『Abstract
　Phosphorus (P) is often a limiting factor of forest growth but our knowledge of the processes governing P availability in forest soils is rather limited. In the present work, we combined a isotopic dilution method with extraction methods to evaluate the P status in Pinus pinaster plantation forests on highly P-deficient soils. Total, organic, and inorganic P, dissolved and diffusive P, i.e. ionic P species that can be transferred from the solid phase to the soil solution due a gradient of concentration, were determined to a soil depth of 120 cm in a gradient of 18 forest sites (seven humid sites, five mesic sites, and six dry sites). Our objective was to assess the potential contribution of organic and inorganic P to plant available P. Based on results and our original assumptions, we observed that the contribution of organic P fractions (mineralization of soil organic P) to P availability related to the contribution of inorganic P fractions (diffusive P for durations up to 1 year) was predominant in litter, less important in top soil horizons, and negligible at depths below 30 cm. This was partly due to a decreasing proportion of organic P and an increasing proportion of diffusive P with soil depth. Owing to a very low amount of diffusive P in the top soils in dry sites, the relative contribution of organic P was actually higher in these sites than in humid and mesic sites, despite a lower overall organic P fraction. The combination of extraction and isotopic dilution methods in our study shed new light on P status in this forest range. In particular, these methods enable assessment of both the size of the pools and their dynamic fractions.

Keywords: Available phosphorus; P-deficiency; Phosphorus stocks; Pinus pinaster; Podzol』

Introduction
Materials and methods
　Site characteristics and soil sampling
　Determination of total, organic and inorganic P
　Gross amount of diffusive P and soil solution P ions concentration
　Incubation experiment
　Data handling and statistics
Results
　Conditions and parameters for the isotopic dilution method
　P ions in solution and diffusive P
　Total, organic and inorganic P contents
　Relative proportions of P fractions
　P stocks
　Incubation experiment
Discussion
　General considerations in the study of P status in forests
　Relative importance of organic and inorganic P to plant P availability
　Implications of the study for local forest management
Conclusion
Acknowledgments
References