Topographic Controls on Soil Nutrient Spatial Variability in a Mango Orchard of China’s Dry-Hot Valley: Effects of Slope Gradient, Position, and Aspect

Abstract

Spatial heterogeneity of soil nutrients in the dry-hot valleys of Southwest China is strongly shaped by topography, yet quantitative evidence remains limited. In this study, we assessed the effects of slope gradient, slope position, and slope aspect on nine soil nutrient indicators in a representative mango orchard in Yanbian County, Panzhihua City, China. Stratified soil samples were collected from two depths (0–10 cm and 10–20 cm) across contrasting topographic conditions. The results showed that: (1) total nitrogen (TN) and organic matter (OM) declined with increasing slope gradient, while available phosphorus (AP) accumulated in the 10–20 cm layer of gentle slopes (0°, 20°). The N:P ratio peaked at 0° slope (0–10 cm), whereas the C:N ratio peaked at 80° slope (10–20 cm). (2) Soil OM and available potassium (AK) increased with higher slope position, while total phosphorus (TP) decreased. TN and AP reached maximum values on hillslope terraces, and total potassium (TK) was highest on piedmont alluvial fans. Summit platforms exhibited the highest C:N, C:P, and N:P ratios (0–10 cm). (3) Sunny slopes had higher TN, OM, and TP, whereas shady slopes had higher TK and AK. The C:N and C:P ratios (0–10 cm) were greater on sunny slopes, while N:P was higher on shady slopes. Principal component analysis indicated that slope gradient, position, and aspect accounted for 60.6%, 68.2%, and 59.6% of the variance in soil nutrients, respectively. Overall, this study highlights the quantitative influence of topography on soil nutrient distribution, providing a scientific basis for more site specific nutrient management in mango orchards of dry-hot valley regions.

1. Introduction

Dry-hot valleys are characterized by high temperatures, low humidity, and strong anthropogenic pressure, which together accelerate soil degradation and desertification [1,2]. In particular, agricultural intensification (frequent tillage, excessive fertilizer application, and irrigation on steep slopes) amplifies topographic controls on soil nutrients [1]. These practices often enhance erosion on upper slopes, promote nutrient leaching down slopes, and lead to heterogeneous nutrient accumulation in piedmont fans, thereby reinforcing the redistribution patterns driven by slope gradient, position, and aspect [1,3]. The surface soil in the southwestern arid and hot river valley is deficient in nutrients. The total nitrogen (TN) is less than 0.10%, and the soil organic carbon (SOC) ranges from 8 to 15 g·kg⁻¹, both lower than China averages (TN, 0.15%, SOC, 20 g·kg⁻¹) [4,5]. Studies on the spatial variability of soil properties are most frequently carried out in homogeneous settings such as arid zones, forests, pastures, and broad acre cropping systems (e.g., grains and oilseeds) [6]. In contrast, there is limited research on soil variability within intensively managed, heterogeneous environments like orchards [7]. In Panzhihua, the unique light and heat resources have been harnessed for large-scale mango cultivation, creating a model of ecological-economic integration [8]. However, fragmented terrain and steep hydrological gradients intensify soil nutrient redistribution and restrict vegetation carrying capacity [3,5]. Therefore, clarifying how topography shapes soil nutrient spatial variability is essential for ecological restoration and sustainable orchard management [4,7].

