[Microbial Community Structure and Functional Genes of Phosphorus Cycling in Cotton Field Soil Under Long-term Saline...

Freshwater resources are scarce in arid regions, and the rational use of brackish water resources can alleviate local freshwater shortages, but long-term brackish drip irrigation increases the risk of soil salinization, which in turn affects soil nutrient transformation and microbial diversity. Soil phosphorus availability is critical for crop growth, yet it is unclear how long-term brackish drip irrigation will affect soil phosphorus transformation. Therefore, to investigate the effects of long-term brackish drip irrigation on soil phosphorus-transforming microorganisms and their functional genes in cotton fields, the experiment was set up with two irrigation water salinities, freshwater （0.35 dS·m^-1, FW） and brackish water （8.04 dS·m^-1, SW）. The results showed that long-term brackish drip irrigation significantly decreased cotton dry matter weight, phosphorus uptake, yield, soil pH, and Ca₂-P and Ca₁₀-P contents but significantly increased cotton phosphorus content and soil water content； electrical conductivity； quick phosphorus； and Ca₈-P, Al-P, Fe-P, and O-P contents. The dominant species in each treatment at the phylum level were Ascomycetes, Actinobacteria, Acidobacteria, Bacillus, and Greenscapes； and at the phylum level, the dominant species were α-Ascomycetes, Actinobacteria, β-Ascomycetes, Oleococcus thermophilus, and γ-Ascomycetes. including Proteobacteria, Actinobacteria, Acidobacteria, Gemmatimonadetes, and Chloroflexi. Select dominant species at the class level included Alphaproteobacteria, Actinomycetia, Betaproteobacteria, Thermoleophilia, and Gammaproteobacteria. Long-term saline drip irrigation significantly reduced the relative abundance of Actinobacteria, Acidobacteria, and Nitrospirae but significantly increased the relative abundance of Proteobacteria, Gemmatimonadetes, and Bacteroidetes and significantly reduced the expression levels of the organic phosphorus mineralization gene phnA, transport gene pit, and polyphosphate synthesis gene ppaC. Moreover, it significantly increased the expression levels of the polyphosphate degradation gene HDDC3； organic phosphorus mineralization genes phnG, phoA, phnH, phnL, phnM, phnN, phnP, and phnW； transport genes phnK, phnE, phnC, and phnD； and the regulatory gene phoB. Correlation analysis showed that soil phosphorus-cycling microorganisms and functional genes were closely related to soil physicochemical properties and soil inorganic phosphorus content. Therefore, long-term saline drip irrigation changes the composition of soil phosphorus-cycling microorganisms by affecting soil physical and chemical properties and inorganic phosphorus content, which in turn drives the expression of phosphorus-cycling-related functional genes to regulate and adapt to salt stress.