Abstract: Soil is the common but precious natural resource that sustains the survival and development of human beings and our society. As a vital link of global carbon cycle, it is the largest carbon pool of terrestrial ecosystem. During the past two decades, Chinese government has been implementing an unprecedented largescale afforestation program that played a key role on the cumulative carbon sequestration. Hence, it is of great importance to study the effects of forest management (e.g. thinning, stand density) on soil organic carbon (SOC) characteristics for mitigating climate changing effects. In this paper, we reported a thinning trial of Pinus tabulaeformis Carr. plantations and analyzed its effects on SOC and soil nutrients characteristics in Mulan-Weichang, Hebei Province of northern China. The Pinus tabulaeformis Carr. plantations with six densities (540, 650, 1 084, 1 104, 1 408, 1 860 stem/hm2) after thinning were selected to study SOC content and density, soil nutrient contents and their correlations. As a prerequisite, site conditions including aspect, slope, slope position, etc, stand age of 40 years old, trees’growth status and forest management approaches were kept the same or similar before the thinning trial. Correlation analysis, single factor analysis of variance (ANOVA) and multiple comparisons were carried out with SPSS 18.0. The study indicated that as follows:(1) the SOC content and density was characteristic of vertically descending, both decreasing significantly with the increase of soil depth; the SOC content and density were not consistent when the stand density increased from 540 stem/hm2 to 1 860 trees/hm2 , and ranged from 10.56 to 21.21 g/kg1，and from 5.48 to 11.70 kg/m2, respectively. (2) Stand density has significant effects on the SOC content and density. Significant differences were found for the Pinus tabulaeformis Carr. plantation at the density of 1 408 trees/hm2 with those of 650 and 1 860 trees/hm2, but there were no significant differences of the SOC content and density between the other stand densities. At the stand density of 1 104 trees/hm2, total N, P, K and available P, K contents were in relatively higher levels in the soil depth of 0- 60 cm, and reached the maximum values of 1.38 g/kg, 0.34 g/kg, 32.75 g/kg, 33.10 mg/kg and 118.85 mg/kg, respectively. (3) There were significant differences of correlations among the SOC content and density, total N, P, K and available P, K in the different depth at different stand densities; in the whole soil profile, the SOC content and density had very significant or significant positive correlations with total N, P and available K contents. (4) According to this study, the stand density of Pinus tabulaeformis Carr. plantation was suggested to be about 1 104 stem/hm2 for the soil carbon sequestration. The innovation and highlight of this paper is the discussion about the effects of stand density on forest SOC content and density.

Key words: Pinus tabulaeformis Carr, plantation, soil organic carbon, soil organic carbon density, soil nutrients, stand density