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Article

The Effects of Land Use and Landform Transformation on the Vertical Distribution of Soil Nitrogen in Small Catchments

by
Yunlong Yu
1,2,3,
Shanshan Wang
1 and
Junping Qiu
1,2,*
1
Academy of Data Science and Informetrics, Hangzhou Dianzi University, Hangzhou 310018, China
2
Chinese Academy of Science and Education Evaluation, Hangzhou Dianzi University, Hangzhou 310018, China
3
State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(17), 7590; https://doi.org/10.3390/su16177590
Submission received: 25 July 2024 / Revised: 25 August 2024 / Accepted: 30 August 2024 / Published: 2 September 2024
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Abstract

:
The diversity of land use and consolidation is fundamental to ensuring sustainable development. However, the impact of diverse land uses and consolidation on the well-known shallow accumulation pattern of soil nitrogen (N) remains unclear. This existence of this knowledge gap severely constrains the sustainable production of newly created farmland. Therefore, the objective of this study was to investigate the effects of land use and gully land transformation on the vertical distribution of soil N in agricultural and nature catchments. Methodologically, soil nitrate (NO3−), ammonium (NH4+) and total nitrogen (TN) were measured to a depth of 100 cm in the hillslope forestland, grassland and gully cropland areas of the treated (gully landform reshaping) and untreated (natural gully) catchments on the Chinese Loess Plateau (CLP). The results indicated that soil N in the hillslope forestland and grassland exhibited a shallow accumulation pattern, while the vertical distribution of soil N in the gully cropland areas displayed a homogeneous, random or deep accumulation pattern. In the hillslope areas, vegetable cover was the dominant factor controlling N variation in the topsoil. In contrast, in the gully areas, the interaction of landform transformation and hydrology was the primary factor influencing the distribution of soil N. In the treated catchment, soil NO3− exhibited a significant deep accumulation pattern in the newly created farmland through gully landform reshaping. In the untreated catchment, soil NH4+ showed a significant deep accumulation pattern in the undisturbed natural gully. This study provides valuable insights into how land use and gully landform transformation affect the soil N profile. This information is crucial for the sustainable development and scientific management of valley agriculture at the catchment scale.

