance (ANOVA). The difference analysis plots and linear regression plots were visualized using the ‘ggplot’ package within R (http://cran.r-project.org/web/packages/ggplot, accessed on 2 May 2023).

3. Results

3.1. Soil Properties

The soil properties measured across different treatments showed ranges of 15.57 to 18.55 g kg⁻¹ for total carbon (TC), 1.19 to 1.39 g kg⁻¹ for total nitrogen (TN), and 0.30 to 0.39 g kg⁻¹ for total phosphorus (TP). Bulk density (BD) values ranged from 0.89 to 0.94 g cm⁻³, indicating slight variations among the treatments. There were no significant differences in soil TC, TN, TP, EC and BD values among the five treatments, which included no grazing, continuous grazing, and different seasonal grazing. However, when compared to the no grazing treatment, grazing in July and September (G79) showed a significant increase in moisture content. On the other hand, grazing in June and August (G68) and G79 exhibited a significant decrease in pH (

Table 1. Characteristics of soil physicochemical properties among the different grazing management practices.

Soil Properties	NG	CG	G57	G68	G79
TC (g kg−1)	16.03 ± 2.21a	17.23 ± 3.12 a	17.15 ± 3.07 a	15.57 ± 2.79 a	18.55 ±1.81 a
TN (g kg−1)	1.25 ± 0.19 a	1.19 ± 0.14 a	1.22 ± 0.22 a	1.30 ± 0.21 a	1.39 ± 0.11 a
TP (g kg−1)	0.34 ± 0.05 a	0.31 ± 0.03 a	0.39 ± 0.13 a	0.30 ± 0.03 a	0.33 ± 0.03 a
Water (%)	10.85 ± 1.15 a	10.78 ± 1.19 a	11.77 ± 2.25 ab	10.72 ± 1.30 a	14.03 ± 3.15 b
EC (mS m−1)	12.32 ±0.53 a	12.87 ±1.07 a	13.18 ±0.68 a	13.30 ±1.71 a	13.18 ±0.68 a
pH	8.70 ± 0.08 a	8.67 ± 0.14 ab	8.61 ± 0.14 abc	8.57 ± 0.04 bc	8.53 ± 0.07 c
BD (g cm−3)	0.89 ± 0.003 a	0.92 ± 0.001 a	0.94 ± 0.001 a	0.92 ± 0.001 a	0.91 ± 0.001 a

Note: The differences among different grazing management practices were analyzed by one-way analysis of variance, and the lowercase letters represent the significant differences (p < 0.05). Abbreviation: total soil carbon (TC), total soil nitrogen (TN), total phosphorus (TP), soil moisture content (Water), soil electrical conductivity (EC), bulk density (BD), no grazing (NG), continuous grazing from May to September (CG), grazing in May and July (G57), grazing in June and August (G68), and grazing in July and September (G79).

).

3.2. Soil Particle and Pore Distribution Characteristic

In both grazed and ungrazed plots, soil particles with particle sizes of 0.005–0.05 mm and 0.075–0.25 mm were most common, accounting for 20% and 60% of the soil particles, respectively. In the seasonally grazed plots (G68 and G79), the proportion of micro particles (0.002–0.005 mm) and particles sized 0.05–0.075 mm was significantly lower compared to the ungrazed plots (NG). The proportion of particles sized 0.25–2 mm increased in the seasonally grazed plots G68 and G79, but the results were not statistically significant (

Table 2. Characteristics of soil particle distribution among different grazing management practices.