Topographic factors, including slope gradient, slope position, and slope aspect, govern soil nutrient patterns through multiple interconnected biogeochemical processes [4,6]. Slope gradient regulates the balance between erosion and deposition, whereby steeper slopes enhance particulate organic matter loss and nitrogen leaching, while gentle slopes promote infiltration and nutrient accumulation [9]. Slope position establishes a downslope transfer pathway: summit soils experience continuous carbon and nitrogen depletion, whereas piedmont fans receive nutrient enriched sediments and phosphorus bound to mineral oxides [5]. Slope aspect modifies microclimate (radiation, temperature, and moisture), which in turn influences vegetation composition and litter quality [10]. On sunny slopes, rapid litter decomposition enhances nitrogen mineralization but accelerates carbon turnover, whereas shady slopes maintain higher soil moisture and microbial diversity, fostering phosphorus and potassium cycling [6,10]. These mechanisms jointly determine the spatial stoichiometry of C, N, and P in dry-hot valley soils [4,8]. Steep karst slopes retain only 30–50% of the SOC present on gentle slopes, owing to bedrock exposure and shallow soil depth [11]. Conversely, gentle slopes, with longer runoff residence times, tend to accumulate nutrients [11,12]. On the slopes of the lowland plains in the Peruvian Amazon region Slope position establishes an erosion-deposition gradient: summit soils often lose 10–40% of SOC and TN, which are subsequently deposited in piedmont alluvial fans [13]. Complex hillslope hydrology plays a crucial role in the soil of the orchard. Convergent flow zones tend to accumulate runoff and soluble nutrients, while divergent flow zones experience greater leaching and nutrient depletion [10,14,15]. These processes can be further intensified by tectonic activity, which enhances material transport [16,17]. Slope aspect regulates microclimate, with shady slopes typically retaining more soil moisture, SOC, TN, thereby accelerating nitrogen and phosphorus cycling [10,18]. In addition to current topographic nutrient interactions, future changes in precipitation regimes may further alter these relationships [16]. Intensified rainfall events are likely to enhance slope-driven erosion and nutrient leaching, whereas prolonged droughts could exacerbate nutrient stratification and limit biological cycling in summit soils [19]. Unlike the nutrient redistribution in semi-arid loess plateaus or arid Sahel regions, which are mainly controlled by rainfall intensity and land use, the steep terrain and hilly microclimate of the arid river valleys generate stronger vertical gradients of temperature and moisture, thereby amplifying the erosion-sedimentation process [20,21]. In a mango orchard on a slope in the northern peninsula of Malaysia, TN varies more vertically than horizontally in the surface layer. The lower and middle parts of the slope have a higher accumulation of OM and nutrient [22]. Another mango orchard located in the karst area of Guangxi has the ratios of C:N, C:P, and N:P as follows: slope > transitional zone between depression and slope > depression > saddle area [23]. In China’s dry-hot valleys, where water is scarce and soils are inherently vulnerable, the lack of site-specific nutrient management strategies tailored to topographic variability likely accelerates this process, threatening the sustainability of this vital economic industry [24].

Therefore, this study investigates the effects of slope gradient, slope position, and slope aspect on soil nutrient variability in mango orchards of Panzhihua’s dry-hot valleys. Specifically, we aim to: (1) characterize the topographic differentiation of soil nutrients; (2) identify terrain-related indicators of erosion risk; and (3) elucidate mechanisms underlying C:N:P stoichiometry under topographic regulation. The results would provide theoretical support for improving soil and water conservation measures and for informing more site-specific nutrient management in economic orchards of dry-hot valley regions.

2. Materials and Methods

2.1. Study Area

The study was conducted in Baiyanzi, Yanbian County, Panzhihua City, Sichuan Province (E 101°52′52″, N 26°38′33″), a typical dry-hot valley area. The mango orchard in the study area (covered with erigeron) has been cultivated for 23 years. Before that, it was a barren hill. Panzhihua’s mango industry is critically important to China’s domestic market supply, with a planting area exceeding 100,000 hectares and an annual output of approximately 400,000 tons. The terrain is dominated by deeply incised mid-mountain canyons with pronounced vertical relief, the sampled sites represent typical orchard soils developed on alluvial–colluvial deposits derived from granitic and sandstone parent materials (Figure 1). Such topography creates marked local climate variations, with sunny slopes experiencing higher radiation and evaporative demand, while shady slopes retain cooler and moister microclimates [9]. These differences, together with slope-controlled airflow and temperature inversions, are crucial drivers of soil formation and nutrient cycling. The region has a subtropical semi-arid monsoon climate, with a mean annual temperature of 19.5 °C (extremes: −1.8 °C to 41.2 °C) and annual precipitation of 760–1200 mm (2022–2024), of which ~85% falls between June and October. During the dry season, actual evaporation exceeds precipitation by a factor of 3–4, further intensified by foehn winds and winter temperature inversions [8]. These conditions shape a unique mango orchard ecosystem typical of dry-hot valleys [24,25].

2.2. Soil Sampling and Physicochemical Assessment

The soils at the study site are developed from granitic and sandstone parent materials under strong topographic control. Field observations revealed shallow profiles on steep upper slopes, consisting of a thin organic horizon (5–10 cm) overlying sandy loam subsoils According to the World Reference Base for Soil Resources, the soils are classified mainly as Cambisols on mid- and lower slopes and Regosols on steep upper slopes [26].

