| In order to alleviate the problem of soil nitrogen pool imbalance and reduced cotton yield caused by nitrogen fertilizer reduction, field trials were carried out from 2022 to 2023. The experiment was set up with six treatments: no nitrogen fertilizer (N0), conventional nitrogen application (300 kg·hm-2, N300), 20% reduction of nitrogen fertilizer (240 kg·hm-2, N240), 40% reduction of nitrogen fertilizer (180 kg·hm-2, N180), 20% reduction of nitrogen fertilizer with biochar (N240+BC) and 40% reduction of nitrogen fertilizer with biochar (N180+BC). Results indicated that (1) soil total nitrogen content of N240, N180, and N180+BC treatments was significantly lower than that of N300 treatment. The soil total nitrogen content in the N240+BC treatment increased significantly by 19.49% and 22.86% compared to the N240 treatment, while there was no significant difference compared to the N300 treatment. (2) The content of soil water\|soluble organic nitrogen, particulate organic nitrogen, and mineral bound organic nitrogen decreased to varying degrees with the increase of nitrogen fertilizer reduction ratio. Compared with N240, the soil water\|soluble organic nitrogen and particulate organic nitrogen content in N240+BC treatment significantly increased by 25.28% and 51.81%, respectively. (3) Nitrogen fertilizer reduction with biochar (N240+BC, N180+BC) significantly increased the nitrogen pool management index by 48.12%~101.13% and 17.97%~41.37% compared to the corresponding nitrogen reduction treatments (N240, N180). (4) The activities of leucine aminopeptidase, N-acetyl-β-D-glucosidase, cellulase, and β-glucosidase in soil treated with reduced nitrogen fertilizer and biochar were significantly higher compared to those in treatments with reduced nitrogen fertilizer alone, with an increase of 30.90%~90.21%, 11.24%~102.23%, 11.45%~51.73%, and 10.92%~32.96%, respectively. (5) Reducing nitrogen fertilizer significantly decreased cotton yield, with the magnitude of reduction increasing over the years. In the second year, cotton yield in the N240+BC treatment significantly increased by 13.79% compared to the N240 treatment. However, there was no significant difference in cotton yield between the two years when compared to the N300 treatment. The results revealed that reducing nitrogen fertilizer by 20% combined with biochar significantly increased total nitrogen, water\|soluble organic nitrogen, and particulate organic nitrogen in drip\|irrigated cotton fields. This also improved the soil nitrogen pool management index and cotton yield, helping to sustain both the nitrogen pool capacity and the soil’s nitrogen supply. |