Quantifying Macropore Variability in Terraced Paddy Fields Using X-Ray Computed Tomography

Abstract

Large soil pores critically influence water and solute transport in soils. The presence of preferential flow paths created by soil macropores can profoundly impact water quality, underscoring the necessity of accurately assessing the characteristics of these macropores. However, it remains unclear whether variations in macropore structure exist between different altitudes and positions of terraced paddy fields. The primary objective of this research was to utilize X-ray computed tomography (CT) and image analysis techniques to characterize the soil pore structure at both the inner field and ridge positions across different altitude levels (high, medium, and low altitude) within terraced paddy fields. The results indicate that there are significant differences in the distribution of large soil pores at different altitudes, with large pores concentrated in the surface layer (0–10 cm) in low-altitude areas, while in high-altitude areas, the distribution of large pores is more uniform. Additionally, as altitude increases, the porosity of large pores shows an increasing trend. The three-dimensional equivalent diameter and large pore volume are primarily characterized by large pores ranging from 1 to 2 mm and 0 to 5 mm³, respectively, with their morphology predominantly appearing spherical or ellipsoidal. The connectivity of large pores in the surface layer of paddy soil is stronger than that in the bunds. However, this connectivity gradually weakens with increasing soil depth. The findings from this study provide valuable quantitative insights into the unique characteristics of soil macropores that vary according to the altitude and position in terraced paddy fields. Moreover, this study emphasizes the necessity for future research that encompasses a broader range of soil types, altitudes, and terraced paddy locations to validate and further explore the identified relationships between altitude and macropore characteristics.

1. Introduction

The Hani terraced fields, located in the Ailao Mountains of southern Yunnan, China, were designated as a World Heritage Site by the United Nations Educational, Scientific and Cultural Organization (UNESCO) in 2013 and recognized as a Globally Important Agricultural Heritage System (GIAHS) by the Food and Agriculture Organization of the United Nations (FAO) in the same year [1]. This recognition is attributed to their unique landscape and water use ecosystem. The ecosystem exemplifies a ‘four-element isomorphism’ characterized by forests at the top, villages in the middle, terraced fields at the bottom, and water flowing through them, thereby forming a complete water cycle within the terraced field wetland [2]. The rice terraced field wetlands play a crucial role in water management by controlling erosion, enriching soil, conserving biodiversity, stabilizing climate, and safeguarding heritage [3,4]. The advancement of tourism in the Hani terraced fields, along with the degradation of the forest ecosystem, has negatively affected rice cultivation. This situation has resulted in several issues, such as alterations in the natural water flow pathways within the terraced fields, uneven distribution of water resources, increased sewage output, and water scarcity [5,6]. The area of paddy fields in Yuanyang, which includes the UNESCO-designated Cultural Landscape of Honghe Hani Rice Terraces, decreased by 23% from 2002 to 2017 [7]. These factors collectively threaten the sustainability of HPT agricultural heritage and the ecosystem services it provides. Notably, water transport within paddy fields is a critical component of the ecological hydrological process in terraced fields, directly influencing rice growth, nutrient cycling, and soil and water conservation functions. The study of soil macropores, which regulate water transport, is of significant importance for promoting sustainable agricultural development and advancing ecological civilization.

Soil macropores are defined as the pores that facilitate the rapid migration of water and solutes within the soil [8]. These macropores are formed through various processes, including animal excavation, the interweaving of plant roots, and the effects of freeze–thaw and dry–wet cycles [9]. Although macropores constitute only 0.1% to 5.0% of the total soil volume, they can conduct up to 90% of water flow through unstable preferred flow pathways [10,11]. On one hand, macropores enhance soil permeability and reduce surface runoff, while simultaneously replenishing water and solutes in deeper soil layers. This phenomenon can lead to decreased efficiency in the utilization of water and fertilizers and exacerbating groundwater pollution [12]. On the other hand, the presence of large pores in the soil improves aeration, thereby promoting water exchange, replenishment, and the growth of crop roots [13]. In the paddy field ecosystem, large soil pores can significantly alter the migration path and rate of water, impacting the water absorption and utilization of rice, as well as the soil and water conservation functions of terraced fields [14].