Given the complex terrain, slope gradient, slope position, and slope aspect were sampled independently without a factorial design. In June 2024, which corresponds to the early rainy season in the dry-hot valley, Soil sampling was conducted along three transects established across representative slope gradients in the study area (Figure 1). The transects were laid out parallel to each other with an interval of approximately 100 m. The height difference between the sampling points is 60 m. Along each transect, sampling points were located at regular intervals of 100 m. The spacing of sampling points was kept consistent across all three transects to ensure comparability: Slope gradient: 0°, 20°, 60°, and 80° (mid-slope, sunny aspect), with 0° for near-flat terrace surfaces, 20° for gentle cultivated slopes, 60° for steep orchard hillsides, and 80° for extremely steep slopes with shallow soils. Slope position: summit platform, hillslope terrace, and piedmont alluvial fan (0° slope, sunny aspect). Slope aspect: sunny (convex) vs. shady (concave) slopes (80° mid-slope). For each topographic category, three independent plots (A distance of 50 m apart) were established as true replicates. Within each plot, five soil cores were collected at random locations within a 10 × 10 m area and composited to form one representative sample per depth (0–10 cm and 10–20 cm). Thus, the three plot-level composites served as replicates, while subsamples within each plot reduced microsite variability. Samples were air-dried, sieved through a 100-mesh screen, and analyzed for the measurement of soil physicochemical properties.

TN was determined by digestion with sulfuric acid-mixed catalyst and then by (FOSS) automatic Kjeldahl nitrogen analyzer; OM was determined by potassium dichromate external calorific capacity method; TP was determined by sulfuric acid-perchloric acid digestion combined with molybdenum antimony colorimetry; TK was determined by flame photometry after melting sodium hydroxide; AK was determined by direct flame photometry after extraction with neutral ammonium acetate; AP was determined by hydrochloric acid-ammonium fluoride extraction method; NH₄⁺-N was determined by potassium chloride extraction- indophenol blue colorimetry; NO₃⁻-N was determined by phenol disulfonic acid colorimetry; pH was determined by potentiometric method, and suspension was prepared according to a water-soil ratio of 1:2.5, following the standard procedures described by Lu [27].

2.3. Data Analysis

Data was processed in Microsoft Excel 2010. All data were first checked for normality (Shapiro–Wilk test) and homogeneity of variance (Levene’s test) prior to statistical analysis. One-way ANOVA was used to evaluate the effect of individual topographic factors (slope gradient, slope position, and slope aspect) on soil nutrient concentrations and stoichiometric ratios (C:N, C:P, N:P). Two-way ANOVA was applied to test the interactive effects of topographic factors and soil depth. In all cases, post hoc comparisons were conducted using Tukey’s HSD test at a significant threshold of p < 0.05. Statistical analyses were performed in SPSS 23. To explore multivariate relationships among soil properties and topographic variables, principal component analysis (PCA) was carried out using Origin 2024. PCA was based on standardized (z-score) data to ensure comparability among variables. Replication was handled at the plot level (n = 3 per topographic category), while subsamples were composited to reduce microsite variability. All statistical procedures were reported in accordance with recommendations for transparency and reproducibility.

3. Results

3.1. Effects of Topography on Soil Physicochemical Properties

3.1.1. Effects of Slope Gradient on Soil Physicochemical Properties

In the 0–10 cm layer, TN and SOC peaked at the 0° slope, reaching 0.12% and 15.11 g kg⁻¹, respectively (Figure 2). In the 10–20 cm layer, SOC was significantly higher at the 60° and 80° slopes than at 0° and 20°. TP reached its maximum at the 20° slope, while TK and AK peaked at 60° in both layers. Within the 10–20 cm layer, NH₄⁺-N was highest at 20° (1.79 mg kg⁻¹). In contrast, in the 0–10 cm layer, NO₃⁻-N was lowest at 0°, and pH was highest at 0° (p < 0.05).

Slope gradient, soil depth, and their interaction significantly influenced TN, AK, AP, NO₃⁻-N, and pH (

Table 1. Effects of slope and soil depth on soil nutrients. Effects of slope gradient and soil depth on soil nutrients. Numbers represent F-values, * and ** indicate significance at p < 0.05 and p < 0.01, respectively.

