『Abstract
　Background, aim, and scope　Despite the contribution of many sequential P fractionation schemes to the study of P transformations in agricultural soils, the nature of P in each fraction remains qualitative rather than mechanistic. This study used the sequential extraction and isotopic dilution techniques to assess the recovery of a tracer (³²P) in soil P fractions and to elucidate the transformation of soil P in different P pools and its lability.
　Materials and methods　Three contrasting soils (Vertosol, Calcarosol, and Chromosol) were collected from paddocks with a long history of P fertilization and from an adjacent virgin area under native vegetation. The soils were labeled with ³²P and then incubated for differing periods before being sequentially extracted for P fractions. Recovery of ³²P in each P fraction was measured.
　Results　The P history increased total and available P in all soils but decreased phosphorus buffering capacity only in the Calcarosol. The previously applied P was distributed into all Pi fractions, and the proportion of the P transformed into individual fractions depends on soil characteristics. Adding P significantly increased the ³²P recovery in the water-Pi fraction of the Calcarosol. In contrast, the higher proportion of the label was recovered in the bicarbonate-Pi of the Vertosol and in the NaOH-Pi of the Chromosol.
　Discussion　The recovery of ³²P in all soil P fractions showed that ³²P had undergone exchange with the native P. The exchange reaction was most dominant in the Pi fractions. The greater level of the ³²P recovered in the water-Pi fraction of the P-amended Calcarosol indicates that the added P transformed into this fraction remains highly exchangeable. In contrast, the significantly greater amount of ³²P recovered in the NaOH-Pi fraction of the Chromosol suggests that this fraction is of great importance in P fertility of this soil type.
　Conclusions　The transformation of soil P fraction was dependent on soil type and P fertilization history. However, during the short-term (42 days), the applied P preferably remained in the form that can be exchangeable with solution P and, therefore, can be plant-available.
　Recommendations and perspectives　Long-term history of P fertilization has resulted in P accumulation which is associated with an increased P availability and decreased sorption. The fertilizer P is shown to distribute into all the P fractions. Further studies are warranted to examine the accessibility of these P fractions by plants. The isotopic dilution technique using ³²P has been verified to be useful for quantifying P transformation and contributes to a further understanding of P dynamics in native and farming systems.

Keywords: Ecosystems; P history; Phosphorus fractionation; Phosphorus exchangeability; Phosphorus transformation; Soil type』

1. Background, aim, and scope
2. Materials and methods
　2.1. Soils
　2.2. Incubation experiment and sequential P extraction of the labeled soils
　2.3. Recovery of ³²P activity in soil P fractions
　2.4. Statistical analysis
3. Results
　3.1. Soil properties
　3.2. Distribution and changes of soil P fractions following phosphorus addition
　3.3. The fate of applied P in three soil types
　3.4. The recovery of ³²P in soil P fractions across three soil types
4. Discussion
　4.1. The distribution of ³²P among Pi fractions
　4.2. Changes in soil P fractions following incubation and P addition
　4.3. Changes in general soil characteristics following long-term P fertilization
5. Conclusions
6. Recommendations and perspectives
Acknowledgements
References