The Hani terraced rice cultivation system is heavily dependent on substantial irrigation water sources for the irrigation and fertilization of fields. However, approximately 80% of the irrigation water is lost through seepage and transpiration, with seepage from soil pores in the rice fields accounting for 50% of the total input water. This loss directly diminishes the efficiency of water and nutrient utilization in rice cultivation [15]. Due to prolonged waterlogged farming practices and the unique irrigation methods employed in terraced fields, the number, characteristics, and connectivity of macropores tend to increase with the duration of farming. This alteration significantly influences the infiltration and redistribution processes of soil moisture, thereby modifying the spatial distribution patterns and temporal dynamics of soil moisture [16].

The primary observation techniques for macropores include the soil column soaking method [17], X-ray computed tomography [18], ground penetrating radar detection method [19], soil penetration curve method [20], and mathematical modeling method [21]. X-ray computed tomography (CT scanning method) is a non-invasive imaging technique that enables high-resolution, three-dimensional, and non-destructive visualization of heterogeneous soils, facilitating the observation of actual soil pore characteristics instead of inferred ones [22,23]. This technique has been effectively employed in assessing the size, shape, distribution, and arrangement of soil pores, as well as evaluating surface area and pore connectivity [24,25].

Based on field experiments, this study employed micro-CT scanning and image analysis techniques to investigate the characteristics of macropores in Hani terraced fields. The objective was to minimize the ineffective loss of irrigation water during rice cultivation and enhance the utilization coefficient of irrigation water, thereby providing references for increasing rice yield, optimizing the irrigation system of terraced fields, and maintaining the ecosystem functions of these fields.

2. Materials and Methods

2.1. Study Area

The soil columns investigated in this study were excavated from the Hani terraced fields (23°5′20″–23°13′18″ N, 102°43′16″–102°50′39″ E) located at Honghe Hani and Yi Autonomous Prefecture, Yunnan Province. This area belongs to the Malizhai River Basin. This region exhibits typical low mountain and hilly landforms, with terraced fields distributed according to the mountainous terrain, at altitudes ranging from 1360 to 2230 m. The climate is classified as a subtropical mountain monsoon climate, characterized by an annual average temperature of 20.05 °C, annual precipitation between 1500 and 2000 mm, an average foggy period of 180 days, an average sunshine duration of 1820.8 h, and an average evaporation rate of 1500.6 mm. The three-dimensional climate characteristics are pronounced; in certain areas, temperature and evaporation decrease with increasing altitude while humidity rises [26,27]. In this study area, rice is the predominant crop, cultivated through a system of one crop per year and employing the farming method known as ‘three plowing and three harrowing’ [28,29]. The ridges of the rice fields are covered with weeds such as Bermuda grass, Polygonum multiflorum, and Centella asiatica, which contribute to soil erosion control, enhance water retention, promote soil conservation, and facilitate field management. The primary soil types in this region are yellow soil and yellow–brown soil.

2.2. Soil Sampling

A total of six undisturbed columns (110 mm diameter, 400 mm depth, and 30 mm thickness) were collected in October 2023. Two cores, one from the inner and one from the ridge of the rice field, were obtained from each of the following three altitudes: Zhulu Village (low altitude: 1491.6 m), Shangzhulu Village (medium altitude: 1642.3 m), and the Quanfu Village Viewing Platform (high altitude: 1877.3 m) (Figure 1). During plot selection, site conditions such as slope, slope aspect, and slope position were thoroughly considered to ensure good comparability among plots. Prior to sampling, the water in the rice field was drained, and the remaining rice roots and weeds were removed. A shovel was then employed to cut off the portion of the field ridge that extended above the field, leveling it with the surface soil. To prevent damage to the original soil, one end of the PVC pipe was sharpened, and a thick wooden board was utilized to assist in inserting it vertically into the soil. Subsequently, the soil column was excavated with a shovel and immediately sealed with cling film and foam to ensure the integrity of the soil structure.

Following the sampling, bubble wrap packing material was utilized to fill the airspace at both ends of each soil column within the PVC pipe. An end cap was securely fitted over the sides of the PVC pipe prior to transport. To minimize disturbance to the soil columns during transport, a cushioning layer of 110 mm wood shavings was placed beneath the soil columns. The samples were subsequently maintained at a temperature of 4 °C until analysis was performed. A uniform moisture condition across all cores [30] was established by saturating all soil columns with water before scanning and allowing them to drain for approximately three days. To reduce variations in the histogram of the X-ray attenuation values, slope positions were compared consistently. Each soil column was assumed to be at approximately field capacity before scanning. The bottoms of each soil core were inspected to ensure they were sufficiently dry to prevent damage during transport for scanning.

