Abstract: Biochar has emerged as a pivotal green technology for agricultural emission reduction, carbon sequestration, and straw resource valorization, demonstrating considerable potential for enhancing arable land quality. However, variations in the physicochemical properties arising from differences in biochar particle size directly influence its mechanisms of soil nutrient adsorption and release, leading to uncertainties in agricultural application. This study investigates the differential mechanisms by which biochar of varying particle sizes (millimeter-scale (MB), micrometer-scale (UB), and nanometer-scale (NB)) enhances soil nutrient retention capacity. Through experiments analyzing biochar structures of different particle sizes and solute adsorption, the effects on soil phosphorus fixation capacity, aggregate water stability, and colloidal stability were systematically compared. Results indicated that the particle size of biochar decreased from MB to NB, and the fatty groups gradually disappeared while the aromatic groups were basically retained, resulting in a higher degree of aromatization. The functional groups of C-C and COOR alkyl and ester compounds decrease, while the functional groups of C-O phenols and alcohol compounds show an increasing trend; (2) The absolute value of Zeta potential of biochar particles increases, and the stability of the colloidal dispersion system is enhanced. In terms of soil structure, MB exhibits a loose and porous structure, which can enhance the structural stability of soil aggregates (reduce the fractal dimension of aggregates); However, UB and NB exhibit nano crystalline or layered structures, tending to aggregate or adsorb on the surface of soil particles, and with increasing application, the stability of aggregates is actually reduced. (3) In terms of phosphorus adsorption and fixation, the Freundlich and Temkin models can well fit the isothermal adsorption process of phosphorus. As the particle size of biochar decreases, the maximum phosphorus uptake (q_max) of soil significantly increases (NB reaches 1.05 mg/g), the phosphorus adsorption index PSI increases by 39.5%, and the phosphorus release risk parameter EPC_o decreases by 34% in NB treatment. Relevant analysis shows that the water stability, fractal dimension, and Zeta potential of aggregates are significantly positively correlated with phosphorus fixation indicators (P<0.01). In summary, reducing the particle size of biochar can synergistically enhance the chemical adsorption and physical fixation of phosphorus in soil by improving colloid stability and optimizing surface chemical properties. For sandy loam soil, the optimal adsorption of phosphorus fertilizer is achieved when the application rate of NB is 2.0%. The above research results provide a theoretical reference for accurately guiding the application and production of biochar, and improving the nutrient retention capacity of dryland farmland soil.