Effects of Biochar on the Yield of Melon and the Diversity of Rhizosphere Soil Microbial Communities Under Saline–Alkali Stress

In this study, the melon variety 'Da Shetou' was used as the test material, and pot cultivation was employed with soil collected from Da'an City to investigate the effects of biochar addition on melon yield and quality, rhizosphere soil physicochemical properties, and soil microbial community. The experiment was set up with five treatments: saline-alkali soil (B0), 1% biochar and 99% saline-alkali soil (B1), 3% biochar and 97% saline-alkali soil (B3), 5% biochar and 95% saline-alkali soil (B5), and 7% biochar and 93% saline-alkali soil (B7). This study found that the addition of 3% biochar increased the fruit yield of melons. Compared to the control, the soil bulk density was reduced by 4.99%, 8.66%, 1.77%, and 7.71% under the 1%, 3%, 5%, and 7% biochar treatments, respectively. Biochar addition increased organic matter, alkaline-hydrolyzable nitrogen, available phosphorus, and available potassium concentrations in the rhizosphere soil. Additionally, the total nitrogen, salt concentration, and exchangeable sodium percentage were also reduced. Compared to the B0 treatment, the concentrations of K⁺, Ca²⁺, and Mg²⁺ increased to varying degrees across different treatments, while the concentrations of Na⁺ and Cl^- decreased. The relative abundance of dominant bacterial phyla in the soil varied across different treatments. The dominant bacterial phyla included <i>Proteobacteria</i>, <i>Actinobacteriota</i>, <i>Acidobacteriota,</i> and a total of 10 others. The dominant fungal phyla included <i>Ascomycota</i>, <i>Basidiomycota</i>, <i>Mortierellomycota,</i> and a total of seven others. Redundancy analysis (RDA) identified key drivers. Available potassium in the rhizosphere soil of melons was the dominant factor influencing bacterial community composition at the phylum level. Soil bulk density, exchangeable sodium percentage, and total nitrogen were identified as the dominant factors influencing fungal community composition at the phylum level. This study confirmed that 3% biochar application synergistically regulated nutrient cycling and microbial functional groups, thereby enhancing yield of thin-skinned melons (yield increase: 45.22%).