Abstract: The size of particles determines the adsorption reaction. In this study, three different particle sizes of biochar (0.25-1 mm, 0.075-0.25 mm, <0.075 mm) were produced from rapeseed straw (SBC) and chicken manure (MBC). The biochar was mixed with high phosphorus (P) soil and low P soil and then incubated for 30 days. We conducted isothermal P sorption and desorption experiments to evaluate the effects of biochar particle size on sorption-desorption characteristics of soil P, and analyzed soil properties associated with P sorption. The results showed that P sorption capacity of SBC and MBC in the water system was highest for the smallest particle size (<0.075 mm) (SBC: 43125 mg·kg^-1, MBC: 20083 mg·kg^-1), followed by the intermediate particle size (0.075-0.25 mm) (SBC: 37376 mg·kg^-1, MBC: 13199 mg·kg^-1) and the largest particle size (0.25-1 mm) (SBC: 27749 mg·kg^-1, MBC: 12251 mg·kg^-1). However, there was little difference in soil P sorption between the three particle sizes of the same biochar in the soil system. In comparison with no biochar treatment, the addition of SBC increased the Langmuir P sorption maximum (S_max) by 236.8%-755.7%, and decreased soil P desorption rate. The addition of MBC increased S_max, but the enhancement was less than that of SBC. Soil P desorption rate was increased by 7.2%-295.9%. Both SBC and MBC significantly increased the contents of soil total P, available P, and exchangeable calcium (Ca) and magnesium (Mg). The increases in Ca and Mg contents due to biochar addition was 64.0%-257.1% (SBC) and 39.1%-205.3% (MBC), respectively. The contents of soil exchangeable Ca and Mg were positively correlated with S_max. These results suggested that biochar particle size had little effect on soil P sorption, but the enrichment of Ca and Mg due to biochar addition played a critical role in regulating soil P sorption. The rapeseed straw biochar had a high adsorption capacity for soil P, making it suitable for improving the P fixation capacity of soil rich in P and reducing the loss of excess P. Chicken manure biochar could be used to improve the P availability of low P soils and increase the contents of available P.

Key words: soil phosphorus, biochar, sorption, desorption, particle size, Langmuir equation