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The investigation was conducted on the spatial and temporal distributions of soil organic mater (SOM) in the mixed plantations of alder (Aluns cremastogyne) and cypress (Cupressus funebris) (MPAC),which distributed in the hilly areas of central Sichuan Basin (HACSB). The results show that:① the spatial distribution of SOM among different sites at the same age are not significant before 15-year-old, but significant at 20-year-old, and not significant again after 25-year-old; ② the SOM contents in 0-15 cm and 15-30 cm layers increase sharply from 10- to 15-year-old, and decline gradually from 15- to 30-year-old; the SOM contents of the 30-year-old PCP were 80.38% and 78.42% higher than that of the 10-year-old, but 29.16% and 53.37% lower than that of 15-year-old in the 0-15 cm and 15-30 cm layers, respectively. The decrease of SOM contents would lead to the degradation of soil fertility and the decline of forest productivity.