A Bacterial–Microalgal–Manure Co-Application Ameliorates Saline-Alkali Soil and Promotes Wheat Growth

Severely saline–alkaline land degradation poses a considerable challenge to sustainable agriculture, owing to high salinity, elevated pH, and nutrient deficiency. To address this, a salt-tolerant nitrogen-fixing bacterium (Bacillus sp.) and a microalga (Chlorella pyrenoidosa) were applied—alone, in combination, or with sheep manure—in a pot experiment with six treatments to examine their individual and combined effects on soil amelioration and wheat (Triticum aestivum L. cv. Jinchun 6) growth. We specifically assessed whether the three-component system outperforms single or dual applications. The bacterial–algal co-inoculation (BA) markedly outperformed single inoculations: shoot biomass increased by 117% and soil organic matter (SOM) by 130%, compared with the control. BA also alleviated oxidative stress, as evidenced by reduced malondialdehyde (MDA) content and elevated superoxide dismutase (SOD) and peroxidase (POD) activities. Scanning electron microscopy (SEM) observations confirmed tight bacterial attachment to algal surfaces. Incorporating sheep manure (BAM) further enhanced these benefits, achieving the lowest pH and electrical conductivity (EC), the highest SOM and available-nutrients, and the greatest wheat biomass. 16S rRNA sequencing showed that BAM increased microbial diversity, shifted community structure, and enriched beneficial genera (Sphingomonas, Flavihumibacter, and Fuscovulum) that were positively correlated with soil nutrient availability and plant stress tolerance, while the halophilic genus Halomonas declined. Collectively, the bacteria–algae–manure co-application establishes positive feedback between soil improvement and functional microbiome recruitment, offering a promising strategy for the remediation of severely saline–alkaline soil.