2.3. CT Scanning and Image Analysis

The soil cores were scanned using medical GE Optima CT670 (GE Healthcare, Chicago, IL, USA) installed in the Yuanyang County People’s Hospital, Yunnan Province. This scanner is equipped with a high-resolution transmitted target X-ray source, a detector, and a scanning system. It accurately reflects the distribution and structural characteristics of soil pores in situ and facilitates three-dimensional visualization of these pores. The machine has the ability to take up to 64 slices in one scan, thus covering 40 mm at 0.625 mm slice thickness in one scan. The scanner was operated with the scanning parameters set to 120 kv, 140 mA, 1 s exposure time, and a voxel size of 0.88 × 0.88 × 1.25 mm³, which provided detailed projections with relatively little noise. A total of 1626 32-bit TIFF format images were obtained.

The images were analyzed using Avizo 2020, a three-dimensional stereoscopic image processing software developed by Thermo Fisher Scientific Inc. (Waltham, MA, USA). Initially, the TIFF format image was imported into Avizo, and the median filtering algorithm from the Filter Sandbox module was employed to eliminate image noise. To reduce the influence of edge effects on the experimental results, the central area of the soil block with a depth of 35 cm was selected for analysis. Pores with an equivalent pore size greater than 1 mm were classified as macropores [31]. The threshold segmentation of macropores was conducted using Interactive Top-Hat and Interactive Thresholding techniques. The segmented image was processed through the Volume Rendering module to generate a three-dimensional representation of the macropores, accurately reproducing their structure. The Label Analysis module was utilized to calculate the number of pores, porosity, pore volume, equivalent diameter, shape factor, and Euler number on a three-dimensional scale. Hierarchical calculations were performed on the exported data using Excel. In this study, the equivalent diameter was categorized into five ranges as follows: 1–2 mm, 2–3 mm, 3–4 mm, 4–5 mm, and >5 mm. The macropore volume was classified into the following five categories: 0–5 mm³, 5–10 mm³, 10–20 mm³, 20–50 mm³, and >50 mm³.

2.4. Extraction of Soil Macropore Structure Parameters

(1)

Porosity. The term ‘porosity’ refers to the total number of independently distributed pores within a three-dimensional model. Avizo 2020 software is utilized to automatically count the number of large pores present in the model.

(2)

Porosity ($N_{3d}$). Porosity is defined as the ratio of pore volume to the total volume of the soil. The calculation formula is as follows:

$$N_{3d}={\displaystyle \frac{V_v}{V_{T^{\prime }}}},$$

(1)

where $V_v$ is the pore volume, mm³; $V_{T^{\prime }}$ is the total volume of the soil, mm³.

(3)

Pore volume ($V_{3D}$). Pore volume refers to the sum of the volumes of voxels occupied by pores in a three-dimensional model. The calculation formula is as follows:

$$V_{3D}=N\times V_0,$$

(2)

where $N$ is the number of pore voxel units; $V_0$ is the minimum voxel unit volume, mm³.

(4)

Equivalent diameter ($d_{3D}$). The equivalent diameter refers to the diameter of a sphere with the same volume as an irregular object in three dimensions. The calculation formula is:

$$d_{3d}=\sqrt[3]{{\displaystyle \frac{6V_{3D}}{\pi }}},$$

(3)

where $V_{3D}$ is the pore volume, mm³.

(5)

Shape Factor ($S{F}_{3D}$). In three dimensions, the degree to which an object approximates a sphere is quantified using the three-dimensional shape factor. A standard sphere has a shape factor of 1, while elongated, flat, or irregular shapes yield higher shape factors. Based on the classification method for soil pore shapes [32] and taking into account the actual conditions observed in the images, large pores with shape factors less than 3 are classified as spherical ellipsoidal pores. Those with shape factors ranging from 3 to 6 are classified as columnar pores, while large pores with shape factors greater than 6 are categorized as branched pores. The calculation formula is as follows:

$$S{F}_{3D}={\displaystyle \frac{{Area}_{3D}^3}{{36\pi V}_{3D\prime }^2}},$$

(4)

where $Are{a}_{3D}$ is the surface area of the pores, mm²; $V_{3D}$ is the pore volume, mm³.