Dependent Variable	Slope Gradient	Soil Depth	Slope Gradient × Soil Depth
TN	20.8 **	164.6 **	31.4 **
OM	2.6	20.9 **	11.1 **
TP	65.1 **	51.2 **	2.1
TK	8.7 **	0.2	0.9
AK	111.8 **	179.3 **	16.0 **
AP	35.0 **	216.5 **	35.0 **
NH4+-N	3.9 *	0.0	2.2
NO3−-N	10.8 **	183.4 **	7.1 **
pH	16.4 **	18.9 **	3.9 *

). OM was significantly affected by soil depth and its interaction with slope gradient (both p < 0.01). TP was influenced by both slope gradient and soil depth (p < 0.01), whereas TK and NH₄⁺-N were significantly affected only by slope gradient (p < 0.01 and p < 0.05, respectively).

3.1.2. Effects of Slope Position on Soil Physicochemical Properties

In the 0–10 cm layer, TN, SOC, and TP reached maxima at the hillslope terrace, summit platform, and piedmont alluvial fan, with mean values of 1.25 g kg⁻¹, 18.6 g kg⁻¹, and 0.77 g kg⁻¹, respectively (p < 0.05; Figure 3). No significant differences in these variables were observed among slope positions in the 10–20 cm layer (p > 0.05). In the 0–10 cm layer, TK was significantly higher in the piedmont alluvial fan compared with the hillslope terrace (p < 0.05). The summit platform had the highest AK (161.33 mg kg⁻¹), while the hillslope terrace recorded the highest AP (8.54 mg kg⁻¹) and NO₃⁻-N (65.62 mg kg⁻¹). NH₄⁺-N and soil pH showed no significant differences among slope positions in this layer (p > 0.05). In the 10–20 cm layer, AK was lowest in the piedmont alluvial fan, while NH₄⁺-N was highest at the summit platform (1.98 mg kg⁻¹). Soil pH at the hillslope terrace was significantly higher than at the summit platform and piedmont alluvial fan (p < 0.05).

TN, AP, and AK were significantly affected by slope position, soil depth, and their interaction (p < 0.05;

Table 2. Effects of slope position and soil depth on soil nutrients. Numbers represent F-values, * and ** indicate significance at p < 0.05 and p < 0.01, respectively.

Dependent Variable	Slope Position	Soil Depth	Slope Position × Soil Depth
TN	32.9 **	117.6 **	21.1 **
OM	31.7 **	115.2 **	1.1
TP	272.2 **	75.7 **	0.3
TK	4.8 *	0.0	1.0
AK	127.2 **	110.7 **	4.9 *
AP	93.0 **	60.6 **	104.3 **
NH4+-N	1.9	1.6	2.2
NO3−-N	56.9 **	184.7 **	10.7 **
pH	2.2	0.0	7.4 **

). SOC and TP were significantly influenced by slope position and depth (p < 0.01), while TK was affected only by slope position (p < 0.05).

3.1.3. Effects of Slope Aspect on Soil Physicochemical Properties

At 0–10 cm depth, TN showed no significant difference between sunny and shady slopes (p > 0.05). However, at 10–20 cm, TN and NH₄⁺-N were significantly higher on sunny slopes compared with shady slopes (p < 0.05; Figure 4). SOC in both layers was significantly higher on sunny slopes (p < 0.05). TP in the 0–10 cm layer was significantly higher on sunny slopes than in the 10–20 cm layer (p < 0.05). By contrast, AP was higher on shady slopes in the 0–10 cm layer, with the opposite trend in the 10–20 cm layer (p < 0.05). TK and AK were consistently higher on shady slopes in both soil layers (p < 0.05). NO₃⁻-N concentrations were significantly higher in the 0–10 cm layer than in the 10–20 cm layer for both aspects (p < 0.05). Soil pH did not differ significantly between aspects or depths (p > 0.05).

TN, SOC, TK, AK, and NO₃⁻-N were significantly influenced by both slope aspect and soil depth (p < 0.01;

Table 3. Effects of slope aspect and soil depth on soil nutrients. Numbers represent F-values, * and ** indicate significance at p < 0.05 and p < 0.01, respectively.

