Abstract:

Objective: This study was designed to characterize the variation in soil phosphorus forms and their contents in differently aged Chinese fir plantations, and analyze the correlations among various phosphorus forms, in order to provide a basis for improving the availability of soil phosphorus in Chinese fir plantations and promoting the sustainable development of Chinese fir plantations. Method: Seven Chinese fir plantations at different ages (3, 6, 12, 18, 25, 32 and 49 years) were selected from the Shanxia Forest Farm in Fenyi County, Jiangxi Province, China. Total soil phosphorus and available phosphorus were detected by molybdenum antimony scandium colorimetric method. The contents of soil aluminum-bound P (Al-P), high-iron-bound P (Fe-P), occluded P (O-P) and calcium-bound P (Ca-P) were detected by a grading method, and the correlations among various phosphorus forms were analyzed, to reveal the dynamic changes of soil phosphorus at different forest ages. Result: The soil inorganic phosphorus contents of differently aged Chinese fir plantations accounted for 55.37%~74.22% of the total phosphorus content, and the amount of inorganic phosphorus fractions was in an order: O-P> Fe-P> Ca-P> Al-P; The content of total phosphorus, available phosphorus, Al-P and O-P in soil showed a trend of first decreasing followed by increasing with age, and the minimum appeared at 12-year-old or 18-year-old, the Fe-P content displayed a gradual increased but not significantly, the Ca-P content displayed an increase first followed by a decrease. The dynamic changes of soil inorganic phosphorus with forest age indicate that the Chinese fir plantation is in the rapid growth stage during the middle forest age and absorbs a large amount of soil nutrients, prompting the activation of O-P and the release of Al-P and Fe-P under the action of root exudates, but due to the different rates of release and absorption, the content of different phosphorus forms varies. From 18-year-old to 49-year-old, the Ca-P content decreased significantly, indicating that starting from the near-mature forest, the Chinese fir plantations increased the Ca-P absorption rate and slowed down the absorption rate of other phosphorus forms, resulting in an increase of the content. The correlation analysis of various phosphorus proved that soil available phosphorus was positively correlated with Al-P, Fe-P, and O-P content (P>0.05), and O-P was positively correlated with Al-P and Fe-P content (P>0.05). Conclusion: The changes of content of different phosphorus forms in the soils of Chinese fir plantations at different ages show certain patterns. The inorganic phosphorus is the main component of the total phosphorus. And correlation analysis shows that different phosphorus forms can be transformed into each other.

Key words: Chinese fir plantation, age of forest, total soil phosphorus, available phosphorus, inorganic phosphorus