1. Introduction

Land consolidation plays a crucial role in sustainable agricultural development by optimizing land use and enhancing productivity [1]. It contributes to the achievement of the Sustainable Development Goals (SDGs) such as ending poverty (SDG 1), ending hunger (SDG 2) and Life on Land (SDG 15) through increasing the economic viability of farming, agricultural productivity and food security, sustainable land management practices that prevent land degradation, and encouraging the preservation of ecosystems [2]. Land consolidation involves reorganizing fragmented land parcels into more efficient and larger plots, facilitating modern farming practices and the use of advanced agricultural technology [3]. Consequently, the landform and land uses are significantly changed after large-scale land consolidation, leading to substantial variations in the distribution of nutrient elements [4]. Soil nitrogen (N) is a vital element for maintaining soil fertility and enhancing ecosystem productivity [5,6]. Understanding the distribution pattern of soil N is crucial for effective field management. Over the past few decades, the vertical patterns of soil N at regional and global scales have been extensively investigated [7,8,9]. However, soil N cycling at reshaped catchments has received less attention, despite being a significant component of the terrestrial N cycling processes. Therefore, strengthening the research on soil N cycling in reshaped catchments is essential to enhance our understanding of soil N dynamics and management in the ecosystem.
Soil N concentration generally reflects the combined effects of climate, vegetation and topography, and its vertical distribution can provide insights into N inputs, outputs and cycling processes [10,11,12]. The vertical distribution of soil N is primarily controlled by four critical processes: atmospheric deposition, parent material weathering, ecosystem biological cycling and physical leaching [13]. Among these, biological cycling (i.e., plant uptake, root function and leaf litter return) is considered the most important factor regulating soil N cycling and profile distribution in terrestrial ecosystems [14,15,16]. Many studies have demonstrated that due to biological N cycling, the soil N profile generally exhibits a shallow accumulation pattern [10,17,18]. Land use is one of the most significant factors affecting biological N cycling and the vertical distribution of soil N [19]. However, the effects of land use are generally limited to the soil surface [20,21,22]. For instance, Wei et al. [23] found that soil total nitrogen (TN) in the 0–40 cm soil layer differed among various land use types, whereas it tended to be similar in the subsoil layer (40–100 cm).
Topography is also an important factor affecting the redistribution of soil N through hydrological and erosional processes [24,25,26]. For example, Kosmas et al. [27] demonstrated that soil nutrients can accumulate in gully areas due to runoff and sediment concentration. Additionally, Xue et al. [28] found that soil nitrate concentration in valley areas was significantly higher than that in top-of-mound crest and mound slope areas due to nutrient transportation. At the catchment scale, hydrology plays a crucial role in controlling the spatial and vertical distribution of soil N [26]. Hydrology often interacts with topography to influence the distribution and cycling of soil N. Different slope positions typically have distinct water regimes, leading to varying patterns of soil nutrient accumulation [29,30]. As mentioned earlier, lower slope positions (i.e., valley or gully, foot slope) generally exhibit higher soil N accumulation compared to upper slope positions (i.e., slope, summit or shoulder) in the watershed due to runoff and sediment concentration [12]. Therefore, soil N distribution at the catchment scale is generally controlled by the combined effects of land use, topography and hydrology.
Since 2000, the Chinese government has initiated the “Grain for Green Program” (GGP) on the Chinese Loess Plateau (CLP) to control hillslope erosion. After nearly 20 years of restoration, the vegetation cover on the CLP doubled, increasing from 32% in 1999 to 60% in 2013 [31]. However, the large-scale vegetation rehabilitation on the CLP has resulted in a substantial decrease in hillslope farmlands. To offset the loss of farmlands and reduce land reclamation on the slopes, the CLP launched a megaproject called “Gully Land Consolidation” (GLC) in 2011 [32,33,34,35]. The main approach of GLC includes reshaping the creek valleys by incising foot slopes, filling stream channels and ditches, constructing drainage canals, dams and reservoirs, and creating flat farmlands. This project improves the sustainable use of gully agricultural land and water resources in the Loess Plateau, which is vital for achieving the SDGs [36]. At the catchment scale, such large-scale modification of the landform can remarkably change the gully topography and, consequently, the hydrological pathways and material circulation. However, how such landform transformation affects the soil N distribution and how it differs from undisturbed catchments remains unknown.
To address the above questions, two adjacent small catchments on the CLP were selected: one catchment with hillslope revegetation and gully landform transformation and one catchment with only hillslope revegetation. The objectives of this study were to (1) characterize the vertical distribution patterns of soil NO3−, NH4+ and TN under different land uses, (2) determine whether land use significantly affects deep soil N variation at the catchment scale and (3) explore the effect of gully landform transformation on the vertical distribution and sustainable management of soil N.

