Abstract: We analyzed C, N, and P concentrations in roots, stems and leaves of different plant species, as well as soil nutrients in different slope aspects in Picea crassifolia forest, Prunus mongolica shrublands, and Ulmus pumila shrublands distributed in same altitude on the shady and sunny slopes of Helan Mountains. The key factors affecting plant nutrient contents were identified, and the relationships between C, N, and P of plants and soil nutrients as well as the slope aspects of different forest types were examined. There were significant differences in soil organic C, total N, and total P contents in Ulmus pumila shrublands and Prunus mongolica shrublands, and soil organic C (37.69 g·kg^-1) and total P (0.65 g·kg^-1) contents were the highest in Picea crassifolia forest on the shady slope. On the same slope aspects, soil organic C, total N, and alkaline hydrolyzable N (AN) of Picea crassifolia forest were higher than those of other two forest types. There were no significant differences in soil C/N and C/P among different forest types and slope aspects, and the N/P was lower than the average level of soils in China, indicating that there was a serious shortage of soil N supply. Leaf C content varied significantly among different forest types, with the highest value of 483.92 g·kg^-1 in Picea crassifolia forest on a shady slope. The pattern of C, N, and P contents in roots were consistent with those in leaves. There was no significant correlation between leaf C content and soil C content, a significant negative correlation between leaf N and soil N, and a significant positive correlation between leaf P and soil available P. Nutrients in leaves and roots are sensitive to slope aspects, but the processes of C assimilation and N and P fixation were not entirely synchronized with soils. Soil N/P and AN are the key factors driving the changes of nutrient contents of roots, stems, and leaves.

Key words: Helan Mountains, forest type, soil nutrient, leaf, root, slope aspect