Soil Particle Distribution (%)	NG	CG	G57	G68	G79
<0.002 mm	0.18 ± 0.35 a	0.02 ± 0.02 a	0.01 ± 0.01 a	0.00 ± 0.00 a	0.01 ± 0.02 a
0.002–0.005 mm	0.58 ± 0.24 a	0.39 ± 0.28 ab	0.31 ± 0.22 ab	0.17 ± 0.20 b	0.15 ± 0.23 b
0.005–0.05 mm	13.41 ± 1.70 a	14.49 ± 2.74 a	13.02 ± 0.93 a	12.61 ± 2.46 a	14.59 ± 1.46 a
0.05–0.075 mm	19.98 ± 0.84 a	20.92 ± 2.06 a	20.13 ± 1.20 a	18.26 ± 0.94 b	19.98 ± 0.89 a
0.075–0.25 mm	60.14 ± 1.87 a	58.13 ± 2.18 a	59.85 ± 1.93 a	60.71 ± 3.83 a	57.66 ± 1.98 a
0.25–0.5 mm	2.42 ± 1.22 a	2.46 ± 1.88 a	2.84 ± 1.54 a	4.08 ± 0.61 a	3.65 ± 0.65 a
0.5–2 mm	3.30 ± 0.63 a	3.59 ± 0.89 a	3.84 ± 0.82 a	4.17 ± 1.40 a	3.96 ± 0.89 a

Note: The differences among different grazing management practices were analyzed by one-way analysis of variance, and the lowercase letters represent the significant differences (p < 0.05). Abbreviation: no grazing (NG), continuous grazing from May to September (CG), grazing in May and July (G57), grazing in June and August (G68), and grazing in July and September (G79).

).

The scanning electron microscopy (SEM) images visually depict the two-dimensional structure and characteristics of soil particles and pores under various grazing methods. Figure 1a illustrates that the ungrazed plot (NG) has a greater proportion of small to medium-sized particles compared to the grazed plot. Additionally, the ungrazed plot exhibits more small pores and fewer large pores. On the other hand, the continuously grazed and seasonally grazed plots display a prominent presence of large particles and a higher abundance of large pores.

The analysis of pore size and morphology revealed that both ungrazed and grazed plots were primarily composed of large pores (Figure 2). In particular, the proportion of large pores in continuously grazed plots (CG) was found to be 0.96 ± 0.01, which is significantly higher than the proportion in ungrazed plots (NG) at 0.86 ± 0.06. Conversely, the proportion of small pores and micro-pores in CG was significantly lower than that in NG. Additionally, the distribution characteristics of soil pores in seasonally grazed plots, grazing in May and July (G57), and G79, showed no significant differences when compared to NG.

The pore area ratio in CG is significantly higher than that in NG, with no significant differences observed between seasonally grazed plots and ungrazed plots (Figure 3a). The average pore diameter did not show a significant difference between grazed and ungrazed plots (Figure 3b). However, the maximum pore diameter in the soil of CG is significantly higher than that in NG. Furthermore, the maximum pore diameters in seasonally grazed plots, G57 and G68, are also significantly higher than those in NG (Figure 3c).

3.3. Relationships between Soil Physicochemical Properties and Soil Particle and Pore Structure

The results of correlation analyses between soil physicochemical properties, soil particle size, and pore structure indicate that there are no significant correlations among TN, TC, TP, water, EC, pH, and soil pores. However, there are correlations with particle distribution. TN, TC, and soil moisture are significantly positively correlated with the proportion of particles in the 0.005–0.05 mm range, with correlation coefficients of 0.41, 0.73, and 0.39, respectively. On the other hand, TN, TC, and the proportion of particles in the 0.075–0.25 mm range show a significant negative correlation. EC is negatively correlated with the proportion of particles in the 0.05–0.075 mm range, while pH exhibits an opposite negative correlation.

The univariate regression equations for total carbon (TC) and the proportion of particles in the 0.005–0.05 mm range are y = 4.37 + 0.55x, with an R2 value of 0.52. Similarly, the univariate regression equation for TC and the proportion of particles in the 0.075–0.25 mm range is y = 70.56 − 0.67x, with an R2 value of 0.45. Moving on to total nitrogen (TN), the univariate regression equation with the proportion of particles in the 0.005–0.05 mm range is y = 7.85 + 4.55x, with an R2 value of 0.14 (Figure 4). Similarly, the univariate regression equation for TN and the proportion of particles in the 0.075–0.25 mm range is y = 67.10 − 6.16x, with an R2 value of 0.17 (

Table 3. Pearson correlation analysis between soil physicochemical properties and soil particles, and pore structure.

