Temperature-Dependency of Phosphorus Sorption by Goethites and Tropical Soils Amended with Woodchip Biochar

Abstract

Core Ideas: Freundlich and Langmuir isotherms described P sorption by oxides and soils, respectively. Biochar increased the extent and relative strength of P sorption by goethite at 15°C. Biochar reduced P sorption but increased P binding strength for sandy loam at 35°C. Phosphorus is a key nutrient in agriculture but also a common contaminant. Bioavailability and transport of P are controlled by, among other things, the presence of Fe and Al oxides in tropical soils. Biochar has been suggested for improving the bioavailability and reducing the off-site transport of P. The objective of this study was, therefore, to evaluate the effect of biochar on P sorption by goethite, aluminum-substituted goethite (Al-goethite), and two tropical soils amended with 0, 20, and 40 g biochar kg−1 at 15, 25, and 35°C. Sorption by goethite and Al-goethite was adequately described by the Freundlich isotherm although the Langmuir isotherm was a better fit for the two soils tested. At 15°C, biochar application increased P sorption by goethite from 9.5 to 11 g kg−1/(mg L−1)1/n as indicated by the increase in the Freundlich constant, Kf, and the increase in relative sorption strength represented by a reduction in the Freundlich exponent (1/n) from 1.2 to 0.8. At 35°C, biochar addition significantly reduced the Langmuir sorption maximum (Smax) from 17.3 to 15.8 g kg−1 (mg L−1)−1 for the sandy loam soil, indicating a reduction in P sorption capacity in the presence of biochar. Biochar addition also significantly increased the Langmuir constant, (KL) for the sandy loam from 0.04 to 1.7 L mg−1 at 35°C, indicating greater P binding strength in the presence of biochar. The sorption data from this study provide insights into the effect of temperature on P sorption by Fe oxides and tropical soils in the presence of biochar.