Deciphering the performance and mechanisms of glycolipids in regulating crop growth in coastal saline-alkali soils: Perspectives on soil properties and microbial communities

Coastal regions face significant challenges in crop production due to soil salinization. Rhamnolipids and sophorolipids are prototypical glycolipid biosurfactants that exhibit considerable promise in soil amelioration and crop growth regulation. Nevertheless, the efficacy and mechanisms of these glycolipids in coastal saline-alkali soils remain poorly understood. In this study, a pot experiment was carried out to examine the impact of rhamnolipids and sophorolipid supplementation at different doses (i.e., 50, 100, and 200 mg/kg) on oleic sunflower growth. Both biosurfactants at appropriate doses effectively reduced the soil alkalinity, elevated the levels and bioavailability of soil nutrients (i.e., available nitrogen, phosphorus, and organic matter), and improved enzyme activities (i.e., catalase, urease, beta-glucosidase, and alkaline phosphatase). Notably, the application of medium-dose rhamnolipids resulted in the most substantial enhancement of crop growth (up to 117 % increase in fresh weight). Besides, an appropriate dose of rhamnolipids enriched the beneficial microbiota (e.g., Glycomyces, Stachybotrys, and Mortierella) while inhibiting the growth of pathogens (e.g., Fusarium) in rhizosphere zones. Conversely, a high dose of sophorolipids accumulated plant pathogens in both the rhizosphere and bulk soils. Correlation-based network analysis and microbial function predictions revealed that the intervention of medium-dosed rhamnolipids enhanced the cooperation within bacterial and fungal communities and activated the community functions related to carbon and nitrogen cycling. Moreover, the increased soil enzyme activity was the main driver for improved crop growth in rhamnolipids-supplemented treatments. The findings provide new insights into the underlying mechanisms of glycolipids-assisted improvement of crop growth in coastal saline-alkali regions.