Investigation of Rainfall Infiltration and Redistribution in Bare Land Within the Black Soil Region of Northeast China Under Traditional Ridge Tillage Practices

Abstract

A prerequisite for the efficient utilization of water and fertilizer in the traditional ridge farming model in the black soil region of Northeast China is the precise elucidation of the small-scale temporal and spatial characteristics of rainfall infiltration and redistribution. However, the existing research findings have yet to fully satisfy this requirement. To investigate soil water infiltration and redistribution at different positions (ridge bed, ridge side, and furrow) before ridge closure in ridge-furrow crops within the black soil regions of Northeast China, indoor simulation experiments and field natural rainfall monitoring were conducted. The indoor test involved rainfall settings of 12, 16, 20, 24, and 30 mm with a rain intensity of 90 mm/h. Field monitoring recorded a natural rainfall intensity of 56 mm/h lasting 22.5 min, with cumulative rainfall reaching 21 mm (randomly measured), to analyze the process of soil water movement post-rainfall. Results indicated that under conventional ridge planting in black soil areas, prior to ridge bed coverage, the infiltration amounts for ridge bed, ridge side, and furrow under 16 mm rainfall conditions equaled the rainfall itself, with ratios close to 1:1:1, showing no significant redistribution of precipitation during infiltration. For rainfall levels of 20 mm, 24 mm, and 30 mm, the ratios of infiltration to rainfall at the ridge bed, ridge side, and furrow positions were 0.92:1.03:1.04, 0.90:1.03:1.06, and 0.89:1.04:1.09, respectively. When rainfall exceeded 20 mm, the infiltration-to-rainfall ratio was approximately 0.9 and 1.04, respectively. Approximately 10% of the rainfall on the ridge platform migrated to the ridge side via splash and runoff, increasing the water volume at the ridge side by about 4%. For rainfall less than 24 mm, the ridge bed, ridge side, and furrow reached a stable state after approximately 50 min of infiltration and redistribution. For rainfall between 24 mm and 30 mm, the ridge platform stabilized within 50 min, whereas the ridge side and furrow required longer stabilization times. These findings elucidate the spatial variation laws of small-scale rainfall infiltration, providing insights for enhancing soil water and fertilizer utilization efficiency.

1. Introduction

As a crucial grain-producing region and commercial grain base in China, ridge tillage is widely practiced in Northeast China to enhance soil moisture content and increase crop yields due to the relatively low annual average temperature [1,2]. Ridge tillage is a prevalent cultivation technique that offers numerous advantages, including improved soil moisture retention, enhanced soil temperature regulation, increased water use efficiency, and higher crop yields [3]. Soil water serves as the link between surface water and groundwater, playing a vital role in the formation, transformation, and consumption of water resources. It is also closely associated with the growth of agricultural crops and changes in arable land quality. The infiltration process of soil water is complex and influenced by various natural factors, such as soil texture, rainfall intensity, and soil water content [4,5,6,7,8,9,10]. Additionally, due to the combined effects of precipitation, topography, soil properties, and land-use types, soil water exhibits significant spatial heterogeneity at small scales.

Ridge tillage patterns are influenced by numerous factors, including soil bulk density, slope type, natural rainfall, surface rainwater convergence, the vertical projected area of crops, as well as the dimensions of ridges and ridge platforms [11,12,13,14]. These factors result in significant variations in infiltration amounts across ridges, furrows, and ridge platforms after rainfall. To date, studies both domestically and internationally have explored the mechanisms of water infiltration in ridge systems [15,16,17,18,19,20,21,22,23,24,25], primarily focusing on furrow irrigation infiltration, sediment production processes, and the erosion characteristics of sloping farmland [26,27,28,29,30,31,32]. In 2022, Zhang et al. investigated the soil and water conservation effects of straw mulching without tillage on black soil sloping farmland [33]. Their findings revealed that the infiltration rates of ridges and furrows decreased rapidly over time and stabilized approximately 10 min after the start of the experiment. The stable infiltration rates of ridges and furrows were found to be similar, with no differences observed between them. Additionally, the initial infiltration rate of the ridge bed was slightly higher than that of the furrow, though this difference was not statistically significant. Guo et al.’s research demonstrated that the stable infiltration rate at the ridge and terrace positions was higher compared to that in the ridges and furrows [34], with the variation process aligning with the findings reported by Zhang et al. [33]. These studies collectively focused on the infiltration rates of ridges and furrows to explore their variation characteristics. However, all infiltration tests were conducted using a double-ring infiltrometer, which neglected the influence of raindrop splashing and runoff. This experimental setup differs from natural rainfall conditions, thereby limiting the direct applicability of the research findings to practical scenarios.

