Please check the following link.

https://www.researchgate.net/publication/254213588_Phosphorus_Mineralization_and_Availability_in_Soil_Amended_with_Biosolids_and_Animal_Manures

Article Phosphorus Mineralization and Availability in Soil Amended w...

Phosphorus exists in soils and minerals, living organisms, and water. Although P is widely distributed in nature, P is not found by itself in elemental form. Elemental P is very reactive and will combine with oxygen when exposed to the air. In natural systems like soil and water, P exists as phosphate, a chemical form in which each P atom is surrounded by 4 oxygen atoms.

Phosphate is taken up by plants from soils, utilized by animals that consume plants, and returned to soils as organic residues in soils. A large amount of the phosphate used by living organisms is incorporated into organic compounds. When plant residues are returned to the soil, this organic phosphate is released slowly as inorganic phosphate or be incorporated into more stable organic forms and become part of the soil organic matter. Mineralization is the release of inorganic phosphate from organic phosphates and is caused by the decomposition of organic materials by microorganisms. The decomposition process is rapid when soils are warm, moist and well drained.

There are already three methods which have already been published to estimate gross P mineralization. These include (1) an isotope dilution method that relies on specific activity, (2) isotope dilution method that uses isotopically exchangeable P as the response variable, and (3) a differential extraction method.  You can find more information from the attached paper.

Anion sinks such as exchange membranes might be useful.  Some commercial types are easily inserted into and removed from moist media.   The quantities are small so you need ICP, preferably MS.  Sequential removal/replacement provides information on fluxes.  The difficulty is that the effective sampling volume is small, so you need to learn how many reps are required for your purpose.   This technique can be used in combination with isotopic labelling 

 I appreciate the important topic raised by Ms.Samantha.I also appreciate good responses by Drs.Arvind,Milham,Agegnehu.As I understand ,the P mineralization from organic P in soil is not so smooth as N mineralization.The amount mineralized is small (as pointed out by Dr.Milham).We may need more sensitive and precise instruments to measure small amount of P (as mentioned by Agegnehu).Anion exchange resins can be used as sinks for P but periodic destructive sampling has to be made to remove /desorb the resin adsorbed P. Also ICP may estimate both inorganic P and soluble organic P but we need the estimation of inorganic P mineralized.Another problem is sorption of mineralized P by soil constituents especially in soils low in available P. With batch methods we can use anion exchange resin strip/membrane.We can also use the normal soil test extractants  in batch methods ( in place of water or 0.01M CaCl2 in leaching column studies).The following method used by us in case of sulphur mineralization can be tried for P mineralization study.

https://www.researchgate.net/publication/230465332_sulfur_mineralization_in_two_soils_amended_with_organic_manures_crop_residues_and_green_manures

Journal of Plant Nutrition and Soil Science165:167-171( 2002).

Article Sulfur mineralization in two soils amended with organic manu...

The information given by Dr. Rao's are very useful. What is you comment on the mineralization of phosphorus on soil pH of different levels (e.g. problematic soils such as, acidic, saline/sodic) compared to normal pH for P?

Dr.Agegnehu,can you please further elaborate your question?

I think Isotope dilution method, CO2 respiration method and Anion exchange resin strip membrane would be useful. Please also go through the attached references which may be helpful.

Plant Science (2016). doi: 10.3389/fpls.2016.01038