Introduction

The application of chemical fertilizers raised maize and wheat yields in China by 58–64%1. However, recent evidence indicates an excessive use of chemical fertilizers (the sum of the NPK nutrients) in China (341 kg per hectare), which is higher than the international environmental safe use limit—that is, 225 kg per hectare2.Correspondingly, it causes non-point source water pollution, and releases nitrous oxide, a greenhouse gas that contributes to climate change3,4.Long-term application of phosphate fertilizers has led to an annual increase of 0.5–1.2% in soil cadmium (Cd) and arsenic (As) concentrations, accounting for over 60% of heavy metal inputs in agricultural soils5. The exceedance rate of Cd concentration in wheat grains (> 0.2 mg/kg) had risen from 5–35%.6. From the aspects of soil nutrient balance and sustainable utilization, we can clearly see that chemical fertilizers have a short-term effect on improving the fertility of cultivated land7. In field trials across the Yangtze River Basin, balanced NPK fertilization increased rice and rapeseed yields by 22.5% and 76%, respectively.8,9. However, long-term unreasonable use had some negative impacts on land health and agriculture. The agricultural products produced from soil pollution have been put on people’s tables. The problems were placed in front of people such as melons are not sweet, vegetables are not fragrant, and fruits are tasteless10,11.

Thus, Excessive application of chemical fertilizers was an important factor in soil pollution, which might have a negative effect on soil health and soil-related ecosystem services12,13,14,15,16,17. Imbalanced use of chemical fertilizers can alter soil pH, and increase pests attack, acidification, and soil crust, which results in a decrease in soil fertility, stunting plant growth and yield18,19. In Weibei dry-land and Guanzhong area, farmers collected surface soil of terrace section and applied it to farmland to increase soil fertility and grain yield. The surface soil was fully exposed to the direct action of natural factors such as sunlight, wind and rain, and nutrients will be enriched in the surface layer, which was a good source of fertilizer. Because, the appropriate temperature and humidity conditions can promote the activities of microorganisms, accelerate the decomposition and transformation of organic matter, eliminate ferrous ions and other toxic substances, and make the soil difficult to dissolve nitrogen, phosphorus, potassium and other substances into soluble substances, thus promoting soil ripening. Sunlight can also indirectly affect the ripening process of soil by affecting temperature and humidity.

This study aims at the current problems of soil fatigue in cultivated land, excessive application of chemical fertilizers, and too single products to improve soil fertility. By analyzing the change process of surface soil quality over time along the terrace section, the mechanism of nutrient enrichment and self-repair of surface soil was revealed, which can be further applied in land regulation projects on a large scale to reduce the use of fertilizer in cultivated land. The terrace topsoil contains nitrogen, phosphorus, potassium and rich trace elements required for crop growth. It can improve the soil structure and nutrient circulation year by year, effectively reduce the pollution of chemical fertilizers and damage to land fertility, thus reducing the risk of pollution to land and surrounding water. It helps preserve biodiversity, enables soils to better store carbon, and enables agricultural activities to better coexist with natural ecosystems. This provides means for pollution control, soil health, and carbon reduction in sustainable land management practices.

Materials and methods

Overview of the study area

Weibei dry-land is in the north of Guanzhong Plain of Shaanxi Province and the south of the hilly and gully region of northern Shaanxi Province. It borders Shanxi Province and Gansu Province respectively. Including the south of Yan 'an, Weinan, Xianyang. Its land area is 3.79 million hm2. Weibei dry-land is a temperate continental climate with an average annual temperature of 8–12 °C, annual sunshine duration of 2200–2500 h, annual rainfall of 300–550 mm and annual evaporation of 1000–2000 mm. Guanzhong Plain is from Baoji in the west to Tongguan in the east, covers an area of about 3.6 million hm2. Guanzhong Plain is a temperate monsoon climate with an average annual temperature of 12–13.6 °C, annual sunshine duration of 2000–2400 h, annual rainfall of 400–900 mm, and annual evaporation of 1000-1600 mm.

