This study investigates the long-term effects of a one-off application of biochar on highly saline-alkali paddy soils, focusing on soil physicochemical properties, salt concentration, nutrient availability, enzyme activity, and rice yield over a 6-year period. The experiment was conducted in a saline-alkali paddy field in Jilin Province, China, using two nitrogen application levels (0 and 225 kg ha\(^{-1}\)) and four biochar application rates (0%, 1.5%, 3.0%, and 4.5% w/w). Key findings include: 1. **Soil Physicochemical Properties**: Biochar application significantly reduced soil bulk density (BD) and increased total porosity (Tp) and saturated hydraulic conductivity (Ks). The highest BD reduction was observed at the 4.5% biochar rate, while the highest Tp and Ks increases were at the 3.0% and 4.5% biochar rates, respectively. 2. **Soil Aggregates**: Biochar consistently improved soil aggregate stability, with macro-aggregate formation and mean weight diameter (MWD) increasing over time. The 4.5% biochar rate showed the most significant improvements. 3. **Salt Concentration and Chemical Properties**: Biochar application reduced Na\(^+\) concentration, Na\(^+/K^+\) ratio, and Na\(^+/Ca^{2+}\) ratio, while increasing K\(^+\), Ca\(^{2+}\), Mg\(^{2+}\), and cation exchange capacity (CEC). The 4.5% biochar rate had the most pronounced effects. 4. **Nutrient Availability**: Biochar application increased soil total nitrogen (TN), available phosphorus (AP), and available potassium (AK) content. The 4.5% biochar rate showed the highest increases. 5. **Enzyme Activity**: Biochar application enhanced soil enzyme activities, particularly catalase, phosphatase, urease, and sucrase. The 4.5% biochar rate had the highest enzyme activities. 6. **Rice Yield**: Biochar application significantly increased rice biomass yield (BY) and grain yield (GY). The 4.5% biochar rate resulted in the highest yields, with a 145.20% increase in GY compared to the control. 7. **Correlation Analysis**: Positive correlations were observed between biochar application and improved soil properties, nutrient availability, and enzyme activities, while negative correlations were noted with soil salinity and Na\(^+\) concentration. These findings highlight the long-term benefits of a one-off biochar application in improving soil quality and rice yield in highly saline-alkali paddy soils, particularly at higher application rates.This study investigates the long-term effects of a one-off application of biochar on highly saline-alkali paddy soils, focusing on soil physicochemical properties, salt concentration, nutrient availability, enzyme activity, and rice yield over a 6-year period. The experiment was conducted in a saline-alkali paddy field in Jilin Province, China, using two nitrogen application levels (0 and 225 kg ha\(^{-1}\)) and four biochar application rates (0%, 1.5%, 3.0%, and 4.5% w/w). Key findings include: 1. **Soil Physicochemical Properties**: Biochar application significantly reduced soil bulk density (BD) and increased total porosity (Tp) and saturated hydraulic conductivity (Ks). The highest BD reduction was observed at the 4.5% biochar rate, while the highest Tp and Ks increases were at the 3.0% and 4.5% biochar rates, respectively. 2. **Soil Aggregates**: Biochar consistently improved soil aggregate stability, with macro-aggregate formation and mean weight diameter (MWD) increasing over time. The 4.5% biochar rate showed the most significant improvements. 3. **Salt Concentration and Chemical Properties**: Biochar application reduced Na\(^+\) concentration, Na\(^+/K^+\) ratio, and Na\(^+/Ca^{2+}\) ratio, while increasing K\(^+\), Ca\(^{2+}\), Mg\(^{2+}\), and cation exchange capacity (CEC). The 4.5% biochar rate had the most pronounced effects. 4. **Nutrient Availability**: Biochar application increased soil total nitrogen (TN), available phosphorus (AP), and available potassium (AK) content. The 4.5% biochar rate showed the highest increases. 5. **Enzyme Activity**: Biochar application enhanced soil enzyme activities, particularly catalase, phosphatase, urease, and sucrase. The 4.5% biochar rate had the highest enzyme activities. 6. **Rice Yield**: Biochar application significantly increased rice biomass yield (BY) and grain yield (GY). The 4.5% biochar rate resulted in the highest yields, with a 145.20% increase in GY compared to the control. 7. **Correlation Analysis**: Positive correlations were observed between biochar application and improved soil properties, nutrient availability, and enzyme activities, while negative correlations were noted with soil salinity and Na\(^+\) concentration. These findings highlight the long-term benefits of a one-off biochar application in improving soil quality and rice yield in highly saline-alkali paddy soils, particularly at higher application rates.