2. Materials and Methods

2.1. Study Site

This study was conducted in two small catchments locating in the Gutun watershed of Ganguyi Town, Yan’an City, Shaanxi Province, China (latitude: 36°45.32′–36°50.60′ N, longitude: 109°46.16′–109°51.30′ E) (Figure 1). The mean annual temperature in the area is 10.3 °C, and the mean annual rainfall is 495 mm, of which approximately 70% falls from June to September. The dominant soil type is loessial soil, which corresponds to Clacis Cambisols in the FAO soil taxonomy [37]. The dominant tree, shrub and grass species are Robinia pseudoacacia, Sophoraviciifolia Hippophae and Artemisia sacrorum, respectively.
The areas of the treated (S1) and untreated (S2) catchments are 44.7 and 47.0 ha, respectively, with similar elevations ranging from 972.8 m to 1188.3 m a.s.l. In 2012, the GLC project was implemented in one of the two catchments (treated, S1); therefore, the land use type of the gully area in the catchment was converted to flat farmland. The untreated catchment (S2) represents a natural catchment, and the gully area in this catchment has not been reshaped by the GLC project. A vegetation rehabilitation project (Green for Grain Project) has been implemented in both catchments since 1999. The hilly slopes and ridges in the two catchments currently consist of forestland, shrubland and grassland. The two catchments present marked differences in land use types (Figure 2). Forestland is the dominant land use type in the treated catchment (67.9%), followed by grassland (15.0%), gully farmland (8.4%), shrubland (5.6%) and other land use types (3.1%) (Table 1). The untreated catchment is composed of 33.2% forestland, 34.0% shrubland, 30.2% grassland and 2.6% other land use types (Table 1).
Because of the same dominant soil type and the relatively small area in the two catchments, we assumed that the effects of atmospheric deposition and parent material weathering on the vertical distribution of soil N in the two catchments are equivalent; so, these effects were not explored and discussed in this study [38].

2.2. Sampling

Figure 3 is the flowchart regarding sampling, laboratory procedures and data analysis. Based on an 80 m × 80 m grid on Google maps in the slope area and a 30 m sampling interval in the gully area, 81 and 72 sampling sites were selected in the treated (S1) and untreated (S2) catchments, respectively, ensuring that the sampling sites were uniformly distributed and represented all land uses in each watershed. In each grid cell, one representative site where the dominant land use appeared was selected to collect soil samples. At each sampling site, the soil sampling depth was 100 cm, and soil samples at the layers of 0–20, 20–40, 40–60, 60–80 and 80–100 cm were collected using soil augers. A total of 725 soil samples were obtained from the two catchments, comprising 385 soil samples from the treated catchment and 340 soil samples from the untreated catchment. During the process of soil sampling, each soil sample was divided into three parts, which were used for measuring soil moisture, inorganic N (NO3− and NH4+) and soil TN.

2.3. Laboratory Analysis

The first part of each soil sample was stored in an aluminum box (diameter, 40 mm; depth, 23.5 mm) to determine the soil moisture content, and the sample was oven-dried at 105 °C to a constant mass [39]. The second part of the soil sample was passed through a 1 mm mesh and then stored at 4 °C to prevent microbial activity that could alter NO3− and NH4+ concentrations. We placed 5 g soil of each sample in an extraction vessel, added 50 mL of 1 M KCl solution to the vessel (at a soil/water ratio of 1:10), then shook the mixture on a mechanical shaker for 1 h to ensure thorough mixing and extraction of NO3− and NH4+, and subsequently centrifuged the mixture and filtered the supernatant through Whatman No. 42 filter paper. The clear extract was used to measure NO3− and NH4+ using the colorimetric method, and this was analyzed by using an automatic continuous flow analyzer (AutAnalyel, Bran + Luebbe GmbH, Krefeld, Germany) [40]. The third part of the soil sample was passed through a 0.25 mm mesh and was used to determine the soil TN concentration using the Kjeldahl digestion procedure in a Kjeltec auto analyzer (Kjeltec 8400, FOSS, Hillerød, Denmark) [41].

2.4. Statistical Analysis

In this study, the number of soil sample sites was limited in the shrublands because they are located in the deep hillslopes and soil sampling is very difficult to carry out in that area (Table 1). Therefore, data analysis of soil N in the shrublands was excluded. In this paper, means and standard errors of soil N are calculated, and significant differences are obtained. The Mann–Whitney (M-W) U test is a nonparametric statistical test used to compare differences between two independent groups. Unlike parametric tests (the t-test), the M-W U test does not assume that the data follow a normal distribution, making it particularly useful when dealing with non-normally distributed continuous data. Therefore, the nonparametric Mann–Whitney U test was used to compare the differences in soil N among the different land uses, soil layers and catchments because most data did not follow a normal distribution [42]. The most common method, Bonferroni correction, was applied for multiple comparisons. The alpha level of the significant difference test was 0.05; that is to say, if the p-value is less than 0.05, we reject the null hypothesis, concluding that there is a statistically significant difference between groups. The results of the M-W U test can reveal the effect of land use and landform transformation on soil N concentration to some extent. Moreover, correlation analysis was used to determine the relationships between soil N and soil moisture. The results of the correlation analysis can provide valuable information about how hydrology affects soil N changes. All calculations and statistical analyses were conducted using R 4.4.0 software (R Development Core Team, 2024).

