Abstract: Soil quality and productivity usually decline with long-term monoculture of Cunninghamia lanceolata. To clarify the soil improvement potential of native broad-leaved tree species in subtropical regions on degraded C. lanceolata plantation, we focused on three kinds of broad-leaved monoculture of Phoebe bournei, Schima superba, and Cyclobalanopsis gilva, using C. lanceolata harvested plantation as control. We examined the responses of soil physicochemical properties in different soil layers (0-20, 20-40, and 40-60 cm) after transforming a pure Chinese fir forest into pure broad-leaved forests. The results showed that broad-leaved pure plantations improved the quality of surface soil (0-20 cm), with P. bournei pure plantation showing the greatest improvement in soil pore structure and water retention. Compared with the C. lanceolata pure plantation, soil total porosity, capillary porosity, capi-llary water-holding capacity and field water-holding capacity in the surface layer of P. bournei pure plantation increased by 6.4%-11.7%, while the mass water content was significantly increased by 28.7%. S. superba pure plantation showed obvious advantages in enhancing soil organic carbon and total nitrogen content, which were 2.8 and 3.6 times that of the C. lanceolata pure plantation, and 2.5 and 2.0 times that of the control, respectively. C. gilva pure plantation performed better in promoting available nutrients accumulation. The contents of alkali-hydrolyzable nitrogen and available phosphorus in the surface layer increased by 89.8% and 204.9% respectively compared with the C. lanceolata monoculture, and increased by 36.4% and 85.8% respectively compared with the control. With the deepening of soil layer, the improvement advantages of broad-leaved pure plantations significantly diminished. In the 40-60 cm soil layer of three broad-leaved pure plantation, soil bulk density increased by 8.7%-11.4% compared with the surface layer, while porosity, water-holding capacity, and the contents of soil organic carbon, total nitrogen, available nitrogen, and available phosphorus decreased by 15.4%-52.4% compared with the surface layer. The content of total phosphorus, available phosphorus, and available potassium in the 40-60 cm soil layer were generally low, indicating that there were still phosphorus and potassium limitations in the deep soil after the broad-leaved transformation of degraded C. lanceolata plantation. Correlation analysis showed that the maxi-mum water-holding capacity, capillary water-holding capacity and field water-holding capacity were significantly positively correlated with total porosity and capillary porosity, but significantly negatively correlated with soil bulk density. Soil organic carbon content was significantly positively correlated with total nitrogen, alkaline hydrolysable nitrogen, and available phosphorus, indicating that soil pore structure determined water holding capacity, while organic matter accumulation contributed to the enhancement of nutrient availability. Therefore, converting degraded C. lanceolata plantation into broad-leaved pure plantation of S. superba, P. bournei, and C. gilva, could significantly improve soil quality of surface layer, but the effect on deep soil remained limited.

Key words: Cunninghamia lanceolata, conversion, precious native broad-leaved tree, soil physical and chemical property