[Distribution of phosphorus fractions in soil aggregates in Chinese fir plantations with different stand ages].

Exploring the distribution of phosphorus (P) fractionsin soil aggregates is helpful to improve soil P availa-bility during Chinese fir planting. In this study, soil samples were collected in the 0-20 cm soil layer from Chinese fir plantations with different stand ages (9 a, 17 a, and 26 a) and one nearby abandoned land (CK) in Rongshui County, Guangxi, China. Soil aggregates were classified into >2 mm, 1-2 mm, 0.25-1 mm, and <0.25 mm size classes through dry-sieving process, and then soil P fractions in different sized aggregates were measured. These results showed that: 1) The composition of soil aggregates showed significant difference among different stand ages. As the major aggregate fractions in soil, the contents of >2 mm aggregates increased firstly and then decreased over time, and peaked in the 17 a Chinese fir plantation. The changes of soil mean weight diameter (MWD) and geometric mean diameter (GMD) during Chinese fir planting were the same as the content of >2 mm aggregates. 2) Soil total P, inorganic P, and organic P contents did not differ among different sized aggregates. However, soil available P content was mainly distributed in >2 mm aggregates with a range of 1.23-7.33 mg·kg. Compared with CK, soil total P, available P, and inorganic P contents were significantly higher in Chinese fir plantations, and their contents increased firstly and then decreased over time. Soil total P (322.40 mg·kg) and available P (7.33 mg·kg) contents were the highest in the 9 a plantations, and soil inorganic P content (114.05 mg·kg) was the highest in the 17 a plantation. Moreover, soil organic P content showed an order of 9 a > 26 a >17 a > CK, with the highest content (210.00 mg·kg) in the 9 a plantation. 3) The distribution of P stock in soil aggregates was related to the contents of different sized aggregates, with >2 mm aggregates having the highest P stock. Except for organic P, soil P stock increased firstly and then decreased with the increases of stand age. In conclusion, Chinese fir planting was helpful to improve soil aggregate stability and to promote the increase of soil P level before the stand age of 17 a. However, Chinese fir planting could result in the degradation of soil aggregates and in the decrease of soil P level after 17 a. The formation and stabilization of >2 mm aggregates played an important role in the maintenance of soil quality and soil P supply level after 17 a Chinese fir planting.