3. Results

3.1. Vertical Distribution Patterns of Soil N under Different Land Use Types

Figure 4 illustrates the vertical distribution patterns of soil NO3−, NH4+ and TN across different land use types in the two catchments. In the forestland and grassland, soil NO3− was depleted from 0 to 60 cm, and then stabilized at greater depths (60–100 cm) (Figure 4a,d). The distribution of soil NH4+ showed a homogeneous or random pattern in the same areas of both catchments (Figure 4b,e). Additionally, soil TN exhibited a typical shallow distribution pattern in the forestland and grassland. However, the vertical distribution patterns of soil NO3−, NH4+ and TN in the gully land were significantly different from those in the forestland and grassland (Figure 4). In the gully farmland of the treated catchment, soil NO3− showed a significant accumulation pattern below 60 cm, increasing by 3.97 mg kg−1 from 60 to 100 cm—much greater than that in the natural gully of the untreated catchment (0.61 mg kg−1). In contrast, the vertical distribution of soil NH4+ in the gully of the untreated catchment revealed significant accumulation below 60 cm, with NH4+ increasing by 4.50 mg kg−1, which was much greater than the increase observed in the treated catchment’s gully land of 0.67 mg kg−1.

3.2. Soil N Variation among Different Land Use Types in the Same Soil Layer

Figure 5 illustrates the variations in soil N concentration among the different land use types in the same soil layer. Land use significantly affected soil NO3− levels in the various soil layers of the two catchments, except for the soil layer of 40–60 cm in the treated catchment. In both catchments, soil NO3− concentrations in the soil layers of 0–20 and 20–40 cm were significantly higher in the forestland compared to the grassland and gully land. However, in the 60–80 and 80–100 cm soil layers, soil NO3− concentrations were much higher in the gully land than in the forestland and grassland. Significant differences in soil NH4+ levels among the different land uses were observed in each soil layer of the two catchments, except for the 0–20 and 20–40 cm soil layers in the treated catchment. The highest soil NH4+ concentrations occurred in the natural catchment’s gully. In both catchments, topsoil TN concentrations (0–20 and 20–40 cm soil layers) in the gully land were significantly lower than those in the forestland and grassland. However, in the untreated catchment, subsoil TN concentration (80–100 cm soil layer) in the gully land was considerably higher than the concentrations in the forestland and grassland.

3.3. Soil N Variation between Treated and Untreated Catchments

Figure 6 illustrates the differences in soil N concentrations between the treated and untreated catchments within the same land use type and soil layer. In the forestland and grassland, surface soil NO3− and both surface and subsoil NH4+ concentrations were significantly higher in the treated catchment compared to the untreated catchment, while no significant difference was observed for TN in these two land uses. However, there were significant differences in soil TN in the gully land between the two catchments, but no significance was found for soil inorganic N. Although subsoil NO3− concentrations showed no significance across all land uses, they were higher in the treated catchment than in the untreated catchment, particularly in the gully land. Conversely, subsoil NH4+ concentrations in the gully land were higher in the untreated catchment compared to the treated catchment.