Dependent Variable	Slope Aspect	Soil Depth	Slope Aspect × Soil Depth
TN	9.2 **	25.6 **	0.4
OM	4.9 **	4.2 **	0.0
TP	6.5 **	2.9	0.2
TK	12.1 **	0.1 **	0.3
AK	7.1 *	13.8 **	1.3
AP	4.6 *	8.4 **	35.3 **
NH4+-N	10.8 **	2.4	3.7
NO3−-N	10.3 **	40.3 **	0.4
pH	0.1	0.8	1.2

). TP and NH₄⁺-N were significantly affected only by slope aspect (p < 0.01). AP was significantly influenced by aspect, depth, and their interaction (p < 0.05).

3.2. Effect of Topography on Soil Stoichiometry

The soil C:N ratio in the 0–10 cm layer was significantly higher at the summit platform than at other slope positions (p < 0.05). Within the piedmont alluvial fan, the 0–10 cm layer had a significantly higher C:N ratio than the 10–20 cm layer. The maximum C:P ratio (25.3) occurred in the 0–10 cm layer of the summit platform. The N:P ratio in the 0–10 cm layer was significantly higher than in the 10–20 cm layer at both the hillslope terrace and piedmont alluvial fan (p < 0.05). In contrast, the summit platform exhibited the highest N:P ratio (2.31) in the 10–20 cm layer.

For slope gradients, the 10–20 cm layer at 80° had the highest C:N ratio compared with all other gradients. Vertical C:N patterns varied with gradient: at gentle slopes (0° and 20°), the 0–10 cm layer had higher ratios than the 10–20 cm layer, whereas at steep slopes (60° and 80°) the reverse pattern was observed (p < 0.05). The C:P ratio was highest (13.0) in the 10–20 cm layer at 80°. For 0°, 20°, and 60°, the C:P ratio was significantly higher in the 0–10 cm layer than in the 10–20 cm layer, while at 80° the opposite trend occurred (p < 0.05). The maximum N:P ratio in the 0–10 cm layer (1.93) was recorded at 0°, whereas in the 10–20 cm layer the highest ratio (1.16) occurred at 80°. Regarding slope aspect, the C:N and C:P ratios in the 0–10 cm layer were significantly higher on sunny slopes than on shady slopes (p < 0.05). No significant aspect-related differences in N:P ratio were detected at either depth (p > 0.05; Figure 5).

3.3. Principal Component Analysis (PCA) of Topographic Factors and Soil Nutrients

PCA quantified the coupling between topographic variables, soil depth, and nutrient distribution. For slope gradient, the first two principal components (PC1 + PC2) explained 60.6% of total variance (Figure 6a). NH₄⁺-N and pH showed strong covariation and were enriched in the 10–20 cm layer, suggesting alkalinity-driven ammonium retention. For slope position, PC1 and PC2 together explained 68.2% of variance (Figure 6b). Piedmont alluvial fans exhibited enrichment of TP, TK, and pH in the 10–20 cm layer, indicating leaching and illuviation processes, whereas summit platforms had higher AK and NH₄⁺-N in the 0–10 cm layer, reflecting stronger biological cycling. For slope aspect, PCA explained 59.6% of nutrient variability (Figure 6c).

4. Discussion

4.1. Topographic Dynamic of Soil Nutrient Distribution

Soil nutrient variability in mango orchards of dry-hot valleys is strongly regulated by topographic features and soil depth. Summit and upper slopes function as donor sites, characterized by rapid runoff and nutrient export; mid-slopes represent normal transit sites, where partial retention and redistribution occur; and footslopes or valley bottoms act as recipient sites, accumulating sediments and nutrient-rich runoff. Compared with flat terrain (0°), steeper slopes (20–80°) exhibited significantly lower TN, likely due to erosion-driven losses of soil fertility under higher gradients [28]. Surface SOC (0–10 cm) displayed a spatial pattern consistent with TN, reflecting localized management practices and disturbance [28,29,30]. TP content showed an initial increase and subsequent decline with slope steepness. This may reflect phosphorus redistribution: on slightly gentle slopes (20°), runoff from flat terrain accumulates and infiltrates, elevating TP, whereas at higher gradients, reduced infiltration and water retention promote TP depletion [23,31]. NO₃⁻-N concentrations were lowest at 0°, possibly due to erosion-induced redistribution downslope, as indicated by the significant slope–position interaction from the two-way ANOVA. Across all topographic settings, nutrient concentrations were generally higher in surface soils than in deeper layers, this phenomenon occurred due to vegetation or human intervention, consistent with previous findings [1,23].