After the cessation of rainfall, soil infiltration water undergoes redistribution driven by gravitational forces and soil matric potential. The process of water redistribution within the soil profile plays a critical role in determining the amount of water available for plant root uptake [35,36]. Furthermore, the essential nutrients required by crop root systems are transported from the soil surface to the root zone via water as a carrier. Consequently, investigating the mechanisms of soil water redistribution is of paramount importance for optimizing crop growth and resource utilization [37,38]. Chen et al. [39] conducted indoor simulated soil trench artificial rainfall experiments to examine the soil moisture redistribution patterns under varying initial soil moisture contents. Their findings revealed that an increase in the initial soil moisture content leads to a prolonged redistribution duration [39]. Similarly, Lv et al. [40] demonstrated that soil moisture redistribution processes differ significantly under distinct tillage practices. Ridge tillage, in particular, enhances the soil surface layer moisture content, thereby ensuring an adequate water supply during critical crop water demand periods [40]. Additionally, the influence of the initial soil moisture content on soil water redistribution patterns is modulated by factors such as micro-topography and terrain characteristics (e.g., slope gradient, slope position, and slope aspect) [41]. Collectively, these studies indicate that soil water redistribution exhibits regularity and spatial small-scale variability. Moreover, the differences in infiltration rates among ridge beds, ridge sides, and furrows during rainfall further exacerbate the spatial heterogeneity of soil water redistribution.

Soybeans in the black soil region of Northeast China are typically sown in the first half of May. The seedling stage extends until mid-June, lasting approximately 20 to 30 days. During this period, multiple rainfall events occur. Rainfall during this phase significantly influences the migration of nutrients from the base fertilizer, thereby affecting the soybean absorption and utilization of fertilizers throughout the growth period. Despite extensive research on the black soil region of Northeast China, many studies have not focused on this critical stage. This has led to a lack of understanding regarding the process of rainfall infiltration and redistribution, particularly the small-scale spatial variations among ridges, ridge sides, and furrows. These factors represent a key gap limiting the efficient use of water and fertilizer resources in this region. The findings of this study aim to address this gap.

Aiming at the conventional tillage mode in typical black soil regions, we conducted indoor simulations and field in situ monitoring experiments by considering the underlying surface conditions and rainfall characteristics in the study area. The research elucidated the water seepage migration process following rainfall events, revealed the small-scale spatial migration patterns at the ridge platform, ridge side, and ridge furrow, and provided a scientific basis for the efficient utilization of soil water and fertilizer resources.

2. Materials and Methods

2.1. Study Location

The field natural rainfall monitoring experiment was carried out at the experimental site of the College of Water Conservancy and Civil Engineering, Northeast Agricultural University, from 6 May 2024 to 10 June 2024 (126°43′06″ E, 45°44′23″ N). The effective growing season for crops lasted between 120 and 130 days, with an effective accumulated temperature of ≥10 °C exceeding 2700 °C during the growing season. The average annual precipitation was approximately 560 mm. The soil in the study area is chernozem. The fundamental agronomic and chemical characteristics of the soil utilized in this study are presented in

Table 1. Typical soil conditions.

