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

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.