Impacts and mechanisms of nanobubbles level in drip irrigation system on soil fertility, water use efficiency and crop production: The perspective of soil microbial community

Rhizosphere hypoxia severely inhibited plant growth, reducing the crop yield and water use efficiency (WUE) in greenhouse crops. Irrigation using nanobubbles (NBs) has been an efficient method to increase crop yield and WUE by ameliorating hypoxic conditions and promoting plant growth. However, the potential effects and mechanisms of different oxygen concentrations in water enriched in nanobubbles (NBW) remain unclear. Herein, this study examined the influence of different total oxygen concentrations (TOCs, ranged between 160 and 280 mg L-1) on tomato agronomic performance, soil fertility, and the bacterial community after a two-season of NBW irrigation. The results indicated that the tomato yield, WUE, soluble sugar, and vitamin C were significantly improved by NBs, showing the trends of rising to decrease with the corresponding oxygen concentration in irrigation water. The total C (TC), organic matter (OM), available N (AN) and K (AK) in soil tended to increase and then decrease with TOC in irrigation water. NB220 obtained the highest soil OM and AN values, and NB250 obtained the highest TC and AK values. The soil bacterial community composition gradually differentiated with increasing oxygen concentration. There were significant differences in relative abundances of Proteobacteria and Nitrospirae across the six oxygen levels. The keystones in the co-occurrence network were dependent on oxygen levels. FAPROTAX results revealed that bacterial functions of methanol oxidation, N fixation, aerobic chemoheterotrophy, and cellulolysis were more abundant in NB220 treated soils, consequently resulting in better crop yield, WUE, and soil fertility. Overall, the increased abundance of taxa participating in soil nutrient turnover contributed to improved soil fertility and crop agronomic performance. Our findings provided a significant opportunity to advance the understanding of oxygen threshold during aerated irrigation, with implications for green and efficient agricultural production.