3.4. Vertical Distribution of Soil Moisture and Its Relationship with Soil N

Figure 7 illustrates the average soil water contents across the different land use types in the two catchments. Mean soil moisture in the forest-dominated catchment (treated, S1) was lower than that in the mixed land use catchment (untreated, S2) (Figure 7a). Additionally, soil moisture in the gully land was significantly higher than that in the forestland and grassland (Figure 7b). To investigate the impact of soil moisture on soil N across different soil layers, correlations between soil N and soil moisture were determined (Figure 8). The relationship between soil NO3− and moisture shifted from negative to positive with increasing soil depth in both catchments. This suggested that less soil NO3− migrates from the topsoil to the subsoil when soil moisture is low. Whereas when soil moisture is high, a significant amount of soil NO3− leaches down to the subsoil and accumulates in the deep layers. The same pattern was also observed in the relationship between soil TN and moisture. However, the correlation between soil NH4+ and moisture was positive across all soil layers in both catchments, with the exception of the 20–40 cm soil layer in the treated catchment.

4. Discussion

4.1. Effects of Land Use on Vertical Distribution of Soil N

Our results showed that soil NO3− and TN in the forestland and grassland located along the slope and ridge areas of the two catchments presented shallow distribution pattern, and significant differences in soil N between the forestland and grassland were limited to the soil layers spanning 0–60 cm in depth in both the treated and untreated catchments. These findings are consistent with many studies [20,23,43]. This corroborated the finding of Carter et al. (1997) [17], who found a similar vertical distribution pattern of soil TN in the 16 different agroecosystem sites, in which approximately 30, 28, 25 and 16% of the soil TN mass was distributed in the 0–10, 10–20, 20–40 and 40–60 cm soil layers, respectively. Previous studies have demonstrated that the shallow distribution of soil N is mostly controlled by biological N cycling under different land uses and land cover [44,45,46,47,48]. Jobbágy and Jackson (2001) [10] demonstrated that the shallow distributions of the most limiting soil nutrients for plants, including N, were dominated by plant cycling (i.e., litter return and root uptake and excretion). This mechanism was supported by Francaviglia et al. (2014) [49], who found crop residues and root turnover to be important factors explaining the higher soil TN concentrations and stocks in the topsoil. Many studies demonstrated that the heterogeneity of soil properties would also significantly affect biological N cycling, especially in the topsoil, which showed higher heterogeneity of soil organic matter, root mass, soil moisture, temperature and microbial activity than in the subsoil and thus showed more significant differences in topsoil N variation [20,21,50].
In this study, the area of gully land showed a different distribution pattern of soil N when compared with the forestland and grassland, which was probably attributed to the concentrated soil nutrients from the surrounding hillslopes. Numerous studies have demonstrated that soil nutrients, including soil N, concentrate in the area of gullies due to the concentration effects of runoff and sediments from the hillslopes [27,51]. Moreover, soil moisture and groundwater tables in the area of gullies are high and thus can result in intensive leaching of soil N, especially nitrate. Tan et al. [52] found that large amounts of soil N could be transported down to the deep soil layer by strong leaching processes in the gully land. In addition, we found that the content of soil N in the subsoil was positively correlated with the content of soil moisture (Figure 8), which further demonstrates that higher soil moisture can lead to greater soil N leaching. Therefore, the area of gully land showed higher subsoil N than that in the forestland and grassland due to high leaching effects (Figure 5).
The comparison of soil N between the treated and untreated catchments revealed that, with the exception of subsoil NH4+ in the gully land, the concentrations of soil inorganic N across all land uses were higher in the treated catchment than in the untreated catchment. Given the similar elevations, soil types and vegetation in the two catchments, these differences can be attributed to the distinct slope aspects between them. Similar findings were reported by Yu et al. [53]. Numerous studies have highlighted that slope aspect is an important topographical factor influencing the spatial variation of soil N [21]. Our results showed that while no significance was found for soil inorganic N in the gully land, notable differences in subsoil NO3− and NH4+ were observed between the treated and untreated catchments. Additionally, soil TN concentrations in the gully land of the untreated catchment were higher than those of the treated catchment. These findings suggest that changes in land use within gully land might alter the patterns of soil N accumulation.