Soil	Particle Size Distribution (%)			Soil Organic Matter (%)	Total Phosphorus (g/kg)	Total Potassium (g/kg)	pH
	Clay <0.002	Silt 0.002–0.05	Sand >0.05
Silt Loam	16.71	67.14	16.15	1.63	1.52	0.68	7.67

. The test plot measured 4.5 m in length and 2 m in width, with a slope of 5°. Soybean planting density was set at 268,000 plants per hectare, with ridge widths of 0.067 m. Prior to seeding, urea was applied at a rate of 55 kg/ha, and diammonium phosphate was applied at the same rate of 55 kg/ha. The high-yield soybean variety Suinong 21 was utilized for the experiment.

The indoor simulation test was carried out at the laboratory of the College of Water Conservancy and Civil Engineering, Northeast Agricultural University, from 1 April 2024 to 20 July 2024. The soil used for the indoor simulation test was sampled from the 0–20 cm soil layer at the field test site. After collection, the soil was sieved through a 4 mm mesh screen to remove plant roots and other debris, then air-dried and thoroughly mixed for subsequent use. A self-fabricated polyethylene soil container (33 cm in length, 19.5 cm in width, and 40 cm in height) was constructed with ridge platforms, ridge sides, and furrows, as depicted in Figure 1. Figure 1 presents a schematic illustration of the indoor testing apparatus (a) along with images of the field testing location (b). The ridge spacing is set at 66 cm, with a ridge height of 15 cm, a ridge platform width of 28 cm, and a ridge furrow width of 28 cm. Additionally, a series of 1 mm diameter holes were drilled along the side walls of the container at 5 cm horizontal intervals to facilitate exhaust.

2.2. Experimental Design

Compared to indoor simulation experiments, natural rainfall in field conditions requires more time and is less controllable due to its unpredictability. Indoor simulation experiments allow for precise control over various influencing factors, thereby enhancing the universality of the results, expediting the experimental process, and enabling the acquisition of research findings within a shorter time frame. This study employed both indoor simulation and in situ field monitoring experiments in tandem to cross-validate the results and enhance the generalizability of the findings. According to the field-measured values, the soil bulk density under the ridge was 1.2 g/cm³, on the ridge platform it was 1.1 g/cm³, and the soil water content was 17.5% (gravity moisture content) [1]. The soil was weighed, calculated, and evenly spread on a plastic film. Water was sprayed and added layer by layer, with each soil layer receiving one spray of water. Subsequently, the plastic film was gathered to cover the soil, preventing water evaporation, and rested for 5 h. After ensuring the uniform mixing of the soil and water, the soil was mixed thoroughly. An appropriate amount of Vaseline was applied to the inner walls of the container to ensure close contact between the soil and the inner wall, thereby minimizing boundary effects. The prepared soil was filled into the container in layers according to the designed bulk density, with each layer being 4 cm thick. Each layer was gently smoothed using a soil hammer, and the contact surface between soil layers was scratched using a steel brush to enhance interlayer adhesion. Based on the average rainfall of the test area over many years, five rainfall intensities were designed: 12 mm, 16 mm, 20 mm, 24 mm, and 30 mm, with a rainfall intensity of 90 mm/h. Three replicates were conducted for each gradient. Using a self-made rainfall simulator, the designated water volume was evenly sprayed onto the soil surface. After precipitation ceased, samples were simultaneously collected from the ridge platform, ridge side, and ridge furrow. The sampling locations are illustrated in Figure 1. The sampling depth ranged from 0 to 20 cm, with one sample taken every 2 cm. Sampling times were set at 0 min, 5 min, 10 min, 15 min, 30 min, and 50 min post-precipitation.

The field rainfall sampling was conducted by randomly selecting the later stage of the seedling period. Through rainfall monitoring, the rainfall event on June 1st was chosen as the research object. This event had a cumulative rainfall of 21 mm, an intensity of 56 mm/h, and an average initial soil moisture content of 18.6%. During this period, the plants were relatively short, resulting in minimal stem and leaf interception effects, as illustrated in Figure 2. The sampling location and timing were consistent with those in the indoor simulation experiment.

