Combined improvement of coastal saline-alkali soils by biochar and Azotobacter chroococcum: Effects and mechanisms

This study presents an innovative approach by combining nitrogen-fixing bacterial strain Azotobacter chroococcum with Solidago canadensis-based biochar to enhance the quality of coastal saline-alkaline soils and promote the resource utilization of invasive alien species, thereby mitigating their environmental impact. The results demonstrated that significant improvements in soil bulk density, water retention, and nutrient retention capacity were realized at the addition concentrations of 2.5 % and 10 mL kg⁻¹ for biochar and Azotobacter chroococcum, respectively. Under 90-day of soil incubation, the co-application of Solidago canadensis-based biochar and Azotobacter chroococcum increased soil organic carbon by 14.65 % and nitrate nitrogen (NO₃⁻-N) content by 60.54 %, compared to individual treatments of biochar. Relative to the bacterial application alone, co-application led to increases of 74.75 % and 18.69 % in soil organic carbon and NO₃⁻-N, respectively, demonstrating a clear synergistic effect. The abundant carbonaceous substances in biochar provided a favorable nutritional environment for Azotobacter chroococcum, significantly increasing the relative abundance of the nitrogen-fixing gene (nifH) and soil nitrogenase activity, thereby enhancing soil nitrogen content. Meanwhile, Azotobacter chroococcum markedly reduced the labile carbon fractions within biochar, thus increasing the overall stability of both the biochar and soil carbon pools. Additionally, the relative abundance of Proteobacteria, Actinobacteria, and Bacteroidetes was increased for enhancing soil nutrient cycling. Redundancy analysis indicated that soil organic matter, Olsen-P, and NO₃⁻-N were the primary drivers of microbial community changes. Variance decomposition analysis revealed that the combined contribution rate of Azotobacter chroococcum and biochar was 27.64 %, exhibiting a significant correlation. These results provide new insights and a scientific basis for sustainable and eco-friendly strategies in coastal saline-alkali soil remediation.