Rules of Changes in Soil Nutrients and Enzyme Activities of Larix principis-rupprechtii in Different Forest Ages

In this study, three different ages of Larix principis-rupprechtii forests in 5, 10 and 20 years were selected as the research objects, and the changes in soil nutrient and soil enzyme activities in different growth stages were analyzed. The results showed that the contents of organic matter and available phosphorus in the soil of different growth stages showed a significant downward trend with the increase of soil depth. For different forest ages in the same soil layer, the soil available phosphorus content declined with the increase of the forest age. The organic matter content of 40-60 cm in 20 years of forest age was the lowest in July, which was 4.17 g/kg, significantly lower than that in other soil layers. Besides, the soil available phosphorus content of 5 years of forest age reached the maximum in July, with an average of 4.44 mg/kg, which was higher than the available phosphorus content in soil in May and September, but the difference between the three months was not significant. The changes of ammonium nitrogen and nitrate nitrogen content in soil with different forest ages were consistent with the changes in the soil depth, showing a downward trend. In the new leaf stage, the ammonium nitrogen content of the L. principis-rupprechtii forest land in 5 years of age at 20-40 cm and 40-60 cm of the soil depth was 13.47 mg/kg and 9.09 mg/kg, respectively, which was 46.9% and 64.2% lower than that at 0-20 cm (25.36 mg/kg) of the soil depth. The soil nitrate nitrogen content of 20 years of forest age was 19.24 mg/kg, which was 25.8% lower than that of 10 years of forest age, showing significant difference (P < 0.05). In addition, with the increase of the age of L. principis-rupprechtii, soil catalase (CAT), alkaline phosphatase (ALP) and urease (Ure) decreased, and the decline of ALP was slow, while CAT and Ure decreased significantly. In summary, it is concluded that the soil fertility of forest land declined with the increase of forest age on the basis of change trend of soil nutrient and soil enzyme activity in the surveyed forest age.