4.2. Effects of Landform Transformation on Accumulation of Soil N

In this study, we found that the most significant differences in the vertical patterns of soil N between the treated and untreated catchments were located at the gully area and these differences were mostly due to gully landform transformation through the GLC project. In the treated catchment that carried out the GLC project, the gully channels had been flattened, and thus, the streamflow was blocked, which could lead to the surface runoff and sediments being concentrated in the channel areas (created flat farmland). Moreover, the organic and inorganic N fertilizers applied in the farmland would also leach down to the subsoil and accumulate, likely further inducing water pollution [54]. Sun [55] found that the GLC project could lead to a 27–45% decrease in surface runoff and a 55–73% increase in soil moisture in the reshaped creek valley. Zhao et al. [38] found that the reshaped catchment could significantly intercept rainfall runoff and increase local infiltration after the implementation of the GLC project. Sun et al. [56] also simulated the effects of GLC on runoff–sediment–nitrogen transportation and found that nitrate output in the surface runoff decreased by 31–48% after GLC. Therefore, gully landform transformation is the primary factor that leads to subsoil NO3− accumulation in the reshaped catchment.
In the catchment without conducting the GLC project, there are no blocking effects and surface runoff is intense during rainstorm events [57]. Therefore, larger amounts of soil nitrate are lost through surface runoff. However, compared with the treated catchment, the area of gully in the untreated catchment exhibited a significant accumulation of NH4+ in the subsoil, which was possibly due to a relatively higher ammonification rate and lower nitrification rate under the high soil moisture environment [58]. Rui and Wienhold [59] also found that when soil moisture levels were greater than 80% (v/v), NO3− concentration declined rapidly and NH4+ concentration increased due to the existence of anaerobic conditions. Moreover, Jackson-Blake et al. [60] found that the high soil NH4+ level under the high soil moisture environment resulted from the high net mineralization input. In this study, we also found that the content of soil NH4+ was positively correlated with soil moisture.

4.3. The Implications of Land Consolidation on Sustainable Agricultural N Management

Gully land consolidation has been implemented in many mountainous agricultural areas worldwide [61,62]. This process involves reorganizing the fragmented and scattered plots into larger, more contiguous units, leading to various environmental and agronomic changes [63]. For example, the potential for increased mechanization and intensification of agriculture could result in higher N inputs [64]. However, few studies have addressed the effect of gully land consolidation on soil N distribution in the Chinese Loess Plateau and how to manage N fertilizers through scientific strategies. This study is a pioneering attempt to explore the impacts of gully land consolidation on the vertical levels of soil N. Our findings showed that agricultural land consolidation alters the vertical distribution pattern of soil NO3−, resulting in subsoil accumulation. The deep pattern of soil NO3− increases the risk of groundwater contamination, which poses a threat to the drinking water source of local villages and towns. Similar results were found by Yu et al. [54]. Soil NH4+ and TN concentrations of surface soil were relatively low in the gully farmland.
Therefore, actions must be taken to prevent drinking water contamination from soil NO3− pollution. First, precision agriculture technologies such as GPS-guided equipment, remote sensing and site-specific soil testing can be easily applied to the uniform and larger-sized consolidated farmland. These technologies ensure that N fertilizers are applied in the right amounts, at the right time and in the right place [65,66]. For example, since corn is the main crop in the study areas, we could monitor maize growth and soil N dynamics using these technologies. Consequently, precision fertilization and optimal use of residual soil N could be implemented to increase yield, reduce nitrogen loss and minimize environmental pollution. Second, consolidated farmland facilitates the logistics of collecting, storing and applying organic amendments, making integrated nutrient management more feasible and effective. This approach can enhance soil fertility and microbial activity, improving the efficiency of N utilization [67,68]. In general, gully land consolidation, coupled with scientific farmland management, could promote more balanced and sustainable agricultural development.