The main soil types in the Weibei dry-land include loessal soil and black loessial soil, while the Guanzhong Plain is predominantly characterized by Lou soil and fluvo-aquic soil. Both regions belong to the loessal soil category. Loess is primarily composed of silt particles, which possess a large specific surface area capable of adsorbing nutrients such as potassium, calcium, and magnesium. The mineral composition of loess mainly consists of primary minerals like quartz, feldspar, and mica, as well as secondary minerals such as montmorillonite, illite, and kaolinite. These minerals gradually decompose under weathering, releasing nutrients including potassium, phosphorus, calcium, and magnesium. Additionally, loess contains carbonate substances, which facilitate the dissolution and release of nutrients. Loess is loose and porous, exhibiting good water permeability and aeration, thereby promoting microbial activity in the soil. It is primarily formed through wind accumulation, and the constant friction among loess particles caused by wind accelerates mineral weathering and nutrient release. Consequently, the inherent properties of loess determine that, under prolonged natural processes, nutrients can become enriched in the surface layer.

Sample collection

The accumulation of terrace topsoil nutrients was affected by natural factors such as time, sunlight and moisture. The age can represent the time factor, the direction can represent the light factor, and the depth and height can represent the temperature and moisture factor, so these variables are chosen. The height of the terrace profile was generally around 1 m, and it was significantly influenced by human activities. It has intensified the influence of natural factors. According to previous studies, Nutrients accumulate within the top 5 cm of the surface layer20. For the above reasons, the sampling points were selected in the study area to surface soil of terrace section (about 2 years, about 6 years, and more than 10 years), at different soil depths (1 cm, 3 cm, 5 cm) and different slope directions (shady slope, sunny slope), at different heights (0.2 m, 0.8 m, 1.0 m from the ground). All collected soils were mixed samples, which were fully mixed and packed into sample bags from three sampling points, and a total of 354 samples were collected (Fig. 1).

Fig. 1
figure 1

Geographic location of sample distribution (The Map created using ArcGIS 10.8, ESRI).

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Measurement indicators and measurement methods

The collected samples were analyzed for indicators such as available potassium, available phosphorus, activated organic carbon, total nitrogen, nitrate nitrogen, ammonia nitrogen, and sulfate ions. Available phosphorus was determined by the 0.5 M NaHCO3 extraction colorimetric method. Available potassium was determined by the 1 M NH4OAc extraction flame photometric method. Soil total nitrogen was digested with K2Cr2O7-H2SO4 and determined by the Kjeldahl method. The active organic carbon was extracted with 333 M KMnO4 and determined by TOC/TN analyzer. Soil nitrate nitrogen and ammonia nitrogen were measured using a flow analyzer. Sulfate ions were measured using the EDTA titration method21.

Data analysis

The study utilized one-way analysis of variance (ANOVA) to assess the independent influences of elevation, soil depth, temporal variation, and slope orientation on terrace nutrient enrichment. Furthermore, multifactorial ANOVA was implemented to evaluate the synergistic impacts of dual-factor and triple-factor combinations on nutrient aggregation dynamics. The data were analyzed using the Statistical Package for the Social Sciences (SPSS, version 22).

Results and analysis

Nutrient distribution of terrace topsoil in Weibei dry-land

Nutrient distribution of terrace topsoil at different years on different orientations

Available potassium, available phosphorus, total nitrogen, and active organic carbon all show an increasing trend with the increase in the year of soil formation (Fig. 2). When terrace section was sunny, the available potassium, available phosphorus, total nitrogen, and active organic carbon in the 10-year increased by 0.85%, 2.65%, 30.56%, and 421.4% relative to that of 2-year, respectively. When it was a in shadow, available potassium, available phosphorus, total nitrogen, and active organic carbon of 10-year terrace section increased by 22.90%, 43.66%, 103.33% and 109.30% respectively compared with the 2-year. We showed that the ratios of available potassium, available phosphorus, total nitrogen and active organic carbon in the 10-year terrace section were significantly different from those in the 2-year (P < 0.05).

Fig. 2
figure 2

Nutrient distribution of terrace section at different years in Weibei dry-land.

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Nutrient distribution of terrace topsoil at different depth on different orientations

When terrace section was sunny, the available potassium, total nitrogen and ammonium nitrogen decreased with the increase of soil depth. Among them, the values of available potassium, total nitrogen, and ammonium nitrogen at 1 cm from the surface soil increased by 13.41%, 13.89%, and 77.32% respectively compared with the values at 5 cm from the surface soil (Fig. 3). The differences in available potassium, total nitrogen, available phosphorus, nitrate nitrogen and sulfate ions at 1 cm from the surface soil are not significant compared with those at 5 cm. the ammonium nitrogen values at 1 cm from the surface soil were significantly higher than those at 5 cm.

Fig. 3
figure 3

Nutrient distribution of terrace section at different depths in Weibei dry-land.

