Objective Salinization inhibits plant growth. Due to the unique properties, biochar has great potential in the improvement of saline-alkali soils. This study investigates the response of mulberry seedlings to salt stress under the addition of biochar, aiming to provide scientific guidance and theoretical support for the improvement of saline-alkali soils and the precise addition of biochar.

Method A pot experiment was conducted with five biochar mass fraction gradients (0%, 1%, 3%, 5%, and 7%) and three NaCl concentrations (0 g·kg⁻¹, 1.5 g·kg⁻¹, and 3 g·kg⁻¹). Morphological characteristics, chlorophyll and proline contents, and other indexes of mulberry seedlings were measured. Salt tolerance was evaluated using the membership function method.

Result Soil salinity stress significantly inhibited the growth of mulberry seedlings, and biochar alleviated the inhibitory effect by reducing the content of malondialdehyde. Under higher salt stress, chlorophyll content reached its peak with 5% biochar addition; while under mild salt stress, the leaf area increased by 21.52% with 3% biochar addition, and the proline content of mulberry seedlings was the lowest, which was 24.32% lower than the control, which indicated that 3% biochar addition is the optimal dosage for mulberry seedlings to improve the tolerance to salt stress. Membership function analysis showed that under mild salt stress, the 3% biochar addition yielded the highest tolerance, while under higher salt stress, 1% and 5% treatments were more effective.

Conclusion Biochar enhances the salt tolerance of mulberry seedlings by regulating the malondialdehyde accumulation and chlorophyll synthesis. The optimal biochar addition rate is 3%~5%, which can provide the basis for precise addition in saline-alkaline soils.