Abstract: Phosphorus (P) is one of the essential nutrients limiting plant productivity. However, how stand age affects soil P fractions remains poorly understood. Here, topsoil samples (0-20 cm) were collected from Cryptomeria japonica plantations with seven stand ages (6, 12, 23, 27, 32, 46, 52 years old) in the Hongya Forest Farm in the rainy area of western China in Sichuan basin. Soil P fractions were measured by Hedley P classification method. The results showed that soil total P concents ranged from 231.93 to 370.27 mg·kg^-1, and were the lowest at young age (6 years old) and the highest at middle-aged forest (12 years old). The contents of soil total inorganic P and total organic P first increased and then decreased and increased respectively with increasing stand age. The proportion of total inorganic P to total P decreased with increasing stand age, while the pattern for total organic P was opposite. The contents of labile inorganic P, labile organic P and moderately labile organic P generally increased with increasing stand age, which may be correlated with the decrease of pH and the increase of organic carbon content. Moderately labile inorganic P fractions of middle-aged (12 years old) and mature (32 years old) plantations were significantly higher than those of other stand ages. Soil non-labile P fraction first increased and then decreased with increasing stand age, with the increase in soil bulk density and the decrease in soil water content as main reasons. Results of multiple linear regression model and Pearson correlation analysis showed that labile inorganic P did not correlate with stand ages. Moderately labile organic P, which affects labile inorganic P, could increase with increasing stand age. The increase of moderately labile organic P with increasing stand age could positively affect the production of labile inorganic P. Our results indicated that topsoil P fractions varied with stand ages. These findings can help propose solutions to the problems of soil P at corresponding stand ages and provide  theoretical basis for improving the quality and efficiency and maintaining soil fertility of Cryptomeria japonica plantations.

Key words: rainy area of western China, stand age, coniferous plantation, soil nutrient, Hedley phosphorus fractionation