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When terrace section was an in shadow, available potassium and ammonium nitrogen decreased with increasing soil depth. The available potassium and total nitrogen values at 1 cm from the surface soil increased by 29.11% and 2.63% respectively compared with the values at 5 cm. It is reduced by 32.94% compared with the value 5 cm away from the surface soil. We found that the total nitrogen, nitrate nitrogen, available phosphorus and sulfate ions at 1 cm from the surface soil were not significantly different from those at 5 cm. The nitrogen value was significantly different from the value 5 cm away from the surface soil (Table 1).

Table1 Nutrient differences of terrace section at different depths in Weibei dry-land.
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Effects of multiple factors on nutrient accumulation of terraced topsoil

Age was the most influential factor affecting soil nutrients, demonstrating significant effects on all measured indicators. Total nitrogen showed the most pronounced response to age (P < 0.001), indicating that nitrogen accumulation or depletion in the soil became more evident with increasing cultivation duration. Additionally, available phosphorus and sulfate ion content also exhibited strong age-dependent variations, due to long-term fertilization or soil mineralization processes. Slope aspect significantly influenced only ammonium nitrogen, with no notable effected on other nutrient indicators (P > 0.05). This suggests that slope aspect primarily regulated soil nutrients through its impact on ammonia nitrogen, which can be attributed to differences in hydrothermal conditions between sunny and shady slopes. Variations in evaporation rates and microbial activity between aspects likely affected ammonium transformation and accumulation. Soil depth significantly affected available potassium and ammonia nitrogen. It was more concentrated in surface soil, consistent with plant root distribution and potassium enrichment near the topsoil. In contrast, ammonia nitrogen exhibited notable variations in deeper soil layers, reflecting changes in nitrification or leaching intensity with depth (Table 2).

Table2 Variance analysis of influences factors on soil nutrients in Weibei dry-land.
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Among two-factor interactions, only the combination of age and slope aspect had a significant effect on ammonia nitrogen. This indicated that the regulatory role of age on ammonia nitrogen differs under varying slope aspects. The higher temperatures on sunny slopes may accelerated organic matter decomposition, increasing ammonia nitrogen release, whereas higher humidity on shady slopes could promote nitrification, reducing ammonia nitrogen accumulation. For multifactorial interactions, the three-way combination of age, slope aspect, and depth significantly influenced ammonia nitrogen, demonstrating that its distribution is regulated by synergistic environmental factors. This complex interaction reflected spatiotemporal heterogeneity in nitrogen transformation processes, where deep-layer ammonia nitrogen accumulation was jointly controlled by slope aspect, moisture movement, and age-dependent organic matter input. In contrast, other interactions showed no significant effects on soil nutrients (P > 0.05), suggesting weaker regulatory roles in nutrient distribution.

In the Weibei Dry Plateau region, age was the dominant driver of soil nutrient dynamics, particularly for nitrogen and phosphorus. Slope aspect and depth exert localized effects by altering hydrothermal conditions and vertical nutrient distribution, significantly impacting specific nutrients. Moreover, multifactorial interactions predominantly influence ammonia nitrogen variations, highlighting nitrogen transformation and accumulation as highly sensitive to environmental factors. These findings provided critical insights for optimizing soil nutrient management in the Weibei dry land.

Nutrient distribution of terraced topsoil in Guanzhong Plain

Nutrient distribution of terrace topsoil at different depth on different orientations

When terrace section was sunny, available potassium, total nitrogen and ammonium nitrogen value decreased with the increase of soil depth. Nitrate nitrogen, available phosphorus and sulfate ion increase with the increase of soil depth. The available potassium, total nitrogen, and ammonium nitrogen at the distance of 1 cm from the surface soil increased by 61.67%, 38.89% and 139.87% respectively compared with the values at the distance of 5 cm. The values of nitrate nitrogen, available phosphorus and sulfate ions at 1 cm were reduced by 16.26%, 13.95%, and 6.77%, respectively compared with those at 5 cm (Fig. 4). We found that the available potassium, nitrate nitrogen, available phosphorus and sulfate ions at 1 cm from the surface soil were not significantly different from those at 5 cm. The value of total nitrogen and ammonium nitrogen at 1 cm from the surface soil was significantly different from that at 5 cm.

Fig. 4
figure 4

Nutrient distribution of terrace section at different depth in Guanzhong.