In this study, the Tukey’s Honestly Significant Difference (HSD) test was employed to investigate the differences in infiltration volume at the positions of ridge beds, ridge sides, and furrows.

3. Results and Discussion

3.1. Analysis of Rainfall Infiltration Characteristics on Ridge Platforms, Ridge Sides, and Ridge Furrows

At the conclusion of the indoor simulation test, the soil profile water content at the ridge platform, ridge side, and furrow locations are depicted in Figure 2. As the rainfall intensity increased, the water content at various depths across the three locations gradually rose, with surface water content increasing from 31.3% to 38.7%. The soil profile water content curves for the 12 mm and 16 mm rainfall scenarios were nearly identical, indicating an equilibrium between the water supply and infiltration without significant redistribution. Under an initial soil water content of 17.5%, the 16 mm rainfall was rapidly immobilized by the soil, generating no runoff and exhibiting a minimal splash effect, which had a negligible impact on the soil water content below 8 cm at all three locations. For rainfall intensities of 20 mm and 24–30 mm, the water content of the soil profiles at the ridge side and furrow positions was significantly higher than that at the ridge platform (p < 0.05), with a boundary set at 30% water content. The infiltration depth at the ridge side and furrow exceeded that at the ridge platform (as shown in the comparisons between Figure 2A–C). The surface soil water content was higher at the ridge side and furrow compared to the ridge platform due to water redistribution, raindrop splash, and increasing surface runoff. Comparing the soil profile water content curves for 20 mm and 24 mm rainfall at the ridge side and furrow (Figure 2B,C), the results were largely consistent. At 30 mm rainfall, the soil profile water content at the ridge platform was lower than that at the ridge side, with surface runoff enhancing the influence on rainwater infiltration position and promoting redistribution, consistent with the observed runoff phenomena during the test. Runoff occurred at the ridge platform under 30 mm rainfall conditions, lasting approximately 3.3 min. Rainfall between 20 mm and 24 mm primarily infiltrated via splash effects, whereas rainfall exceeding 30 mm was predominantly influenced by surface runoff.

Figure 3 illustrates the comparative relationship between the infiltration and rainfall ratios for indoor versus outdoor conditions, specifically focusing on the ridge platform, ridge side, and ridge furrow. As previously discussed, during laboratory testing with rainfall amounts of 12 mm and 16 mm, the ratios of infiltration to rainfall at the three locations were 0.97:1:1 and 0.98:1.01:1.03, respectively, which are nearly equivalent to 1:1:1 (Figure 3A). With an increasing rainfall intensity, the proportion of infiltration at the ridge platform gradually decreases relative to the ridge side and furrow. For rainfall amounts of 20 mm, 24 mm, and 30 mm, the corresponding ratios of infiltration to rainfall were 0.92:1.03:1.04, 0.90:1.03:1.06, and 0.89:1.04:1.09, respectively. When rainfall exceeds 20 mm, the ratio of infiltration to rainfall at the ridge platform stabilizes near 0.9. Approximately 10% of precipitation infiltrates into the ridge side and furrow via splashing and runoff, resulting in an approximate 4% increase in infiltration at the ridge side. For rainfall exceeding 24 mm, the proportion of infiltration water at the ridge side increases progressively (Figure 3A). A comparison of the infiltration-to-rainfall ratios from the field in situ monitoring test is presented in Figure 3B. A monitored rainfall event occurred on June 1, with a total rainfall of 21 mm, a rainfall intensity of 57.3 mm/h, and an average initial soil water content of 18.6%. The infiltration-to-rainfall ratios at the ridge platform, ridge side, and furrow were 0.91:1.05:1.04, closely aligning with the results of the indoor simulation test conducted with 20 mm rainfall.

