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.