(6)

Euler number ($\chi (X)$). Euler’s number serves as an indicator for assessing the connectivity of three-dimensional complex structures. It quantifies the connectivity of the structure by evaluating the degree of multiple connections among each component of the object [33]. A stronger connectivity within the soil structure corresponds to a smaller Euler value, while a weaker connectivity results in a larger Euler value. The calculation formula is as follows [34]:

$$\chi \left(X\right)={\beta }_0-{\beta }_1+{\beta }_2,$$

(5)

where ${\beta }_0$ is the number of connecting components; ${\beta }_1$ is the number of holes; and ${\beta }_2$ is the number of closed pores.

2.5. Statistical Analysis

Avizo 2020 software was utilized for the three-dimensional processing of CT scan images and the extraction of data. The extracted data were subsequently analyzed and organized using Excel 2019 software. Statistical analyses, including analysis of variance and correlation analysis, were performed using SPSS 22.0 software. The results of the data analysis were graphically represented using Origin 2021 software.

3. Results

3.1. Visualization and Three-Dimensional Characteristics of Soil in Different Altitudes and Field Position

Figure 2 illustrates the three-dimensional spatial structural characteristics of macropores in paddy fields and ridges at varying altitudes. Significant differences are observed in the structure and distribution of soil macropores between paddy fields and ridges. In paddy fields, a greater number of soil pores is present at depths of 0–10 cm, whereas deeper layers exhibit a scarcity of large pores, which are predominantly small and manifest as minor patterns. In contrast, ridge areas display macropores that extend from the surface to the deeper layers, with a higher overall content compared to paddy fields. The pore diameters in ridge areas are relatively larger, and the distribution of these pores is more pronounced than in the fields.

The distribution of soil pore volumes across the three altitudes decreases gradually with increasing soil depth. Conversely, as altitude increases, the pore volume tends to rise. In low-altitude rice fields, large soil pores are primarily concentrated in the 0–10 cm soil layer, predominantly forming inclined elongated strips. In medium- and high-altitude rice fields, large pores are evenly distributed within the 0 to 20 cm soil layer, resulting in a complex interlaced pore network. Below 20 cm, macropores in paddy field soil predominantly manifest as isolated, sporadic large pores, which lead to poor connectivity among macropores. In ridge areas, numerous transverse tubular pores are observed in the surface soil at low altitudes. The surface and middle sections of mid-altitude soil are predominantly characterized by flake pile structures. In high-altitude regions, large soil pores are densely packed, predominantly taking on an inclined tubular shape, demonstrating strong connectivity.

3.2. Three-Dimensional Porosity Distribution Law

Figure 3 illustrates the trend of three-dimensional macroporosity in rice fields and field ridges as it varies with depth at different altitudes. Overall, macroporosity in both paddy fields and ridges exhibits a gradual decrease with increasing soil depth. In low-, medium-, and high-altitude areas, the macroporosity of the soil profile at the field ridge is 2.2 times, 2.5 times, and 1.3 times greater than that of the soil within the field, respectively. The variation patterns of macroporosity in paddy fields and field ridges at different altitudes demonstrate significant differences in the vertical direction. In the 5–10 cm soil layer of the rice field, macroporosity is highest in the high-altitude area, approximately 10.56%, while it is relatively lower in the low-altitude and mid-altitude areas. Below the 10 cm soil layer, macroporosity generally decreases with increasing depth. When the soil layer depth exceeds 25 cm, macroporosity stabilizes at approximately 1% across all altitude areas. The macroporosity of the soil at the field ridge is relatively high in the surface layer but gradually decreases with increased depth until it stabilizes. At the same depth, the trend of macroporosity is observed as follows: high altitude > medium altitude > low altitude. In the 0–5 cm soil layer, macroporosity in the low-altitude area is the highest at 14.78%, which is 1.4 times and 2.2 times greater than that of the medium-altitude and high-altitude areas, respectively. In soil layers exceeding 15 cm, soil porosity in low-altitude areas fluctuates around 0.5%, while peaks occur in the mid-soil layers of high-altitude and medium-altitude areas, which are 20.5 times and 22.8 times greater than that in low-altitude areas, respectively.

