The adsorption and fixation of potassium in agricultural soils are important as they influence K availability for crops. Soil organic matter (SOM) and ammonium (NH4+) exist in soils and play indispensable roles in soil fertility and crop yield; however, the effects of SOM and NH4+ on K retention in soil aggregate remains unclear. This study aimed to evaluate the effects of SOM and NH4+ on K adsorption and fixation in soil microaggregates (<0.25 mm). Soil microaggregates were extracted from three long-term fertilization treatments under rice-wheat rotations: no fertilizer (CK), fertilized with inorganic NPK (NPK), and inorganic NPK fertilizers combined with straw return (NPKS). Long-term fertilization, particularly the application of inorganic NPK combined with straw return, significantly improved the SOM content in microaggregates. Both NPK and NPKS treatments increased K adsorption but decreased K fixation, and SOM oxidation of microaggregates reduced K adsorption but increased K fixation in all treatments, indicating the positive and inhibitory effects of SOM on K adsorption and fixation, respectively. NH4+ significantly inhibited K adsorption and fixation, and this inhibitory effect was more significant in microaggregates with a higher SOM content. Although NH4+ reduced the positive effect of SOM on K adsorption, it enhanced the inhibitory effect of SOM on K fixation. Conclusionally, long-term fertilization increases K adsorption but reduces K fixation by improving SOM content, where NH4+ enhances SOM inhibited K retention in soil microaggregates, which is considered to improve K availability in soils amended with K fertilizers. Keywords: soil organic matter, NH4+, K, adsorption, fixation