The aforementioned research findings differ from those reported by Zhang et al. in 2022 [33]. According to their experimental results, the initial infiltration rate of the ridge platform was 24.2 mm/h, while that of the ridge furrow was 21.8 mm/h, indicating a higher infiltration rate at the ridge platform compared to the ridge furrow [33]. The initial infiltration rate rapidly decreased within the first five minutes and stabilized approximately ten minutes later. Based on these findings, it can be inferred that the total infiltration volume of ridges and terraces exceeds that of furrows and ridges. Zhang et al. [33] conducted their experiments in August, during which the bulk density of the furrow soil was higher than that of the ridge platform, thereby relatively weakening its infiltration capacity. The double-loop infiltration meter utilized in the experiment measured the surface infiltration rate under conditions without rainfall splash or runoff effects. Rainfall splash and runoff are key driving factors contributing to the differences in infiltration volumes among ridge platforms, ridge sides, and furrows. The conclusions drawn from this study contradict those of Zhang et al. [33], further underscoring the necessity of this investigation. Guo et al. demonstrated through experimental research that the stable infiltration rate at the ridge platform of rotary tillage ridging is higher than that at the ridge furrow [34]. Integrating these findings with the results of the present study, it can be concluded that the influence of raindrop splashing and runoff on the spatial distribution of rainfall infiltration in ridge platforms, ridge sides, and ridge furrows in black soil regions surpasses the impact of soil infiltration rates. This study concludes that the threshold for differences in rainfall infiltration at ridge platforms, ridge sides, and furrows lies within the range of a single rainfall event measuring 16–20 mm. For precise moisture calculations or simulations, if the cumulative single rainfall reaches or exceeds approximately 20 mm, considering the rainfall at the ridge platform position to be 90% of the actual rainfall can enhance calculation or simulation accuracy.

3.2. Redistribution Characteristics of Infiltration Water on Ridge Platforms, Ridge Sides, and Ridges

Upon the cessation of rainfall, water begins to redistribute within the soil. Figure 4 illustrates the relationship between the redistribution process and the initiation time of redistribution under varying rainfall conditions at ridge platforms, ridge sides, and furrows. As the redistribution duration progresses, soil water at the ridge platform position (Figure 4(A-1–A-5)) migrates to deeper layers. The volumetric soil water content data points at 5, 10, and 15 min range from 25% to 30%. Line 1 shifts laterally upward, with some data points indicating a soil water content below 20%, which suggests that redistributed water moves downward but does not significantly infiltrate into the deeper sections (Figure 4(A-1–A-3)). In Figure 4(A-4,A-5), the lowest soil water content point increases from approximately 20% to about 25%, and the depth of the redistributed water extends down to 20 cm. With an increasing water supply, the soil water content of the shallow profile after redistribution deviates progressively from the initial values, with data points located below the 1:1 line, indicating greater divergence. The data points for rainfall redistribution at 30 min and 50 min at different ridge platform locations are closely aligned, suggesting that rainfall below 30 mm at ridge platforms can reach a stable state within an additional 50 min, and redistribution may be considered nearly complete. Over time, the redistribution pattern for rainfall below 24 mm aligns with that observed at ridge platforms, achieving stability within 50 min. Conversely, when rainfall reaches 30 mm, neither the ridge side nor the furrow positions attain a stable state within an additional 50 min, as the redistribution process remains significant (Figure 4(B-5,C-5)). A comparison of the redistribution process and initiation time under 30 mm rainfall conditions across the ridge, ridge side, and furrow positions is shown in Figure 4(A-5,B-5,C-5). The soil water content data points for the ridge, ridge side, and furrow positions sequentially cluster in regions of higher water content, indicating that cumulative infiltration volumes are lower at ridge positions compared to ridge sides and furrows.

The findings in this study regarding the redistribution time of infiltration water differ from those reported by Chen et al. [39]. According to Chen’s experimental results, the redistribution of infiltration water tends to stabilize after approximately 3 days. This is primarily attributed to the initial soil moisture content of 4% in Chen’s experiment, a cumulative rainfall of 60 mm, and the soil’s strong water absorption and infiltration capacity, which resulted in a substantial volume of infiltration water and an extended redistribution period [39]. These research findings deviate significantly from the actual conditions observed in the study area of the present investigation. Conversely, the conclusions drawn by Lv et al. [40] align closely with those in this study, as both indicate that the infiltration volume in furrows exceeds that in ridges. However, the redistribution time in Lv et al.’s study was longer than that observed in this study due to a higher water supply [40].

