| From 2005～2008, a split-plot field experiment was carried out to study the tree's growth, foliar nutrient concentrations, the soil organic carbon contents and the dynamic changes of soil nutrient (N, P, K) contents in three soil layers (i.e.,0-20cm,20-40cm and 40-60cm) of the triploid (cv. B304, B305, B331 and B333) and diploid (cv.1319) Populus tomentosa plantation under five growth densities (including 2m×2m,2m×3m,2m×4m,2m×5m and 3mx4m). The results showed that:(1) Significant increases of diameter at breast height (DBH), the total plant height, individual volumes (IDV) and growing stock (GRS) were found in triploid P. tomentosa. The largest IDV were found in B331 by 2m×5m and B305 by 2m×4m which were the optimal growth densities for triploid clones. Forevermore,2m×2m (2500 tree·hm-2) density for the large-scale planting for triploid clones can get maximum GRSs.(2) Singificant difference on soil pH values were found in different growth denisites and soil layers. Soil pH value increased after 4a and the largest increment were found in 3m×4m growth density with 0.51 for 0-20cm layer,0.74 for 20-40cm layer and 0.87for 40-60cm layer.(3) More soil nutrient contents decreased in the intensive growth densities, such as organic matter in 2m×2m and 2m×3m; available N in 2m×2m; available P in 2m×2m and 2m×5m.(4) Significant decreasing in the contents of organic matter, total and available N were found in 0-20cm soil layer in 2006. Growthing year showed significant effect on pH value, the content of P and K. The organic matter, total and available N and total and available K decreased first and then increased while the contents of P exhibited the totally different tendency.(5) Significant differences were indicated in the soil organic C content insulted by the treatments with growth density, sampling year and their interaction. Organic C content decreased in 2006 and then increased during the following two years. Form 2005 to 2008, more significant effects of growing density on soil organic C content was found in triploid clone than diploid one. The maximum content of organic carbon was found for the 2m×3m which was optional for the triploid clone to enhance the C sequestration in plantation soil.(6) The content changes in the ratio of soil organic C to total N had the same dynamic tendency of soil organic C, and the lowest C:N ratio was in 2006 for all the treatments of the triploid and diploid clones with seven growth densities. Significantly positive correlations between the soil organic C and the total N were found in triploid clone by 2mx2m and 2mx3.5m and diploid clone by 3mx3m and 2mx5m. Significantly negative correlation was also observed in the 2mx5m in 1319 which reflects the complex in the changes of soil organic C and total N in the plantation of triploid P. tomentosa.(7) Significant differences were found in foliar nutrients' contents between the triploid (B304, B305, B331 and B333) and the diploid 1319 clones respectively, and an obvious phonological law was also found for them. Compared with the diploid clone 1319, significantly higher foliar P and K contents were found in B304 representing that B304 has stronger anti-drought capacity. The contents of foliar N, P, K, Fe and Cu all decreased from spring to fall while increasing for the foliar S content. Foliar Ca and Mg were also indicated remarkable phenological dynamics during the growing seasons. Moreover, significant positive correlations between the foliar N and P, the foliar Cu and S/Fe, and the foliar Zn and S/Cu/Fe, while significant negative correlations between the foliar Mg and Ca/Mn, and the foliar K and Ca/Mn were also found. According to the phenological change and correlations among the foliar nutrients elements, triploid Populus tomentosa B304 was the optimal one for local introduction and topdressing in spring was strongly recommended as well as the rational ratio of fertilizer should be considered during the plantation management.(8) Higher P, K, Mg, S, Fe, Cu and Zn contents were found in B304 and it was recommended to further study for significant higher P and K content in foliage.(9) According to DRIS, the optimum ratios were 13.07±1.76 for N/P,6.27±1.77 for N/K, and 0.48±0.10 for P/K for trioloid P. tomentosa.plantation. Based on means of the high-yielding group, the optimum N:P:K was 1:0.18:0.12.(10) Balanced fertilization was recommended for the triploid clone B304 P. tomentosa trees that are spaced at 2×2 and 2×3 m and that have low foliar nutrient concentrations. K was the limiting factor for the triploid clone B304 P. tomentosa plantation with spacings of 2x3.5,2x4,2x5, and 3x3 m, and the order of nutrient deficiencies was K>P>N.(11) Soil N concentrations and foliar N concentrations were positively correlated with spacings of 2x2 and 3x4 m but negatively correlated with a spacing of 2x5 m. Foliar P and K concentrations were negatively correlated with tree DBH with a spacing of 2x3 m, and foliar P concentrations and tree height were positively correlated with a spacing of 3x3 m.
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