Sequential Phosphorus Extraction of a 33 P‐Labeled Oxisol under Contrasting Agricultural Systems

Chemical sequential extraction is widely used to divide soil phosphorus (P) into different inorganic (P i ) and organic (P o ) 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 33 P and extracted after incubation times of 4 h, 1, and 2 wk. Phosphorus concentrations and 33 P recovery in fractions sequentially extracted with anion exchange resin (P i ), 0.5 M NaHCO 3 (Bic‐P i , Bic‐P o ), 0.1 M NaOH (P i , P o ), hot concentrated HCl (P i , P o ), and residual P were measured for each incubation time. Resin‐P i , Bic‐P i , NaOH‐P i , and hot HCl‐P i were increased with P fertilization, with the highest increase for NaOH‐P i The recovery of 33 P in the treatments with annual P fertilization clearly exceeding P outputs indicate that resin‐P i , Bic‐P i , and NaOH‐P i represented most of the exchangeable P. In these treatments, label P transformed with increasing incubation time from the resin to the Bic‐P i and NaOH‐P i 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 33 P was recovered in NaOH‐P o and HCl‐P o fractions 2 wk after labeling, showing that organic P dynamics are important when soil P i reserves are limited.