	Mean Pore Diameter	Maximum Pore Diameter	Micropore	Fine Porosity	Mesoporosity	Macrovoid	Particle <0.002 mm	Particle 0.002–0.005 mm	Particle 0.005–0.05 mm	Particle 0.05–0.075 mm	Particle 0.075–0.25 mm	Particle 0.25–0.5 mm	Particle 0.5–2 mm
TN	0.09	−0.17	−0.20	0.12	−0.27	0.13	−0.30	−0.38	0.41 *	−0.20	−0.43 *	0.54	0.00
TC	0.04	0.09	−0.46	−0.23	−0.47	0.37	−0.22	−0.05	0.73 *	0.18	−0.69 *	0.24	−0.23
TP	−0.30	−0.06	0.19	−0.11	0.05	−0.10	−0.20	0.16	0.09	0.30	−0.14	−0.14	−0.10
Water	0.56	−0.12	−0.30	0.39	−0.03	−0.02	−0.21	−0.24	0.39 *	−0.07	−0.34	0.26	−0.06
EC	−0.06	0.12	−0.13	−0.14	−0.20	0.25	−0.27	0.05	−0.14	−0.38 *	0.21	0.31	−0.09
pH	−0.16	−0.10	0.31	0.04	0.47	−0.39	0.24	0.31	0.14	0.43 *	−0.01	−0.59	−0.22

Note: The relationships between soil physicochemical properties and soil particles, and pore structure were analyzed by Pearson correlation analysis. * p < 0.05. Abbreviation: total soil carbon (TC), total soil nitrogen (TN), total phosphorus (TP), soil moisture content (water), and soil electrical conductivity (EC).

).

4. Discussion

4.1. Changes in Soil Physics Caused by Livestock Trampling

Previous studies have shown that livestock trampling negatively affects soil, leading to soil compaction in the surface layer. This compaction restricted water infiltration and root growth and extends to deeper soil layers, resulting in a denser soil structure and increased bulk density, which further inhibits root growth [14]. Brito et al. (2018) characterized soil compaction using soil resistivity (RP) and found a significant increase in RP values due to grazing, leading to reduced soil aeration and permeability [17]. Frozzi et al. (2020) observed that livestock trampling rearranges soil particles, altering soil structure and potentially disrupting soil aggregation, which made the soil more vulnerable to erosion and water runoff [18].

Brito et al. (2018) also used the mean weight diameter (MWD) of soil aggregates to examine the impact of grazing on soil structure. They found that grazed soils had a higher MWD compared to soils from cultivated grasslands, although higher MWD values do not necessarily indicate an ideal soil structure [17]. The increased MWD in grazed soils could be due to soil compaction and greater resistance to fracturing. Conte et al. (2011) suggested that larger aggregates in grassland areas might result from animal trampling, which brings mineral particles closer together. MWD values are influenced by clay and total organic carbon (TOC) content, both of which play significant roles in the formation and stability of soil aggregates [19].

The structure of grazed soils was influenced by grazing management and climatic conditions [20]. High grazing intensity, especially during periods of elevated soil moisture like the rainy season from October to June, can cause increased structural damage to the soil. Concentrated animal weight in small areas, particularly hooves, exerts high pressure on the soil. Animal trampling could impose contact pressures of 350 to 400 kPa on the soil, and these pressures can increase when animals were in motion, as the entire weight was concentrated on a single claw, distributing the force over a smaller surface area [21].

