『Abstract
　Chemical sequential extraction is widely used to divide soil phosphorus (P) into different inorganic (Pi) and organic (Po) fractions, but the assignment of these fractions to pools of differing plant availability, especially for low P tropical soils, is still matter of discussion. To improve this assignment, the effect of land-use systems and related P fertilizer inputs on size of P fractions and their isotopic exchangeability was investigated. A Colombian Oxisol, sampled from a long-term field experiment with contrasting management treatments, was labeled with carrier free ³³P and extracted after incubation times of 4 h, 1, and 2 wk. Phosphorus concentrations and ³³P recovery in fractions sequentially extracted with anion exchange resin (Pi), 0.5 M NaHCO3 (Bic-Pi, Bic-Po), 0.1 M NaOH (Pi, Po), hot concentrated HCl (Pi, Po), and residual P were measured for each incubation time. Resin-Pi, Bic-Pi, NaOH-Pi, and hot HCl-Pi were increased with P fertilization, with the highest increase for NaOH-Pi. The recovery of ³³P in the treatments with annual P fertilization clearly exceeding most of the exchangeable P. In these treatments, label P transformed with increasing incubation time from the resin to the Bic-Pi and NaOH-Pi fractions. The organic or recalcitrant inorganic fractions contained almost no exchangeable P. In contrast, in soils with low or no P fertilization, more than 14％ of the ³³P was recovered in NaOH-Po and HCl-Po fractions 2 wk after labeling, showing that organic P dynamics are important when soil Pi reserves are limited.』

Abbreviations
(Introduction)
Materials and methods
　Soil characterization
　Sequential P fractionation of labeled soils
　Isotopic exchange kinetics
　Statistical analysis
Results and discussion
　Total soil P and P balance induced by the different treatments
　Isotopic exchange characteristics
　P concentrations in different fractions of the sequential extraction
　Distribution of ³³P among P fractions and dynamics over time
　Total ³³P label recovery
　Specific activities in the fractions determined by the sequential extraction
Conclusions
Acknowledgments
References