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When terrace section was a in shadow, available potassium and total nitrogen decrease with increasing soil depth. The values of available potassium and total nitrogen at 1 mm from surface soil were reduced by 32.48 and 31.25% respectively compared with 5 cm. We found that the available potassium, nitrate nitrogen, ammonium nitrogen, available phosphorus and sulfate ions at 1 cm from surface soil were respectively higher than that of 5 cm, but there was no significant difference in phosphorus and sulfate ions. The total nitrogen value at 1 cm from surface soil was significantly different than of 5 cm (Table 3).

Table3 Nutrient differences of terrace section at different depths in Guanzhong.
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Effects of multiple factors on nutrient accumulation of terraced topsoil

Further ANOVA revealed that environmental factors such as age, slope aspect, and depth selectively influenced the accumulation of different soil nutrient indicators in the surface soil of terraced fields, exhibiting significant spatial heterogeneity. Among the single factors, slope aspect had a significant effect on nitrate nitrogen, a phenomenon attributed to differences in light intensity caused by slope orientation. Stronger sunlight on sunny slopes may promote nitrification, while shady slopes were more conducive to nitrogen preservation. Depth significantly affected total nitrogen and ammonium nitrogen, with the most pronounced variation in total nitrogen content across depths, reflecting the decomposition and migration processes of organic matter within the soil profile. Elevation significantly influenced ammonium nitrogen and sulfate ions, demonstrating its regulatory role on specific nutrients, particularly ammonium nitrogen and sulfate ions, due to the redistribution of hydrothermal conditions caused by altitude variations.

Among the two-factor interactions, slope aspect and depth had a significant impact on ammonium nitrogen. In other combinations of factors, slope aspect, depth, and elevation collectively showed significant effects on ammonium nitrogen. The results indicated that the dynamic changes in ammonium nitrogen were most sensitive to the synergistic effects of environmental factors, which was related to its complex and variable transformation processes in the soil (Table 4).

Table4 Variance analysis of influences factors on soil nutrients in in Guanzhong.
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In the Guanzhong region, slope aspect and depth were the primary factors influencing soil nutrient variations, particularly for nitrogen (total nitrogen, nitrate nitrogen, and ammonium nitrogen). Elevation, by altering local hydrothermal conditions, significantly affected ammonium nitrogen and sulfate ions. Additionally, multi-factor interactions were mainly reflected in the changes of ammonium nitrogen, indicating that the transformation and accumulation of nitrogen were highly sensitive to environmental factors.

Discussion

The Factors affecting surface accumulation of terrace profile

Activation is the main factor of nutrient accumulation in terraced surface soil. According to the analysis of variance in this study, the influence of years on soil nutrients was the most significant compared with other factors. With the increase of the depth, the available potassium and ammonium nitrogen values decrease gradually, which indicated that the available potassium and ammonium nitrogen can easily accumulate in terraced topsoil. Ammonium nitrogen is a fast-effective nitrogen fertilizer and one of the necessary nutrients for plant growth and development21. It has been found that ammonium nitrogen was sensitive to slope direction, age, height and depth or superposition in this study. Affected by many factors, ammonium nitrogen was easy to enrich in the soil surface, which proved the feasibility of soil as a nitrogen fertilizer. It has been found that available potassium easily accumulates in the surface layer of soil in this study. This is because the organic matter in the surface soil decomposes quickly and provides more available potassium. The influence level of potassium in crops in season mainly depends on the content of soil available potassium22,23. The feasibility of it as potash fertilizer was proved.

Migration is another important factor in soil nutrient accumulation. The nutrient migration speed is fast, and its utilization degree of crops in season is high, which is conducive to crop growth, but at the same time, it is easy to lose, which is not conducive to the long-term utilization of crops24. In this study, depth had a significant impact on total nitrogen and ammonium nitrogen, because raindrop sputtering results in increased nutrient diffusion, resulting in accelerated nutrient migration along with runoff25. Raindrop sputtering improves the turbulence of runoff, promotes the integration of runoff and nutrients, and further interferes with deep soil nutrients, resulting in rapid soil nutrient loss and migration to deep soil26. Height had a significant effect on ammonium nitrogen and sulfate ions, because the vertical migration of nutrients with water infiltration was higher than that of other nutrient indexes27,28.

The Practical agricultural applications and sustainable soil management strategies

For land nutrient management, it is essential to fully consider the influence of topographic factors such as slope aspect, gradient, and position. Significant differences in light, temperature, and moisture conditions exist between sunny and shady slopes, leading to variations in soil nutrient transformation and crop uptake efficiency29. In sunny slope, where sunlight is ample and temperatures are higher, soil organic matter decomposes more rapidly. Therefore, it is advisable to moderately increase the frequency of nitrogen fertilizer application, adopting a "little and often" approach. In contrast, shady slope should prioritize a higher proportion of basal fertilizer, combined with the use of slow-release fertilizers. For regions with steeper slopes (> 15°), soil and water conservation measures should be emphasized, such as constructing terraces along contour lines or adopting strip fertilization methods.

