『Abstract
　In forest soils where a large fraction of total phosphorus (P) is in organic forms, soil micro-organisms play a major role in the P cycle and plant availability since they mediate organic P transformations. However, the correct assessment of organic P mineralization is usually a challenging task because mineralized P is rapidly sorbed and most mineralization fluxes are very weak. The objectives of the present work were to quantify in five forest Spodosols at soil depths of 0-15 cm net mineralization of total organic P and the resulting increase in plant available inorganic P and to verify whether net or gross P mineralization could be estimated using the C or N mineralization rates. Net mineralization of total organic P was derived from the net changes in microbial P and gross mineralization of P in dead soil organic matter. We studied very low P^sorbing soils enabling us to use lower extractants to assess the change in total inorganic P as a result of gross mineralization of P in dead soil organic matter. In addition, to enable detection of gross mineralization of P in dead soil organic matter, a long-term incubation (5178 days) experiment was carried out. At the beginning of the experiment, total P contents of the soils were very low (19-51 μg g^-1) and were essentially present as organic P (17-44 μg g^-1, 85-91％) or microbial P (6-14 μg g^-1; 24-39％). Conversely, the initial contents of inorganic P were low (2-7 μg g^-1; 9-15％). The net changes in the pool size of microbial P during the 517 days of incubation (4-8 μg g^-1) and the amounts of P resulting from gross mineralization of dead soil organic matter (0.001-0.018 μg g^-1 day^-1; 0.4-9.5 μg g^-1 for the entire incubation period) were considerable compared to the initial amounts of organic P and also when compared to the initial diffusive iP fraction (＜0.3 μg g^-1). Diffusive iP corresponds to the phosphate ions that can be transferred from the solid constituents to the soil solution under a gradient of concentration. Net mineralization of organic P induced an important increase in iP in soil solution (0.6-10 μg g^-1; 600-5000％ increase) and lower increases in diffusive iP fractions (0.3-5 μg g^-1; 300-2000％ increase), soil solid constituents having an extremely low reactivity relative to iP. Therefore, soil micro-organisms and organic P transformations play a major role in the bioavailability of P in these forest soils. In our study, the dead soil organic matter was defined as a recalcitrant organic fraction. Probably because gross mineralization of P from this recalcitrant organic fraction was mainly driven by the micro-organisms' needs for energy, the rates of gross mineralization of C, N and P in the recalcitrant organic fraction were similar. Indirect estimation of gross mineralization of P in dead soil organic matter using the gross C mineralization rate seems thus an alternative method for the studied soils. However, additional studies are needed to verify this alternative method in other soils. No relationships were found between microbial P release and microbial C and N releases.

Keywords: C/N/P stoichiometry; Dead soil organic matter; Forest Spodosols; Low P-sorbing; Microbial biomass; Organic P mineralization; Phosphorus availability』

1. Introduction
2. Materials and methods
　2.1. Description of study sites and soils
　2.2. Experimental design
　2.3. Incubation experiment
　2.4. Analytical methods during the incubation experiment
　2.5. Soil P reactivity and Freundlich kinetics equation
　2.6. Data handling and statistics
3. Results
　3.1. Physico-chemical soil properties
　3.2. Initial phosphorus status
　3.3. Carbon and nitrogen mineralization
　3.4. Phosphorus mineralization
　3.5. Concomitant increase in available inorganic P
4. Discussion
　4.1. General considerations in the measurement of organic P mineralization
　4.2. C/N/P stoichiometry in organic fractions and mineralization fluxes
　4.3. Net mineralization of total organic P and available inorganic P
　4.4. Conclusion
Acknowledgements
Appendix. Supplementary data
References

※リンの抽出法は、Grierson et al.（1998）とPolglase et al.（1992）による。