Phosphorus sorption and desorption characteristics of soils as affected by biochar

Knowledge of phosphorus (P) sorption and desorption properties of different soils can help predict the availability of added P, which is of importance in terms of managing soil fertility problems and P environmental impacts. As biochar has become widely utilized as an amendment in agricultural soils over the past decades, the present study mainly aims at investigating the effect of wheat straw-derived biochars produced at 300 °C (B300) and 600 °C (B600) on P sorption and desorption via batch experiments in four soil samples with a dissimilar reaction. The sampled soils were incubated with B300 and B600 at three rates of 0%, 3%, and 6% (w/w) for 90 days. Isotherm equations were also applied to evaluate which model presents the best fit of the predicted data with experimental results. The results revealed that in the acidic soils (pH 4.6 and 6.0) P sorption decreased, whereas P desorbability increased with increasing biochar pyrolysis temperature and application rates compared to the untreated soils. A similar trend was observed for soil with a pH of 7.4 following biochars addition excluding, the sample that initially received 200 mg P L−1 at a 6% rate of B600 treatment. Reduction of P sorption in another alkaline soil (pH 8.3) upon biochar addition was evident at lower concentrations of added P and was markedly following B300 addition at the application rate of 6%. In this soil (pH 8.3), P desorbability decreased with increasing biochar pyrolysis temperature and application rates. Fits of isotherm equations showed that Freundlich and Langmuir equations did describe P sorption in most of the studied soils. The Langmuir equation did not sufficiently describe P retention data. However, the Freundlich equation showed satisfactory fits to most of the retention data. As sorption and retention isotherms failed to coincide, hysteresis was observed for all the studied soils which was most predominant for the alkaline soil (pH 8.3) following B600 addition at the rate of 6%. This study presents insight into the contradictory effects of biochars on P dynamics in different soil types and it emphasizes the importance of considering the impact of biochar pyrolysis temperature, application rates, soil properties, and initial P concentration when applying biochar as a soil amendment for agricultural P management. All this can bear implications for further studies on biochar utilization and soil P sorption and desorption characteristics.