CO₂ micro-nano bubble irrigation primes the rhizosphere microbiome to enhance soil phosphorus availability and cotton productivity in saline-alkali soils

Saline-alkali soils lock phosphorus (P) availability and crop productivity, highlighting the need for irrigation strategies that improve nutrient acquisition. However, the microbial and enzymatic responses associated with CO₂ micro-nano bubble water (CO₂-MNBW) irrigation remain poorly understood. We evaluated cotton responses to CO₂-MNBW irrigation over three growing seasons (2021–2023) and examined associated soil nutrient and microbial responses in 2023. Under identical irrigation and fertilizer inputs, CO₂-MNBW increased aboveground dry matter by 10.6%–33.1% and seed-cotton yield by 4.1%–6.7% relative to conventional irrigation. These responses were accompanied by reduced soil electrical conductivity and increased rhizosphere nutrient availability. CO₂-MNBW also altered microbial community composition and increased nutrient-acquiring enzyme activities. Core microbial ASVs were associated with soil N and P fractions and enzyme activities, linking community variation with nutrient transformations. Higher alkaline phosphatase activity and lower NaOH-extractable organic P were consistent with enhanced organic P mineralization, while increased alkaline protease and glutaminase activities suggested greater potential for organic N transformation. Exploratory partial least squares path modeling using matched soil and crop observations from 2023 explained 76% of the variation in the rhizosphere cotton productivity construct, with a positive path coefficient from available P ( β = 0.92). These findings highlight coordinated rhizosphere chemical and biological responses associated with improved cotton nutrient acquisition and productivity, supporting CO₂-MNBW as a promising irrigation strategy for saline–alkali cropland.