3.3. Three-Dimensional Equivalent Diameter and Volume Distribution Law

The overall trend of the proportion curves for the equivalent diameters of large pores in paddy fields and field ridges at different altitudes is consistent (Figure 4), indicating a decreasing trend with increasing equivalent diameter. The equivalent diameters of large pores in both rice fields and field ridges are primarily concentrated between 1 and 2 mm, with pores in this range accounting for 60.01% to 65.10% of the total number of pores. Pores measuring 2–3 mm account for 21.78% to 23.67%, while those measuring 3–4 mm represent 7.84% to 9.50%. Pores ranging from 4 to 5 mm constitute 2.25% to 4.16%, and pores larger than 5.0 mm account for 1.89% to 3.39%. In terms of the number of large pores, field ridges generally exhibit a greater quantity than rice fields, in the following order of altitude: high > medium > low. Specifically, the number of large pores with an equivalent diameter of 1–2 mm in the high-altitude rice field area is 1549, which is 3.85 times and 1.56 times greater than that in the low-altitude and medium-altitude areas, respectively. In the field ridge area, the number of large pores with an equivalent diameter of 1–2 mm in the low-altitude area is 1067, which is double that found in the rice field area. The number of macropores in the medium-altitude and high-altitude areas is 2120 and 2192, respectively.

The classification of large pores was conducted based on pore volume, as illustrated in Figure 5. The data from the rice fields and ridges indicate a trend where the number of large pores is greater in the ridges than in the fields. Furthermore, the phenomenon is more prevalent at high altitudes compared to medium altitudes, and the abundance of large pores at medium altitudes exceeds that at low altitudes. In the rice fields, the volume of large pores—categorized as 0–5 mm³, 5–10 mm³, 10–20 mm³, 20–50 mm³, and those larger than 50 mm³—constituted between 65.49% and 68.38%, 12.30% and 13.68%, 9.42% and 9.94%, 5.42% and 7.20%, and 3.15% and 4.21% of the total volume, respectively. In contrast, the proportion of large pores measuring 0–5 mm³ in the field ridges ranged from 65.07% to 70.16%, which is 15.77 to 24.28 times greater than that of pores larger than 50 mm³. This indicates that the predominant contribution to the number of pores in the soil comes from those measuring 1–2 mm and 0–5 mm³. Furthermore, the influence of altitude on pore volume is significant; as pore volume increases, the benefits associated with high-altitude areas diminish, leading to a rapid decline in the number of large pores.

3.4. Distribution Law of Shape Factors and Connectivity of Large Pores in Paddy Fields and Ridges

The three-dimensional shape factor is a crucial indicator for assessing the spatial morphology of pores, exhibiting distinct characteristics under varying water transport and storage conditions. As illustrated in Figure 6, an increase in the shape factor is associated with a continuous decline in the proportion of macropores. In both rice fields and ridges, the shape factor for large pores predominantly ranges from 1 to 3, indicating that most pores are spherical or ellipsoidal. In rice fields, the average proportion of macropores with a shape factor between 1 and 3 is 81.12%, whereas those with a shape factor greater than 3 comprise only 18.88%. In the ridges, the average proportion of large pores with a shape factor ranging from 1 to 3 is 80.86%, compared to only 19.14% for those with a shape factor exceeding 3. Spatially, both spherical and ellipsoidal pores are distributed within the 0–35 cm soil layer of paddy fields and field ridges. Columnar pores are predominantly located in the 0–20 cm soil layer of both rice fields and ridges, with only a limited presence in the 20–35 cm soil layer of rice fields; however, a significant quantity is observed in the 20–35 cm soil layer of the ridges. Branched pores are primarily concentrated in the 0–20 cm soil layer, showing negligible distribution in the 20–35 cm soil layer of rice fields, while a small quantity is found in the field ridges.

The three-dimensional connectivity of macropores was quantified using Euler numbers (Figure 6). It was observed that the connectivity of macropores in paddy fields diminishes with increasing soil layer depth. Notably, macropores within the soil layer ranging from 20 to 35 cm exhibit minimal connectivity. The connectivity of large pores on the field ridges fluctuates. Furthermore, the connectivity of large pores in the soil layer below 15 cm is significantly reduced compared to that in the 0–15 cm soil layer. In the 0–5 cm soil layer, the connectivity of large pores in the field is stronger than that of the field ridges. However, in the 5 to 35 cm soil layer range, the connectivity of large pores in the field is weaker than that of the field ridges.

