| Pinus tabulaeformis Carr., a prominent species in forest plantations of Shanxi Province, is a common species of coniferous in the cool temperate zone of North China and plays a very important role in soil and water conservation and improving soil fertility in North China Mountainous Districts. This paper studied the carbon storage and its environmental response in P. tabulaeformis plantation in Shanxi province in northern China. In addition, based on the data collected from short-term located observation, the absorption, storage and release rates of carbon in soil ecosystem of20-year-old P. tabulaeformis plantation with four stand denstidies were investigated. The main results of the research are as follows:(1) The soil organic carbon content of P. tabulaeformis plantations decreased with soil depth increasing. The change of average soil organic carbon content of different site conditions was:shady> sunny; foot of the slope> mesoslope> top of the slope. The average soil organic carbon content in lm soil horizon increased with stand age, but did not increased with stand density. The soil organic carbon content correlated with soil bulk density, soil moisture content, soil pH and soil total K contents and this correlation depended on soil depth.(2) Total soil organic carbon density of P. tabulaeformis plantations varied from57.83to121.88t·hm-2with nearly half part of soil organic carbon being present in the top30cm of soil. The soil organic carbon density correlated with slope, slope position, stand age, litterfall mass, the annual litter, soil moisture content, bulk density, soil total K content and soil total N content and this correlation also depended on soil depth.(3) The result showed that aboveground litterfall decreased with stand density, whereas belowground litterfall was the opposite. The decomposition rate of litterfall increased as stand density decreased. The total element returns amount had notable different among four density stand, having the order of CK>MT>HT>LT. The element returns of litter had the order of C>N>P>K. The release rates and dynamic changes of nutrient elements (N, P, K and C) between the leaf litters and fine root litters exists some differences. The decomposition of fine root litters is easier than that of leaf litters, and also the nutrient elements rease more easily in the formers than in the latter. The fine root annual decomposition amount contributes28.6%～40.8%to the total litter decomposition. Amount of N, P, K and C return to soil from fine roots was32.3%～48.5%,24.4%～39.5%,26.6%～40.7%and28.8%～52.6%of total return amount. Therefore, fine root decomposition play an important role in C and nutrients cycling,(4) We found that immediately following thinning treatments, soil respiration increased by8%～21%compared with the unthinned control plots during both growing seasons. There were significant differences in soil respiration and its components among the various treatments. Compared with Ra, Rh made the major contribution to Rs during the growing season at all sites. The relative proportion of Rh to Rs averaged71.6%～79.7%over the four stands. During the growing seasons, the soil respiration and its components were positively correlated with the soil moisture. Correlations between Rh and soil moisture were more significant than that of Ra and soil moisture. Meanwhile, a positive correlation was found between soil temperature and soil respiration and its components at all sites. Rh and Ra have been shown to respond differently to increasing temperature, exhibiting different levels of sensitivity to temperature (Q10). Fitted Q10values of Rh ranged from2.16to2.75in the various thinning intensities, with the highest Q10value in the control sites; Rh is more sensitive to temperature than Ra in all stands. The model with the best fitted temperature and moisture factors explained18.6%～44.3%of the variation in Rs, 66.7%～77.3%of the variation in Rh and30.2%～46.4%in Ra between the four thinning sites during both growing seasons. Overall, soil respiration is better predicted by soil moisture, soil organic C, live fine root biomass and soil temperature when data are pooled for all thinning treatments over the two growing seasons. The best regression model explained74.7%of the total variation in soil respiration over the different thinning intensities for the two sampling periods.(5) After a carbon balance formula was formulated, the dynamics and balance of carbon was studied in the soil of P. tabulaeformis plantations. Results showed that CO2budget value increase with the decrease of the stand density. It is concluded that the decrease stand density could contribute to carbon sequestration, but that moderate and higher disturb would enhance the C sequestration more significantly. The results of this study can be used to help understand how forest management of P. tabulaeformis plantations affects carbon sequestration in forest soils.The paper study on soil carbon pool and effects of tree density on soil carbon cycling of P. tabulaeformis plantations, the analysis and results have certain contribution and practical application values to the forestry managerial and production activities in the Taiyue Mountain area as well as the current forest soil carbon assessment in China. |
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