From iron to sugar: How aerated drip irrigation increases soluble sugar content in citrus fruit through soil microbial communities

Abstract

Root-zone hypoxia is common in citrus orchards on red soils and limits sugar accumulation in fruit. Aerated drip irrigation (ADI) may improve the soil redox environment, but whether it regulates fruit soluble sugar through microbially driven iron transformation is unclear. A field experiment was conducted in a citrus orchard in Nanfeng County, Jiangxi Province, China. The soil was a red soil derived from Quaternary red clay, strongly acidic and rich in free iron oxides but low in available iron. Non-aerated irrigation served as the control. Three ADI treatments delivered dissolved oxygen in the irrigation water of 10, 20, and 30 mg·L −1 . Soil iron fractionation, metagenomic sequencing, and untargeted metabolomics were combined to examine how ADI affects soil iron transformation, microbial communities, and fruit quality. ADI increased soil Eh and DTPA-Fe while decreasing Fe²⁺, thus raising iron availability in the root zone. It also enriched Proteobacteria and siderophore synthesis and transport genes, including entB , fepA , and pvdA . The 20 mg·L −1 treatment showed the highest microbial network connectivity and greatest abundance of key taxa. The 20 mg·L −1 further increased ascorbic acid and glutathione, improved fruit redox homeostasis, and promoted accumulation of metabolites in glycolysis and starch-sucrose metabolism. As a result, soluble sugar content rose by 14.2%. Structural equation modeling confirmed that ADI raised iron availability by altering the redox environment and microbial communities, thereby improving citrus redox homeostasis and soluble sugar. ADI is thus an effective practice for improving the root-zone environment and fruit quality, with 20 mg·L −1 being optimal. These results identify the optimal dissolved oxygen concentration for high-quality citrus cultivation under ADI.