We studied relationship between soil nutrient distribution and tree species diversity of super wet tropical rainforest at West Sumatra, Indonesia. A long-term ecological observation plot (ca. 1 ha) comprising 115 subplots (one subplot: 10 m by 10 m), has been established in Pinang Pinang since 1981. The observation plot had 892 trees, comprising 231 identified and 241 unidentified species. We selected 15 subplots for the detailed survey according to tree species diversity evaluated by the Shannon-winner index (SWI). These subplots were categorized into tree diversity groups in which each group was composed of 5 subplots: the high diversity (SWI=1.838±0.27), intermediate diversity (SWI=1.742±0.66), and low diversity (SWI=1.015±0.51). Each subplot was further sub-divided into 9 sub-subplots. Surface (0-5 cm) and sub-surface (10-15 cm) soil samples were taken from each sub-sub plot. The soil samples were analyzed for pH, total C and N, available P, and K⁺, Ca⁺⁺, Mg⁺⁺ and Na⁺. The average values of these parameters and coefficient of variations (CV) respectively were 5.10 and 21.7% for pH, 42.87 g kg^-1 and 61.0% for total C, 3.51 g kg^-1 and 53.22% for total N, 4.87 mg kg^-1 and 42.6% for available P. Others were 0.18, 6.21 0.37 and 0.07 cmol+ kg^-1 and 69.7, 133.2, 97.3 and 43.5% for K⁺, Ca⁺⁺, Mg⁺⁺ and Na⁺, respectively.

The relationship between the soil nutrient niche and tree species diversity were analyzed by correlation coefficients and ω-index of Iwao (1977). SWI showed positive significant correlation with CV for soil horizontal difference. Moreover that was negatively correlated with soil vertical difference in the contents of all nutrients. These results suggest that the difference in soil fertility niche was associated with tree species diversity and that the horizontal difference in soil nutrient niche might be more important than the vertical distribution in maintaining tree species diversity of tropical rainforest.