【Objective】Biochar application in soil has a positive effect on reducing nitrogen loss and improving nitrogen utilization efficiency. However, information on the interaction between naturally aged biochar and nutrients in the field is limited. Exploring the influence mechanism of aged biochar on soil nitrogen retention is helpful to further understand the environmental and chemical process of biochar in soil.【Method】The method of in-situ embedding of biochar in nylon bags in the field and batch equilibrium adsorption experiment in the laboratory were carried out. The field experiment included four treatments, namely fresh biochar (A0), aged biochar for one year(A1), two years(A2) and three years(A3). The dynamic changes of surface structure characteristics and physicochemical properties of aged biochar and their effects on NH₄⁺-N adsorption were investigated using scanning electron microscopy, BET-specific surface, differential thermal and thermogravimetric analysis techniques.【Result】The results showed that the surface pore structure of biochar after naturally aging for three years in the field collapsed. With the increase of field aging time, the BET-specific surface area, total pore volume, mesoporous pore volume and average pore size of biochar increased, while the micropore surface area decreased. Compared with the A0 treatment, the BET-specific surface area, total pore volume and average pore size of biochar in the A3 treatment increased by 18.93%, 42.31% and 20.71%, respectively, and the micropore surface area decreased by 26.17%. With respect to the chemical structure of aged biochar, the contents of C and N elements increased by 7.92% and 95.61%, respectively, and the aromatization degree and thermal stability of biochar decreased. With the increase of field aging time, the amount of NH₄⁺-N adsorbed by biochar decreased in the order A0 > A1 > A2 > A3, although aged biochar maintained a strong adsorption capacity. Also, batch equilibrium adsorption experiments showed that the adsorption rate of biochar to NH₄⁺-N complied with the pseudo-second-order adsorption kinetics and Langmuir models. 【Conclusion】The present study suggests that the adsorption of NH₄⁺-N to biochar mainly occurred through the single-molecular layer mechanism, followed by physical adsorption. Therefore, studies on the enrichment effect of biochar on carbon and nitrogen and the adsorption mechanism of NH₄⁺-N are significant in exerting carbon sequestration and improving nitrogen utilization efficiency in the soil ecosystem for biochar.