The field-monitored water redistribution process for natural rainfall is illustrated in Figure 5. In this figure, the ridge position has been adjusted from a bulk density of 1.2 g/cm³ to 1.1 g/cm³, aligning it with the ridge platform and ridge side. Due to the poor heterogeneity between the ridge side and ridge position, significant variations in the soil water content and substantial fluctuations in the water redistribution curve across layers are observed. When the vertical depth of the ridge platform, ridge side, and ridge furrow exceeds 7 cm and the soil moisture content surpasses 30%, the order of soil moisture content is: ridge platform < ridge side < ridge furrow. Below 7 cm, the soil moisture content is largely consistent, indicating that infiltration water on the ridge platform is lower than that on the ridge side and the ridge. In the conventional ridge cultivation mode for soybean farmland, raindrop splashing and surface runoff result in approximately 10% less rainfall infiltration water on the ridge platform compared to the ridge side and ridge positions before bed closure. The top-down redistribution process in the soil layer at the same location demonstrates that the soil water content evolves over time, with the upper soil water content decreasing due to downward movement and the middle and lower soil water content gradually increasing, which is consistent with the findings of the laboratory simulation tests.

The redistribution process of infiltration water observed in this study is consistent with the findings of prior studies [38,39,40]. However, a key distinction lies in the presence of a plough layer in the soil profile examined here, which acts as a barrier to water infiltration. This obstruction accelerates the transition of the redistribution process into a stable phase. Additionally, variations in the spatial redistribution of water were noted across ridge platforms, ridge sides, and furrows, aligning with the conclusions drawn by Parkin et al. [41]. These differences can be attributed to micro-topographic variations, which induce spatial heterogeneity in rainwater infiltration and redistribution through mechanisms such as raindrop splashing and runoff.

4. Conclusions

In the traditional ridge tillage system in black soil regions, for rainfall amounts below 16 mm the infiltration volumes at the ridge top, ridge side, and furrow positions were nearly identical to the rainfall volume, resulting in an infiltration ratio of approximately 1:1:1 among these three locations. No significant redistribution of precipitation infiltration occurred under these conditions. For rainfall amounts of 20 mm, 24 mm, and 30 mm, the ratios of infiltration to rainfall at the ridge top, ridge side, and furrow positions were 0.92:1.03:1.04, 0.90:1.03:1.06, and 0.89:1.04:1.09, respectively. When rainfall exceeded 20 mm, the infiltration-to-rainfall ratio at the ridge top approached 0.9, with approximately 10% of the precipitation infiltrating into the ridge side and furrow via splash and runoff. When performing precise calculations or simulations of water and nutrient migration, if the cumulative rainfall in a single event exceeds 20 mm, the rainfall at the ridge position should be adjusted to 90% of the actual rainfall, thereby enhancing the accuracy of the calculation or simulation.

Following the cessation of rainfall, the soil water began to redistribute. The vertical infiltration process at a given location demonstrated that the soil moisture content decreased over time as the water moved downward. Consequently, the upper soil layer experienced a reduction in the moisture content due to the downward movement, while the middle and lower layers gradually accumulated more water. When rainfall was less than 24 mm, the redistribution process at the ridge top, ridge side, and furrow positions stabilized within approximately 50 min. For rainfall amounts between 24 mm and 30 mm, seepage water initially accumulated at the ridge top before redistributing over a 50 min period to reach a stable state. However, the ridge side and furrow positions required additional time to achieve stability. Additionally, based on the results of this study and considering the rainfall characteristics in the black soil region of Northeast China, the application depth of base fertilizer for soybean farmland can be optimized to improve fertilizer utilization efficiency.