Phosphorus sorption and desorption in a Brazilian ultisol: Effects of pH and organic anions on phosphorus bioavailability

Phosphorus bioavailability is a major limitation to plant productivity in weathered Brazilian soils. Soil pH change and organic anions such as oxalate and citrate produce known rhizosphere effects that influence P sorption and desorption. However, few data document their influence on P bioavailability. Quantitative measurement of bioavailable P pools is not well-defined. Our study focused on the surface soil developed under native Atlantic Forest in Bahia. Brazil. Objectives of our study were as follows: (1) To investigate a method of developing P desorption isotherms using multiple strips of anion exchange membranes (AEMs). (2) To evaluate the effect of pH on P sorption and desorption isotherms. (3) To separate ligand-desorbable P into ligand-exchangeable and ligand-dissolvable P. (4) To estimate the relative contributions of disequilibria- and ligand-desorbable P to the total desorbable P.;The Multiple AEM Method was suitable for developing well-defined isotherms and superior to sequential extraction methods over a range of solution concentrations in our study. Increasing pH from 4.7 to 6 and 7 decreased P sorption (up to 21% and 34%, respectively) and increased P desorption for all P application rates. Using less than 1 μmol oxalate or citrate g−1 soil and using 1 min contact time allowed us to desorb P solely by ligand exchange without surface dissolution. Nevertheless, dissolution played the major role in measuring ligand-desorbable P. Although most of P sorbed was found in the ligand-desorbable pool, P was preferentially found in the disequilibria-desorbable pool after P fertilization. The percentage of the total sorbed P that was desorbable was a function of time since fertilization; 13 to 22% on day 1 and 39 to 45% on day 14. This ageing process was due to a shaking artifact (30%), microbial immobilization (57%), and sorption of P onto the soil in a non-oxalate accessible form (13%). Liming and organic anion loading improved P bioavailability in this Brazilian soil. Long-term ageing and continuous exudation of organic anions also influence P bioavailability. Management options that recognize this will improve bioavailability of native and applied P sources in this important agricultural region of northeastern Brazil.