5. Conclusions

Soil NO3−, NH4+ and TN concentrations at a depth of 0–60 cm were significantly higher than those at a depth of 60–100 cm in the forestland and grassland, which exhibited a shallow distribution pattern in the treated and untreated catchments due to biological cycling. However, the gully land, characterized by superior hydrological conditions, displayed different vertical distributions of soil N in the two catchments. In the treated catchment, where valley reshaping had occurred, higher NO3− levels were observed in the 60–100 soil layer, showing a pattern of subsoil accumulation. Conversely, the untreated catchment, which features a natural gully landform, showed significantly higher subsoil NH4+ concentrations compared to the surface soil, which displayed a pattern of subsoil accumulation. The GLC project changed the gully landform and, consequently, the hydrological pathways within the catchment, intensifying soil NO3− leaching and leading to subsoil NO3− accumulation. Overall, land use and landform changes significantly affect the vertical distribution of soil N. Precision agriculture technologies and integrated nutrient management can address the subsoil NO3− problem and support sustainable agricultural development in the reshaped farmland.

Author Contributions

Conceptualization, Y.Y., S.W. and J.Q.; methodology, Y.Y.; software, Y.Y.; validation, Y.Y., S.W. and J.Q.; formal analysis, Y.Y.; investigation, Y.Y.; resources, Y.Y.; data curation, Y.Y., S.W. and J.Q.; writing—original draft preparation, Y.Y.; writing—review and editing, Y.Y., S.W. and J.Q.; visualization, Y.Y.; supervision, Y.Y.; project administration, S.W. and J.Q.; funding acquisition, Y.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the open-end funds of the State Key Laboratory of Loess and Quaternary Geology (SKLLQG2026), the Research Startup Fund of Hangzhou Dianzi University (KYS265624173), and the Basic Research Expenses for Provincial Colleges and Universities—Special Project for Young Teachers’ Research and Innovation (General Project, KYS268224002).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the author Y.Y. at [email protected].

