Objective Mixing with broad-leaved trees is an important measure to enhance soil fertility in Chinese fir(Cunninghamia lanceolata) plantations forests. Exploring the effects of mixing with broad-leaved trees on soil nitrogen mineralization at different depths in Chinese fir plantations can provide a reference for the effects of mixing with broad-leaved trees on soil nutrient cycling in Chinese fir plantations.

Method Mixed forests of two classical mixed patterns（Mixed forests formed by replanting broad-leaved trees in the understory of pure forests after intercutting and mixed forests formed by simultaneous planting of Chinese fir and broad-leaved trees.）and pure forests of Chinese fir were selected for the study. Physicochemical properties, effective nitrogen content and nitrogen mineralization rate of soil at different soil depths in the understory (0～5 cm, 5～20 cm, 20～40 cm and 40～60 cm) were determined.

Results The net N mineralization rate was significantly higher in the 0～5 cm soil layer than in the other soil layers. Mixing and intercropping with broad-leaved trees significantly increased the soil effective N content and net N mineralization rate in the 0～5 cm layer. Compared with the pure forest, the net soil N mineralization rate increased by 111.19% in the mixed forests formed by replanting broad-leaved trees in the understory of pure forests after intercutting and by 70.73% in the mixed forests formed by simultaneous planting of Chinese fir and broad-leaved trees. Redundancy analysis showed that the amount of total carbon and total nitrogen content was the important factor affecting the rate of nitrogen mineralization, water content, carbon to nitrogen ratio, dissolved organic carbon and nitrogen, and also had an effect on the rate of nitrogen mineralization. Correlation analysis showed that the soil carbon to nitrogen ratio in 0～5 cm was an important influencing factor on nitrogen mineralization, whereas soil physical and chemical properties had different effects on nitrogen mineralization among different soil layers.

Conclusion Soil nitrogen mineralization is closely related to soil depth, mainly occurring in the shallow surface soil and affected by a variety of soil properties. Mixing and interplanting with broad-leaved trees can promote soil nitrogen mineralization in Chinese fir plantations, and the positive effect on the soil nitrogen mineralization process is beneficial to maintain and improve soil fertility of Chinese fir plantations.