4. Discussion

4.1. The Influence of Different Altitude Gradients on Soil Macropores Characteristic

Elevation plays a crucial role in shaping mountain landscapes and significantly influences climate [35]. Air temperature, precipitation, and water balance dictate the direction and magnitude of physicochemical processes at all soil depths [36,37,38]. Terraced field soil serves as a vital foundational environment for mountain ecosystems. Variations in temperature and precipitation among terraced fields at different altitudes lead to significant differences in the physical and chemical properties of the soil. Previous studies have indicated that with an increase in altitude, there is a decrease in temperature, an increase in above ground biomass, a decline in microbial activity, and a reduction in the life cycle of vegetation roots [29,39]. These factors contribute to an increase in plant litter and soil organic matter, with the contents of soil organic matter, total nitrogen, and alkali-hydrolyzable nitrogen gradually rising.

Soil macropores are a crucial component of soil structure that significantly influences soil water transport, nutrient retention, and plant root growth, ultimately determining soil productivity [40]. In the low-altitude regions of the Hani terraced fields, macropores are predominantly concentrated in the topsoil layer and exhibit a longitudinal strip-like distribution (Figure 2). This phenomenon can be attributed to the elevated temperatures in low-altitude areas and the lush vegetation present in rice fields and field ridges. The abundant vegetation significantly contributes to the formation and development of large pores in the surface soil through root activities and the addition of organic matter. As altitude decreases, nutrient supply gradually diminishes, leading to a reduction in both the quantity and activity of soil microorganisms, as well as a decrease in macropores [41]. Conversely, with increasing altitude, temperatures gradually decline, which slows the activities of soil microorganisms and the growth of plant roots. Nevertheless, due to the effects of fertilization and irrigation, nutrient content in high-altitude areas remains relatively high. This not only promotes root growth and increases the number of microorganisms but also maintains a relatively stable soil structure, conducive to the formation and maintenance of large soil pores. Consequently, in medium- and high-altitude rice fields, large soil pores are evenly distributed within the 0–20 cm soil layer, and their quantity is significantly higher than that in low-altitude areas. However, in the soil layer below 20 cm, the number of macropores is significantly reduced.

4.2. Evaluation of Soil Macropores Characteristic at Different Field Positions

The distribution of large pores in rice fields is less dense than that of the ridges, and the porosity of the soil at the ridge is greater than that in the paddy field (Figure 3). Tillage activities are the primary factors influencing the distribution characteristics of macropores within the field. Prolonged tillage and soil turning disrupt the original soil structure of the surface layer, facilitating the development of macropores [42]. Compared to uncultivated soil, the topsoil layer of paddy fields following hydroponic cultivation is looser and contains a higher proportion of large pores [43], which enhances water seepage. In contrast, below the plow layer, the soil experiences the combined effects of gravitational and tillage compaction, resulting in increased soil density and inhibiting the formation of macropores [44,45]. Kukal et al. [46] conducted tillage experiments on sandy loam soil, revealing a significant increase in bulk density at depths of 14–20 cm.

The compact nature of the bottom layer of the plow in agricultural fields hinders the vertical infiltration of water. Additionally, field ridges are less influenced by farming activities compared to the interior of the fields, and the activity of burrowing animals in the soil leads to a broader distribution of large pores. Research indicates that during the rice growth period, water loss from the field ridge area can exceed 30% of the total irrigation and rainfall [47]. Furthermore, water primarily seeps rapidly through the field ridges via two pathways: vertical infiltration and lateral seepage [48,49]. In these rice fields, the predominant mode of water loss occurs in the ridge areas, where rapid water seepage is observed [50,51]. These processes not only result in significant irrigation water loss but also adversely impact the quality of surface and groundwater. Therefore, addressing water loss in the ridge areas is essential for enhancing irrigation water utilization efficiency in this region and for mitigating groundwater pollution caused by fertilizer seepage.

