INFLUENCE OF SPECIFIC ORGANIC COMPOUNDS ON PHOSPHORUS SORPTION AND DISTRIBUTION IN A TROPICAL SOIL

Abstract

Effects of organic matter on phosphorus (P) sorption are not completely understood, among others, because the molecular composition of organic matter seldom has been considered. Our objective was to investigate how surface modification of a tropical soil with specific organic compounds altered the P sorption and distribution of the sorbed P. Samples of an ando-humic Nitisol, original and treated with H2O2 to remove native organic matter, were amended by 50 g C kg−1 from lipids, lignin building blocks, glucose, albumin, tryptophan, histidine, and a mixture of these compounds. Phosphorus sorption was studied by equilibrating 1 g of the treated soil samples in 30 mL of 1 mM CaCl2 · 2H2O containing 0 to 645.71 μmol P L−1. The P concentrations in the soil solution after 24 h followed the order: histidine > mixture > lignin > albumin > lipid > tryptophan > glucose > control in the original sample and albumin > lignin > lipid > mixture > histidine > tryptophan > glucose > control in the H2O2-treated sample. Phosphorus sorption was lower in the H2O2-treated than in the original sample, which was explained by removal of aluminum and an increase in negative surface charges after native soil organic matter oxidation. The sorbed P distribution, studied by sequentially fractionating the samples with the highest P load, showed that 73 to 96% (original) and ≈100% (H2O2 treated) of sorbed inorganic P was extracted by the first three fractions (anion exchange membrane-Pi, NaHCO3-Pi, NaOH-Pi). From calculations of P requirements for sufficient P concentrations for plant growth, it was concluded that surface modification by the addition of locally available organic compounds with great molecular complexity and organic P constituents may help to reduce the need for mineral P fertilizer and increase its efficiency in tropical soils.