Slope position also exerted a strong influence on nutrient distribution. In this study, SOC and TN increased with elevation, likely because higher positions promote greater photosynthetic activity, enhancing organic carbon inputs. Conversely, piedmont alluvial fans had lower SOC and AK, as surface runoff and concentrated flushing remove soluble nutrients [32]. Higher NO₃⁻-N concentrations at hillslope terraces are consistent with Yang et al. [33], reflecting accumulation from surface runoff and lateral subsurface flow. Denitrification in poorly aerated microsites can lead to gaseous N losses, especially in concave or convergent flow areas [33]. At the same time, active plant uptake during the mango growing season reduces inorganic nitrogen availability in surface soils, while leaching transports nitrate to deeper horizons or downslope positions [33,34]. TP declined with elevation due to intense erosion at summits, while enrichment at piedmont fans may result from stable phosphorus complexes bound to Fe and Al oxides that resist leaching [35]. The increase in AP at 10–20 cm in piedmont fans may reflect downward redistribution through infiltration following surface evaporation [36,37]. Slope aspect further modified nutrient dynamics. Sunny slopes contained higher TN, SOC, TP, NH₄⁺-N, and NO₃-N. This finding differs from the commonly reported pattern that shady slopes, due to greater soil moisture retention, typically accumulate more organic matter and nitrogen [10,18]. The apparent contrast may be attributed to site-specific factors: (i) enhanced mango growth and management intensity on sunny slopes, which increased litter inputs and fertilization efficiency; (ii) stronger microbial mineralization under higher temperatures that temporarily elevated available nutrient pools and shallow soil depth on shady slopes, which may have constrained nutrient storage despite higher moisture [38,39]. Correlation analysis confirmed a strong positive relationship between SOC and TN, reflecting nitrogen immobilization during humification and consistent with precipitation-driven SOC sensitivity [6,40]. TN was also positively correlated with NO₃⁻-N, TP, and NH₄⁺-N, indicating coupled nutrient cycling. By contrast, TK and AK were higher on shady slopes, likely due to deeper soils with more favorable conditions for litter decomposition and potassium release [40,41].

Across all topographic factors, SOC, TN, and TP declined with soil depth, with SOC showing the steepest decline, consistent with Zhang et al. [41]. This vertical stratification reflects nutrient return from litter and root inputs concentrated in surface horizons, where ~90% of root biomass is located [42]. TP displayed lower vertical variability than SOC and TN due to its mineral origin, low mobility, and relatively uniform distribution [43].

4.2. Topographic Effects on Soil Stoichiometry

Topography not only controls nutrient concentrations but also shapes soil stoichiometry. Steeper slopes enhance erosion and nutrient losses, thereby modifying C:N:P ratios [44,45]. In this orchard, C:N ratios were lower on gentle slopes but higher on steep slopes (Figure 4). This pattern may reflect microbial dynamics: high microbial activity at gentle slopes accelerates SOC decomposition and nitrogen mineralization, reducing C:N, whereas reduced activity on steep slopes slows decomposition, allowing relatively greater carbon accumulation and higher C:N [44]. C:P and N:P ratios showed a trend of decreasing and then increasing with slope, being higher in surface soils at gentle slopes and in subsurface soils at steep slopes. This may be attributed to greater phosphorus mobilization and loss from surface soils on steep gradients [45].

Among slope positions, the summit platform exhibited the highest C:N, C:P, and N:P ratios, indicating strong carbon accumulation relative to nitrogen and phosphorus supply. Similarly to Feng et al. [46], this suggests limited downward transport of phosphorus, leading to elevated C:P and N:P. Notably, all stoichiometric ratios observed here were below the national averages for Chinese soils: C:N (12.3), C:P (52.7), and N:P (3.9) [47]. This suggests that the C and N content of this sample site is lower than that of other orchards in China, warranting increased application of carbon- and nitrogen-rich fertilizers.

