<p>Rice husk biochar (RHB) is widely used as a soil amendment; however, the influence of particle size on nutrient dynamics, microbial function, and crop productivity in sandy loam soils requires further investigation. Therefore, this study aimed to (i) compare the effects of RHB particle size on soil physicochemical properties, (ii) investigate variations in soil microbial functional activity and metabolic profiles under different RHB particle size treatments, and (iii) assess the effects of different RHB particle sizes on water retention and nitrogen leaching, nutrient uptake, and crop productivity. A completely randomized design with eight treatments was established to examine small, medium, and large RHB particles applied with or without chemical fertilizer, including two controls (soil alone and soil with fertilizer), using sandy loam columns. RHB application significantly improved soil physicochemical properties, particularly when combined with fertilizer. Notably small RHB particles significantly increased soil water retention and reduced total nitrogen leaching by more than 50% relative to the fertilizer-only treatment. Although unfertilized RHB treatments promoted greater microbial functional diversity, the combined application of small RHB particles and fertilizer effectively enhanced soil physical properties and nutrient availability, resulting in improved nutrient uptake and biomass production of <i>Brassica alboglabra</i>. Overall, the findings suggest that optimizing RHB particle size may serve as an effective approach to improve nutrient management and mitigate nitrogen loss in sandy loam soils.</p> Graphical Abstract <p></p>