ObjectiveLong maintenance of soil quality after clearcutting is a primary research content of sustainable forest management in plantation. This study aims to explore the effects of natural regeneration of Pinus tabuliformis on soil fertility and enzyme activities after clearcutting, in the meantime, analyze the variation of soil quality during succession, which has great significance for maintaining the site producivity of Pinus tabuliformis plantation.

Method The variations of soil fertility and enzyme activities in 0−10 cm and 10−20 cm soil layers were investigated through chronosequence approach in the Pinus tabuliformis plantation (middle age forest of 32 years, near-mature forest of 40 years and mature forest of 53 years), natural regeneration forest after clearingcutting (5, 10, 24 years after clearcutting) and wasteland after cutting as a contrast to natural regeneration in Pingquan of Hebei Province. Canonical correlation analysis was conducted on soil fertility and enzyme activity indicators.

Result (1) Soil fertility and enzyme activities were continuously improved from middle-aged forest to mature forest. Soil water content, total nitrogen content and invertase activity in 0−10 cm soil layer were increased by 161.5%, 379.1% and 181.3% respectively, and the indexes were also significantly increased in 10−20 cm soil layer (P < 0.05). (2) There was no significant difference in soil fertility and enzyme activity between 5 years natural regeneration forest after clearcutting and mature forest (P > 0.05). However, soil fertility and enzyme activities decreased gradually from 5 years after clearcutting. Compared 24 years with 5 years after clear cutting, soil capillary moisture capacity, total nitrogen content and invertase activity in 0−10 cm soil layer decreased by 24.4%, 61.0% and 28.8%, respectively, and the indexes in the 10−20 cm soil layer also decreased significantly (P < 0.05). However, soil fertility and enzyme activities in 24 years after cutting were still higher than middle-aged and near-mature forests. (3) The soil fertility and enzyme activities of natural regeneration forest were significantly higher than those of wasteland in 10 years after clear cutting (P < 0.05). Compared with wasteland, the soil capillary moisture capacity, organic matter content and urease activity of natural regeneration forest in 0−10 cm soil layer increased by 43.7%, 145.7% and 116.6%, respectively, the soil indexes in 10−20 cm soil layer were also significantly higher than those of wasteland as well (P < 0.05). (4) Canonical correlation analysis showed that there was an extremely significant correlation between soil fertility and enzyme activity (P < 0.01). Soil fertility was mainly affected by bulk density, capillary moisture capacity, capillary porosity, organic matter and total nitrogen, while soil enzyme activity was mainly affected by urease and invertase. The canonical variables of soil fertility and enzyme activity were as follows: mature forest > 5 years after cutting > 10 years after cutting > 24 years after cutting > near-mature forest > wasteland > middle-aged forest.

Conclusion The soil quality of Pinus tabuliformis plantation was significantly improved at the mature forest stage. The certain extent declining of soil fertility and enzyme avtivity would be caused by clearcutting. However, timely vegetation restoration on clear cutting land will significantly reduce the degree of decline, and the soil quality was better than the plantation forest at the same developmental stage. Natural regeneration of Pinus tabuliformis plantation after clear cutting can effectively alleviate the problem of soil degradation.