Temporal dynamics and optimal dose effects of biochar on soil properties, cotton growth, and bacterial community assembly in saline-alkali soils

Biochar is widely recognized as an effective soil amendment, yet its residual effects on saline soils remain underexplored. This study investigates the residual impact of a single biochar application on soil properties, bacterial community assembly, and cotton growth over three years. A controlled pot experiment was conducted using four biochar treatments − 0%, 1%, 3%, and 5% (w/w) - along with two cotton varieties: salt-sensitive and salt-tolerant. Results showed that biochar reduced soil salinity by 0.80–1.39 g·kg−1 in the second year, with no effects observed in years 1 and 3. By the third year, biochar at 5% increased soil pH by 0.34–0.44 units. Biochar application enhanced soil organic carbon by 135.15%–763.19%, total nitrogen by 12.88%–241.92%, and available potassium by 22.00%–168.84%, in a dose-dependent manner. However, available phosphorus was unaffected by biochar and showed a gradual decline over time. Biochar altered bacterial community composition but had limited impact on alpha diversity. A shift from homogeneous to heterogeneous selection in bacterial community assembly was observed in the third year, correlating with soil nutrient changes. Cotton growth exhibited an “A-shaped” response, with Limited benefits in the first and third years but improvement in the second year. The 1% biochar promoted cotton growth by at least 12.07%, whereas higher concentrations (3% and 5%) had no effect, emphasizing an optimal dose effect. These findings highlight biochar’s potential as a tool for improving saline soils, while underscoring the importance of optimizing application rates and considering its long-term residual effects.