Slope aspect is considered to be a key topographic factor that changes the content and stoichiometry of soil C, N and P [48]. Sunny slopes exhibited higher C:N and C:P but lower N:P compared with shady slopes. This pattern is consistent with the effects of greater radiation and temperature on sunny slopes, which enhance plant photosynthesis and biomass input, leading to higher SOC and associated stoichiometric ratios [30,49]. Shady slopes, with cooler and moister microclimates, favor decomposition of labile carbon fractions (e.g., sugars, proteins) by mesophilic microbes while limiting lignin accumulation [50]. These conditions, combined with greater nitrogen deposition and retention, promote higher N:P ratios relative to sunny slopes [51].

Stoichiometric ratios also vary with depth. In surface soils, higher organic matter and root-derived inputs typically elevate C:N and C:P ratios, while greater microbial turnover enhances N:P ratios [51]. In deeper layers, reduced organic inputs, slower decomposition, and stronger mineral associations with phosphorus tend to lower C:N and C:P ratios, while N:P ratios may increase due to the relative stability of inorganic P compared with labile N [47,51].

4.3. Implications for Mango Production in China’s Dry-Hot Valley

The strong link between topography and soil nutrients supports more site-specific nutrient management. A uniform fertilization approach is inefficient and worsens imbalances [47]. Instead, strategies should adapt to specific slopes: Steep slopes, with depleted nitrogen and organic carbon, need nitrogen-rich organic amendments to reduce erosion loss and rebuild soil [9]. Gentle slopes, where phosphorus accumulates underground, may still benefit from targeted phosphate supplementation to maintain surface fertility [23]. Sunny slopes, with lower potassium levels, likely require potassium-rich amendments to sustain yield, unlike potassium-rich shady slopes [37].

While conducted in a specific orchard in Panzhihua, the implications of this study extend to other mango-growing regions within China’s dry-hot valleys [24,25]. The topographic processes governing nutrient redistribution erosion on steep slopes, deposition on gentle slopes, and microclimatic variation between aspects are fundamental and universal [13]. The Honghe, Jinshajiang, and Yuanjiang dry-hot valleys share similar geomorphic, climatic, and edaphic challenges [4]. Therefore, the mechanistic understanding derived from this study is widely applicable. The specific fertilizer application rates may need local calibration, but the principle of tailoring management to slope gradient, position, and aspect is a transferable strategy for all sloping orchards in these regions, offering a model for enhancing precision agriculture and soil conservation efforts across southwestern China.

5. Conclusions

This study demonstrates that topography exerts a strong influence on soil nutrient distribution in mango orchards of the dry-hot valleys of Panzhihua. The observed decline in TN and SOC with increasing slope steepness can be attributed to enhanced erosion and surface runoff, which preferentially remove organic matter and associated nitrogen from upper slope positions. On gentle slopes and flat terrain, longer water residence times promote organic matter accumulation and nitrogen retention, whereas steeper slopes experience accelerated loss of both particulate and dissolved organic fractions. The concurrent increase in C:N ratios suggests that carbon inputs from litter and root residues are relatively less affected than nitrogen losses, leading to a relative enrichment of carbon compared with nitrogen. Phosphorus and potassium exhibited non-linear responses, first increasing at gentle gradients and then decreasing with steeper slopes. Slope position also played an important role: SOC and AK increased with elevation, TN and AP were enriched on hillslope terraces, and TP and TK accumulated in piedmont alluvial fans due to sediment deposition. Summit platforms displayed the highest surface stoichiometric ratios of C:N, C:P, and N:P, highlighting their relative imbalance between carbon accumulation and limited nitrogen and phosphorus supply. Slope aspect produced further contrasts, as sunny slopes contained higher TN, SOC, and TP but lower TK and AK compared with shady slopes, with surface soils on sunny aspects also showing higher C:N and C:P ratios.

These findings provide a basis for developing more site-specific nutrient management strategies tailored to heterogeneous topography in dry-hot valley orchards. Nutrient limitations on steep slopes call for the addition of nitrogen-rich organic amendments, while the phosphorus deficit on gentle slopes can be alleviated through targeted phosphate supplementation. Piedmont alluvial fans, already enriched by sedimentation, require reduced fertilizer inputs to avoid nutrient excess, whereas shady slopes would benefit from potassium-rich amendments to improve stoichiometric balance. In addition, establishing cover crops, particularly on upper slopes, can mitigate erosion-driven nutrient translocation and promote more even distribution across the landscape. Collectively, these measures offer practical guidance for balancing productivity with ecological sustainability in dry-hot valley fruit orchards.