The Assembly of Alginate–Chitosan–Xanthine Hydrogel and Halomonas Improves Soil Quality of Protected Crop Growth Under Saline–Alkali Stress

Soil saline–alkali stress has become a great risk, leading to significant decline in crop yields, especially in protected agriculture that is frequently threatened by excessive chemical fertilization. There is an urgent need to develop friendly strategies for improving the quality of saline–alkali soil. Microbial inoculants are promising agents in attenuating soil stress, but their colonization ability in the crop rhizosphere is often limited. To improve the efficiency of microbial inoculants, this study constructed the assembly of a salt-tolerant bacterium Halomonas (Homs) and a hydrogel composed of sodium alginate, chitosan and xanthine with the assistance of artificial Homs-binding protein (Schx). The effect of this Homs + Schx assembly on the growth of crops and the rhizosphere microecology under saline–alkali stress was systematically evaluated by pot experiments. The results indicate that the application of Homs + Schx enhanced the Shannon index of the rhizosphere bacterial community and increased the relative abundance of key bacterial genera related to biofilm formation and nitrogen cycling (e.g., Curvibacter and Nitrospira). Furthermore, Homs + Schx enhanced the activity of soil urease, peroxidase, and sucrase, reducing the sodium ion content, increasing the potassium ion content, and effectively lowering the soil pH and salt contents. Physiologically, this treatment induced a root osmotic regulatory response, resulting in an increase in the proline contents and a decrease in malondialdehyde contents. Consequently, Homs-Schx promoted the growth of tomatoes and wheat in saline–alkali soil. This study provides new ideas for enhancing the rhizosphere colonization of plant growth-promoting bacteria, regulating the structure of microbial communities, and enhancing crop stress tolerance with the aid of green material strategies.