『Abstract
　Transformations of phosphate (Pi) in different soil fractions were tracked using the stable isotopic composition of oxygen in phosphate (δ¹⁸OP) and Pi concentrations. Clay soil from Israel was treated with either reclaimed waste water (secondary, low grade) or with fresh water amended with a chemical fertilizer of a known isotopic signature. Changes of δ¹⁸OP and Pi within different soil fractions, during a month of incubation, elucidate biogeochemical processes in the soil, revealing the biological and the chemical transformation impacting the various P pools. O in the soil solution is affected primarily by enzymatic activity that yields isotopic equilibrium with the water molecules in the soil solution. The dissolved P interacts rapidly with the loosely bound P (extracted by bicarbonate). The oxides and mineral P fractions (extracted by NaOH and HCl, respectively), which are considered as relatively stable pools of P, also exhibited isotopic alterations in the first two weeks after P application, likely related to the activity of microbial populations associated with soil surfaces. Specifically, isotopic deletion which could result from organic P mineralization was followed by isotopic enrichment which could result from preferential biological uptake of depleted P from the mineralized pool. Similar transformations were observed in both soils although transformations related to biological activity were more pronounced inn the soil treated with reclaimed waste water compared to the fertilizer treated soil.

Keywords: Phosphate; Soil fractionation; Oxygen isotopes; Reclaimed waste water 』

1. Introduction
2.Materials and methods
　2.1. Experimental design
　2.2. Soil samples used in the experiment
　2.3. The ‘irrigation event’
　2.4. Soil incubation and sampling
　2.5. Soil sequential extraction and preparation of samples for isotopic analysis
　2.6. P concentration determination
　2.7. Mass-spectroscopy analysis
　2.8. Calculations isotopic mass balance of soil P fractions
3. Results
　3.1. Concentration and isotopic composition of soil P prior to the experiment and following the ‘irrigation event’
　3.2. Isotopic mass balance calculations
4. Discussion
　4.1. Water extractable and most labile Pi-DDI extraction
　4.2. Loosely bound Pi-Bicarbonate extraction
　4.3. Fixed Pi by oxides and minerals - NaOH and HCl extraction
　4.4. A unifying model
5. Conclusions
Acknowledgment
References