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The locations of the study area and soil sampling sites in the treated and untreated catchments. S1: the treated catchment; S2: the untreated catchment.
Figure 1. The locations of the study area and soil sampling sites in the treated and untreated catchments. S1: the treated catchment; S2: the untreated catchment.
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Figure 2. Land use types in the treated and untreated catchments. (a) The treated catchment (S1); (b) the untreated catchment (S2).
Figure 2. Land use types in the treated and untreated catchments. (a) The treated catchment (S1); (b) the untreated catchment (S2).
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Figure 3. The technology roadmap.
Figure 3. The technology roadmap.
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Figure 4. Average distribution patterns of nitrate (NO3−), ammonium (NH4+) and total nitrogen (TN) under different land use types in the soil profile of the treated and untreated catchments. S1: the treated catchment; S2: the untreated catchment.
Figure 4. Average distribution patterns of nitrate (NO3−), ammonium (NH4+) and total nitrogen (TN) under different land use types in the soil profile of the treated and untreated catchments. S1: the treated catchment; S2: the untreated catchment.
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Figure 5. The differences in soil nitrate (NO3−), ammonium (NH4+) and total nitrogen (TN) among different land uses under the same soil layer in the treated and untreated catchments. Different letters in the same soil layer indicate the significant difference in soil N concentration under different land use types (p < 0.05). S1: the treated catchment; S2: the untreated catchment.
Figure 5. The differences in soil nitrate (NO3−), ammonium (NH4+) and total nitrogen (TN) among different land uses under the same soil layer in the treated and untreated catchments. Different letters in the same soil layer indicate the significant difference in soil N concentration under different land use types (p < 0.05). S1: the treated catchment; S2: the untreated catchment.
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Figure 6. The differences in soil nitrate (NO3−), ammonium (NH4+) and total nitrogen (TN) between the treated and untreated catchments. The star (*) indicates a significant difference in soil N concentration between the two catchments (p < 0.05). S1: the treated catchment; S2: the untreated catchment.
Figure 6. The differences in soil nitrate (NO3−), ammonium (NH4+) and total nitrogen (TN) between the treated and untreated catchments. The star (*) indicates a significant difference in soil N concentration between the two catchments (p < 0.05). S1: the treated catchment; S2: the untreated catchment.
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Figure 7. Soil moisture in the soil profiles of the treated and untreated catchments. Different letters in the same soil layer indicate a significant difference in soil moisture under different land uses (p < 0.05). S1: the treated catchment; S2: the untreated catchment. a: Soil moisture of all samples in the two catchments. b: Soil moisture among different land uses.
Figure 7. Soil moisture in the soil profiles of the treated and untreated catchments. Different letters in the same soil layer indicate a significant difference in soil moisture under different land uses (p < 0.05). S1: the treated catchment; S2: the untreated catchment. a: Soil moisture of all samples in the two catchments. b: Soil moisture among different land uses.
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Figure 8. Pearson correlation analysis of soil N and soil moisture in different soil layers in the treated and untreated catchments. NN: nitrate (NO3−, mg kg−1), AN: ammonium (NH4+, mg kg−1), TN: total nitrogen (g kg−1) and SM: soil moisture (%). The Arabic numerals 1, 2, 3, 4 and 5 following the capital letters represent the 0–20, 20–40, 40–60, 60–80 and 80–100 cm soil layers, respectively. The color and direction of the symbols represent the negativity or positivity of the corresponding correlation coefficients; and the areas of the symbols mean the absolute values of the corresponding correlation coefficients (the larger the area, the smaller the value). (a) The treated catchment (S1); (b) the untreated catchment (S2).
Figure 8. Pearson correlation analysis of soil N and soil moisture in different soil layers in the treated and untreated catchments. NN: nitrate (NO3−, mg kg−1), AN: ammonium (NH4+, mg kg−1), TN: total nitrogen (g kg−1) and SM: soil moisture (%). The Arabic numerals 1, 2, 3, 4 and 5 following the capital letters represent the 0–20, 20–40, 40–60, 60–80 and 80–100 cm soil layers, respectively. The color and direction of the symbols represent the negativity or positivity of the corresponding correlation coefficients; and the areas of the symbols mean the absolute values of the corresponding correlation coefficients (the larger the area, the smaller the value). (a) The treated catchment (S1); (b) the untreated catchment (S2).
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Table 1. Land use types and number of soil sampling sites in treated and untreated catchments.
Table 1. Land use types and number of soil sampling sites in treated and untreated catchments.
CatchmentLand UseNumber of
Sample Sites		CatchmentLand UseNumber of
Sample Sites
Treated
(S1)		Forestland (67.9%)47Untreated
(S2)		Forestland (33.2%)24
Shrubland (5.6%)3 Shrubland (34.0%)4
Grassland (15.0%)13 Grassland (30.2%)32
Cropland (8.4%)17 Wasteland (1.2%)12
Others (3.1%)- Others (1.4%)-
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Yu, Y.; Wang, S.; Qiu, J. The Effects of Land Use and Landform Transformation on the Vertical Distribution of Soil Nitrogen in Small Catchments. Sustainability 2024, 16, 7590. https://doi.org/10.3390/su16177590

AMA Style

Yu Y, Wang S, Qiu J. The Effects of Land Use and Landform Transformation on the Vertical Distribution of Soil Nitrogen in Small Catchments. Sustainability. 2024; 16(17):7590. https://doi.org/10.3390/su16177590

Chicago/Turabian Style

Yu, Yunlong, Shanshan Wang, and Junping Qiu. 2024. "The Effects of Land Use and Landform Transformation on the Vertical Distribution of Soil Nitrogen in Small Catchments" Sustainability 16, no. 17: 7590. https://doi.org/10.3390/su16177590

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