In terms of vertical profile management, attention should be paid to the distribution and migration patterns of nitrogen within the top 1 m layer of deep soil. Soils developed from loess parent material often exhibit thick soil layers. However, investigations have revealed that excessive fertilization leads to significant nitrate accumulation in deep soil layers (60–100 cm), which not only wastes resources but also poses risks of groundwater pollution30. It is recommended to adopt deep loosening tillage to improve soil permeability and promote deeper root growth for better utilization of deep-layer nutrients. A precision fertilization system based on soil testing should be established, coupled with drip irrigation or deep placement techniques during critical crop growth stages to enhance nitrogen use efficiency. Irrigation volume should be carefully controlled to prevent water infiltration from driving nutrients into deeper layers. Leguminous crops should be strategically incorporated into rotation systems to replenish soil nitrogen pools through biological nitrogen fixation, thereby reducing reliance on chemical nitrogen fertilizers.

The potential environmental impacts of surface soil removal

Terrace nutrient management contributes to increased crop yields and reduced soil erosion, but it is crucial to recognize that topsoil is not an infinite resource. Long-term soil exploitation, removal, or improper management may lead to severe environmental consequences, including intensified soil erosion, fertility degradation, biodiversity loss, and ecosystem imbalance31. Therefore, when formulating nutrient management strategies, the sustainable use of soil resources must be comprehensively considered.

While current nutrient management measures can enhance short-term productivity, neglecting the regenerative capacity of soil may result in long-term soil depletion, threatening food security and ecological stability32. To prevent irreversible soil degradation, it is advisable to minimize tillage and adopt practices such as straw mulching and no-till seeding to reduce erosion and improve the water and nutrient-holding capacity of topsoil. The incorporation of livestock manure, green manure, or crop residues can gradually increase soil organic matter content and reduce reliance on chemical fertilizers. Additionally, a long-term soil health monitoring network and database should be established to regularly assess key indicators such as organic matter and pH. Ecological compensation or fallow rotation systems should be implemented in areas suffering from excessive soil removal or severe erosion.

Applicability of the findings

Although this study was based on soil and agricultural management practices in the Guanzhong region and the Weibei dry-land, its core findings hold certain reference value for other areas of the Loess Plateau and even similar dryland farming regions worldwide. Spanning multiple provinces in China, the Loess Plateau shares common characteristics, including loess parent material and an arid to semi-arid climate. However, variations in soil properties, climatic conditions, and cropping systems across different regions may influence the suitability of specific management measures. Therefore, adaptive adjustments should be made based on regional characteristics, considering factors such as precipitation, soil type, and local farming systems.

Conclusion

In this study, the accumulation effect of nutrients in terraced topsoil was studied. It was found that with the increase of years, under the action of natural factors such as temperature, light and water, the nutrients in terraced topsoil were gradually activated and enriched, and the enrichment effect of nutrients in terraced topsoil was the most significant in 10-year terraced topsoil. Available potassium, ammonium nitrogen and total nitrogen were easily enriched in the surface soil of terrace, which can be used as a good soil fertilizer to supplement potassium and nitrogen fertilizer. In this study, only nitrogen, phosphorus and potassium elements necessary for plant growth were studied. The rational application of medium and trace element fertilizers and bio-organic fertilizers had become an important symbol of intensive economic utilization of agriculture. In the next step, attention should be paid to the changes of trace element and microbial activity in the surface soil of terraced fields. Further, the activity state of soil fertilizer process carried out by nutrients, organisms and microorganisms in terraced surface soil was fully revealed.

Human factors can promote the activation of surface soil nutrients and accelerate the formation of natural fertilizers. In local soil management of terraced fields, farmers should implement engineering slope protection to maintain the stability of terraced slopes. Construction of water conservancy measures such as intercepting ditches and cisterns, water retention and drainage, to keep the soil moderate moisture. Adopt biological measures, plant climbing crops, implement vegetation cover, reduce erosion, improve soil aggregate structure and water retention capacity. In this way, the accumulation rate of nutrients in terraced topsoil will be accelerated, and the wide application of natural fertilizer in land regulation and soil ripening will be realized.