Soil chemical properties effect on phosphorus fractions and maximum adsorption capacity in Brazilian tropical soils

Abstract

Phosphorus (P) availability in tropical soils is strongly limited by high adsorption capacity, primarily driven by mineralogical and chemical characteristics. This study aimed to evaluate the influence of soil properties on P sorption kinetics, maximum phosphorus adsorption capacity (MPAC), and P fraction distribution across 17 soil samples from São Paulo, Brazil. Soil samples from multiple classes and textures were characterized for chemical, physical, and mineralogical attributes, and subjected to Langmuir and Freundlich isotherm modeling, pseudo-first and pseudo-second-order kinetics, Fourier transform infrared (FTIR) spectroscopy, and sequential P fractionation. The MPAC values varied widely, ranging from 75.95 to 1149.49 mg kg⁻¹, with higher values in clay-rich Inceptisol (aq) and Oxisols. The pseudo-second-order kinetic model consistently provided the best fit outperforming the pseudo-first-order model, confirming chemisorption as the main mechanism. FTIR spectra revealed the presence of reactive functional groups (Al–OH, Fe–OH, and carboxylic acids), associated with P retention. P fractionation showed predominance of occluded forms (>60% of total P in most soils), with labile P being minimal (<8%). The degree of P saturation and P legacy index indicated potential environmental risks in Inceptisol (aq) and P occlusion in Oxisols. Principal component analysis and Spearman correlation demonstrated strong associations between organic matter, clay, Fe₂O₃, Al₂O₃, and P fractions. These findings underscore the central role of soil mineralogy and organic matter in modulating P dynamics, highlighting the need for targeted fertilization strategies in tropical agroecosystems.