4.2. The Impact of Seasonal Grazing on Soil Structure

Our findings confirmed previous reports that there were no significant differences in soil particle composition and pore characteristics between different grazing methods and non-grazed grassland soils under moderate grazing conditions [19]. This aligned with studies using GPS data to analyze livestock movement trajectories under continuous and seasonal grazing [20]. These studies established connections between grazing patterns and soil structure, indicating that grazing cattle’s movement patterns were dependent on pasture area rather than shape, orientation, topography, or selective attention. Both continuous and seasonal grazing systems exhibited similar soil compaction, evidenced by higher bulk density and soil penetration resistance induced by grazing. Soares et al. (2021) demonstrated that soil cone penetration resistance (RP) in grazed areas ranged from 1.40 to 1.49 MPa, higher than in non-grazed areas, but not leading to soil compaction [14].

The impact of grazing on soil structure was influenced by factors including grazing intensity, animal movement patterns, soil clay content, and others [21]. Seasonal grazing tended to exert lighter grazing pressure compared to year-round continuous grazing. Recent research suggested that shorter grazing periods followed by longer recovery periods can reduce soil compaction, allowing more time for natural soil recovery. Seasonal grazing might also positively impact vegetation growth and distribution, contributing to soil structure maintenance. Moderate seasonal grazing could promote the expansion of vegetation roots, improving soil stability and reducing erosion rates [22,23]. Additionally, seasonal grazing affected soil microbial communities, creating a more intricate soil microenvironment that supports greater microbial diversity and contributes to soil ecological balance.

4.3. Relationships between Soil Structure and Soil Ecological Functions

Our findings demonstrated a significant correlation between soil carbon–nitrogen content and soil particle composition. Soares et al. (2021) observed a decrease in soil cone penetration resistance (RP) in grazed areas, attributed to higher levels of organic matter [14]. This increase in organic matter led to a decrease in soil bulk density and an increase in macroporosity. Some studies found a strong association between total organic carbon (TOC) and mean weighted diameter (MWD) of aggregates. Higher levels of TOC, microorganisms, and robust biological activity, including root systems, can lead to the formation of channels and biopores, consequently altering the soil structure [24,25]. Abundant organic matter promoted soil structure development, facilitating a balanced distribution of particles (sand, silt, clay) and creating pores for water and air storage, providing favorable conditions for plant root growth. Organic matter also retained moisture, preventing it from acting as a lubricant between mineral particles, and enhances the cohesion between soil particles, establishing connections among them [26,27].

Soil permeability and compaction status were influenced by organic matter content, microbial activity, roots, exchangeable cations, and soil texture [28]. Recent studies suggested that soils with higher sand content are more prone to compaction, whereas soils with higher clay content tend to resist compaction due to clay’s ability to adsorb organic anions, increasing colloid surface charge and enhancing the diffusion layer of associated with the surface [29,30,31].

5. Conclusions

The investigation into soil characteristics under different grazing management showed no significant differences in particle distribution and pore structure. The prevalence of specific particle sizes and pore characteristics in both grazed and ungrazed plots indicates the complex relationship between grazing practices and soil composition. Continuous grazing appears to have a more noticeable impact on soil structure compared to seasonal grazing, as it significantly increases the pore area ratio and maximum pore diameter in the soil. In the case of seasonal grazing plots, particularly G57 and G79, there are no significant differences in soil particle structure or soil pore structure compared to ungrazed soil. Correlation analyses between soil physicochemical properties and particle size, as well as pore structure, reveal interesting connections that emphasize the influence of grazing on soil dynamics. These findings provide valuable insights for sustainable land management practices, highlighting the intricate interactions between grazing, soil properties, and ecological resilience. Specifically, our results suggest implementing seasonal grazing practices over continuous grazing, as seasonal grazing maintains soil structure and minimizes adverse changes in pore characteristics, thereby preserving soil health and function. Land managers should consider these practices to promote sustainable grazing that supports both agricultural productivity and environmental conservation.