The effect of humic acid and inoculation of actinomycetes isolates on phosphorus solubilization in laboratory condition and phosphorus content in maize (Zea mays)

Actinomycetes are among the highly populated microbial groups in the soil, and they have significant positive effects on ecosystem preservation. Providing nutrients, especially phosphorus, in the plant rhizosphere, where its absorption is challenging for plants due to various reasons, is one of the positive effects of using growth-promoting actinomycetes. Humic acid is among the organic compounds that act as growth stimulants. Due to its significant role in soil fertility and the enhancement of soil microbial communities, it promotes the growth and activation of beneficial soil microorganisms residing in the rhizosphere of plants. It also improves the efficiency of phosphorus fertilizers. Therefore, the objectives of this study was as follows: 1) Screening the phosphate solubilization capacity of actinomycete isolates in various culture media, 2) investigating the effect of adding humic acid on soluble phosphorus concentration in different media culture, 3) evaluating the effect of inoculation with selected isolate on growth, physiological parameters, and phosphorus content of the maize variety 'Single Cross 704' in the presence of humic acid and different levels of phosphorus fertilizer.

In this research, a total of 20 actinomycete strains were isolated and purified from various agricultural and horticultural ecosystems in Golestan province, based on their morphological characteristics. These strains were utilized for screening purposes. The phosphate solubilization ability of bacterial strains was assessed in GA, NBRIP, and SMM media culture in the presence of commercial humic acid at a concentration of 0.5%. This experiment was conducted in a factorial design within a completely randomized design, with the following factors: bacterial strain (20 purified strains), culture media (three common phosphorus solubilization media), and humic acid (application, non-application) with three replications under laboratory conditions. To evaluate the effects of a selected actinomycete isolate and its combined effects with different levels of phosphorus and the application of humic acid, a pot experiment was conducted under light and ambient temperature conditions in a completely randomized design with factorial treatments of actinomycete inoculation (control, inoculation), phosphorus fertilizer (control, 20 and 40 kg pure phosphorus per hectare), and humic acid with three replicates on Single Cross 704 maize plants.

The screening results revealed that the NBRIP culture medium was more efficient for phosphorus release, and strains 47, 46, 79, 74, and 24 showed significant increases in phosphorus release. The application of humic acid resulted in an increase in the dissolution of tricalcium phosphate and the release of phosphorus by 11.12-, 118.06-, 4.76-, and 9.69-fold, respectively, in the NBRIP culture medium of the isolates mentioned. Isolate 47 exhibited the highest phosphorus solubility of 247.05 mg/litre with the addition of humic acid, and was selected as the superior isolate for the pot test. The sequencing results revealed that the superior isolate had the highest homology with the Streptomyces chartreusis isolate registered in the NCBI database with accession number KJ152149. The results of the pot experiment indicated that the optimal treatment was the combined application of high phosphorus content (40 mg/kg) together with humic acid and Streptomyces inoculation (P2B1H1), which resulted in the highest amount of shoot biomass (13.97 g per pot), root biomass (8.2 g per pot), plant height (32.26 cm), chlorophyll (32.87 SPAD numbers), and phosphorus content (0.23 percent).

Conclusion:

 Based on the results of this study, inoculation with the screened streptomyces isolate and its combined application with humic acid improved the efficiency of phosphorus fertilizer utilization and provided it effectively for the plant.