Effects of slope position, aspect and cropping system on soil nutrient variability in hilly areas

Human activities and topography are main factors affecting soil nutrient variation. However, the relationships between these factors are both site- and scale-specific. In hilly areas of south-western China, the dominant cropping systems are rice, vegetables, oranges, Chinese red pepper and maize–sweet potato intercropping. In the present study, slope position (valley, low slope, flat slope, middle slope, upper slope, ridge) and aspect (north, east, south, west) were derived to investigate the relationships among cropping system, terrain, and soil nutrients at county scale. Crops were mainly planted at middle or flat slope positions. Rice and orange plants were evenly distributed across the aspects whereas vegetables were mostly planted on the northern aspect. Red pepper and maize–sweet potato plants were mainly grown on the western and southern aspects. Rice sites had higher contents of organic matter and available nitrogen (N) and lower contents of available phosphorus (P) and available potassium (K). For dryland cropping systems, vegetable sites had higher contents of organic matter, available N, and available P. Red pepper sites had higher contents of available K. Contents of organic matter and available N were generally higher at lower landscape positions. Contents of available K were higher at lower and flat slope positions. Contents of available P were higher at higher landscape positions. Contents of organic matter and available N were higher on the northern and eastern, and lower on the western aspects. Contents of available P were higher on the western and lower on the northern aspects. No significant differences were found for available K across the aspects. Classification tree algorithms indicated that relative importance of the variables on soil nutrient variation was in the order: (i) cropping system, (ii) slope position, and (iii) aspect.