4.3. The Number of Large Pore

The number of large pores in the soil of the study area exhibited a significant decreasing trend with an increase in equivalent diameter and pore volume (Figure 4). Among these, large pores measuring 1–2 mm and those with a volume of 0–5 mm³ dominate, while extra-large pores with a radius exceeding 2 mm and a volume greater than 5 mm³ constitute a relatively small proportion. Overall, the macropore morphology of the paddy fields and ridges in the study area is similar. The soil is characterized by a limited number of large-radius pores and a substantial number of small-radius pores, with the macropore shapes primarily being spherical and ellipsoidal (Figure 6). In their study of macropores in organic and traditional orchards, Deurer [52] concluded that different management practices did not influence the morphology of soil pores, but they did affect the distribution of macropores. This finding is consistent with the research of Musso, A. [53], which indicated that over time, the richness of soil functions continuously increases, promoting the formation of soil aggregates and leading to a gradual rise in the proportion of small-pore and small-volume pores. This suggests that during the development of soil pores, while pore morphology tends to stabilize, pore diameter and volume gradually decrease.

However, significant differences exist in the macropore connectivity of the Hani terraced fields. The soil below 20 cm in the paddy fields has become compacted due to prolonged cultivation, resulting in the degradation of the macropore structure and a marked reduction in both pore development and connectivity. In contrast, the height difference between the ridges of the rice field and the interior facilitates the lateral seepage of water and fertilizer from the field into the ridges, thereby enhancing the nutrient content of the soil along the ridges [10]. Furthermore, the root system in the soil of the field ridges is abundant. The synergistic effects of the root system and the activities of soil organisms promote the formation of larger pores. Particularly in the soil layer between 20 and 35 cm, a greater number of large-diameter pores, large-volume pores, and branched pores are formed, significantly improving pore connectivity [54,55].

4.4. Effect of Analyzed Soil Volume on Macropore Characteristics

To achieve higher scanning resolution, numerous researchers have opted for soil cores with diameters smaller than 150 mm [16,56,57]. Consequently, the diameter of the soil column selected for this study was 110 mm. However, the diameter of soil cores chosen for computed tomography (CT) analysis is a critical factor that affects the accuracy of soil porosity characteristics. Rab et al. (2014) [58] indicated that reducing the volume of a 50 mm diameter soil core from 100% to 20% of its original volume significantly influenced (p < 0.05) the average pore diameter. Piccoli et al. (2019) [59] observed that when they sub-sampled a 100 mm diameter core into progressively smaller sub-volumes, there was a notable decrease in total porosity as the volumes decreased. Budhathoki et al. (2022) [60] discovered that sampling with smaller diameter soil cores can introduce bias in macropore measurements due to a significantly higher coefficient of variation compared to larger 150 mm diameter cores. These variations in measurements underscore the necessity of using larger diameter cores to better capture the variability in soil pore characteristics. Future investigations should broaden the scope to explore the effects of various sampling strategies, particularly the impact of soil core size on CT analysis and its subsequent effect on soil pore characteristics.

5. Conclusions

This study quantified the variation in soil macropore characteristics across different field positions at high, medium, and low altitude locations within terraced paddy fields. The results provide clear and consistent evidence that the distribution of macropores varies significantly across different gradients and altitudes. Large pores are predominantly concentrated within the topsoil, with a greater number of large pores observed on the ridges of fields compared to those in paddy fields. At equivalent depths, porosity exhibits a trend of high altitude > medium altitude > low altitude. The three-dimensional equivalent diameter primarily ranges from 1 to 2 mm, with a relatively substantial proportion of large pores ranging from 0 to 5 mm³. As the shape factor increases, the proportion of macropores gradually decreases, and the morphology of macropores is predominantly spherical or ellipsoidal. In the soil surface layer (0–5 cm), macropore connectivity in paddy fields is stronger than that in field ridges; however, in the 5–35 cm soil layer, macropore connectivity in field ridges is significantly stronger than that in paddy fields.

Due to the significant variability of macropore traits among different soils, additional replicates for each topographic position should be included to strengthen the evidence supporting altitude effects. The pore structure is influenced by both soil physicochemical properties and root characteristics. Therefore, the CT-based three-dimensional reconstruction of terraced paddies will elucidate how cultivation practices modify macropore structures. Furthermore, the generality of the observed topographic patterns should be evaluated across various slopes, materials, irrigation methods, crops, and soil types.