Effects of different water and fertilizer management measures on the growth of safflower and screening and identification of phosphorus-solubilizing bacteria in safflower rhizosphere

Abstract:

[Objective] To study the effects of different water and fertilizer conditions on the biomass of safflower and the phosphorus and microorganisms in the rhizosphere soil, and isolate the strains with high phosphorus-solubilizing ability from the rhizosphere soil samples, so as to provide a theoretical basis for the reasonable water and fertilizer management of safflower and excellent strains for the research on safflower growth and rhizosphere microenvironment. [Methods] The safflower plants were treated with phosphorus fertilizer at different gradients, and the plant biomass was determined at the rosette, jointing, blooming, and seed maturity stages. At the same time, the microbial community composition, total phosphorus, available phosphorus, and phosphatase activity in the rhizosphere soil were determined. The strains with high phosphorus-solubilizing ability were screened by soil shaking and dilution coating methods and preliminarily identified by comparative analysis of 16S rRNA gene sequences. The phosphorus-solubilizing abilities of the strains in different media were determined by the molybdenum antimony colorimetric method. Furthermore, we inoculated the dominant strains by using the root irrigation method and dilution coating method to analyze the abilities of the strains to colonize the rhizosphere and promote the growth of safflower. [Results] W3-P2 water and fertilizer treatments were conducive to the biomass accumulation of safflower. As the application amount of phosphorus fertilizer increased, the available phosphorus content and phosphatase activity showed a trend of first increasing and then decreasing. The effects of water on soil total phosphorus, available phosphorus, and phosphatase activity were correlated with the safflower development stages. Bacteria were the dominant microorganisms in the rhizosphere soil of safflower, with the highest counts of 3.017×10⁷ CFU/g and 3.021×10⁷ CFU/g at the seed maturity stage in the water treatment and phosphorus fertilizer treatment groups, respectively, which were much higher than those of fungi and actinomycetes in the same treatment group. Five efficient phosphorus-solubilizing strains (accession number C1:OR493125; C2:OR493126; C5:OR493127; C6:OR493128; C7:OR493129) were screened out from the rhizosphere soil of safflower. All of them had the ability to solubilize phosphorus and lowered the pH of the medium with inorganic and organic phosphorus as the only phosphorus source. Strain C6 had the strongest ability to solubilize phosphorus, with the amounts of phosphorus solubilized in the inorganic phosphorus media of tricalcium phosphate, aluminum phosphate, ferric phosphate, and calcium phytate being 380.00, 269.32, 7.15, and 48.16 mg/L, respectively. It solubilized 18.19 mg/L phosphorus in the medium with organic phosphorus (lecithin) as the only phosphorus source. C6 was identified by 16S rRNA gene sequencing analysis as Pseudomonas sp. and C1, C2, C5, and C7 as Sinorhizobium sp. After inoculation of 2% suspensions (10⁸ CFU/mL) of the dominant phosphorus-solubilizing bacterial strains C1, C5, and C6 around safflower plants, the cell count remained 10⁵ CFU/g on day 21, and C6 demonstrated stronger colonization. The strains increased the leaf number, plant height, stem diameter, stem weight, and root length of safflower at the blooming stage, and the C6 strain group presented stronger effects, with the indicators above being 122 tablets, 115.96 cm, 12.49 mm, 43.36 g, and 21.17 cm, respectively. [Conclusion] Water and fertilizer management affects the available phosphorus content and microbial community structure in the rhizosphere microenvironment of safflower and promotes the growth and development of safflower roots, thus directly or indirectly affecting the biomass of safflower. The irrigation at 3 000 m³/hm² and phosphorus fertilizer application 375 kg/hm² were suitable for the growth of safflower. C6 is an efficient phosphorus-solubilizing strain, capable of decomposing insoluble organic and inorganic phosphorus. Pot experiments showed that C6 could colonize the rhizosphere of safflower and significantly promoted the growth of safflower.