In this study, the effects of biochar and compost on the amelioration of coastal saline soil were investigated through indoor leaching experiments and soil culture experiments. The results revealed that the multivoid structure of biochar and compost, when applied to soil, effectively improved soil hydraulic conductivity, promoted the leaching of salt ions, and reduced soil electrical conductivity. Owing to the high pH value of biochar and the lower pH value of compost, the combined application of the two has a complementary effect on improving the pH value of coastal saline soils. The calcium (Ca²⁺) and magnesium (Mg²⁺) contained in biochar and compost are exchanged with Na⁺ adsorbed by soil colloids, which reduces the sodium (Na⁺) adsorption ratio (SAR) value of the soil. Biochar and compost improve the physical properties of the soil, and the organic matter they contain helps soil particles aggregate with each other and form stable clusters, thus promoting the formation of soil agglomerates, which are conducive to the formation of clusters with a diameter of ≤0.25 mm. Biochar and compost are rich in nutrients, and their application significantly increased the contents of available nutrients and organic matter as well as the activities of urease, phosphatase, and dehydrogenase in saline soils. However, too high of an application rate of biochar increases the soil pH value, and excessive application of compost can lead to greater soil conductivity, which inhibits the activities of soil urease, phosphatase and dehydrogenase. Therefore, rational control of application rates is essential for improving coastal saline soils. Future research should further explore the synergistic effects of biochar and compost in improving soil structure, nutrient effectiveness, and microbial activity to promote their effective application in coastal saline–alkaline soil improvement.

Soil salinization is a major global issue that inhibits plant growth, disrupts rhizosphere microbial ecology, and compromises plant health. Utilizing salt-tolerant, plant-growth-promoting microorganisms offers a promising strategy for mitigating salt stress and improving soil productivity. In this study, a salt-tolerant, phosphate-solubilizing fungal strain was isolated from coastal saline-alkaline soil and designated as Penicillium oxalicum PF1. Its salt tolerance and phosphorus mobilizing capacity were characterized, and pot experiments were conducted to elucidate its effects on plant salt tolerance and the underlying mechanisms. The results showed it could survive up to 17 % (w/v) NaCl in culture medium and solubilized different insoluble phosphorus sources in the order Ca-P > Mg-P > Fe-P, with a maximum solubilization of 980.09 mg/L when Ca-P was supplied. Pot experiments and metagenomic analysis revealed that PF1 significantly promoted alfalfa growth in saline soil, it triggered significant restructuring of the alfalfa rhizosphere microbiome, promoting the functional transformation of rhizosphere microbial communities, thereby alleviating salt stress imposed on alfalfa. In summary, P. oxalicum PF1 exhibits robust salt tolerance and high phosphate-solubilizing activity, restructures the alfalfa rhizosphere microbiome, enhances host stress resistance, mitigates salt-induced physiological damage, and ultimately promotes plant growth in saline soil.