Enhancing Phosphorus Availability and Altering Soil Biochemical Responses Through Application of Humic Acid and Streptomyces Inoculation

Document Type : Research Article

Authors

• N. Khalili
• R. Ghorbani Nasrabadi
• M. Barani Motlagh
• R. Khodadadi

Department of Soil Science and Engineering, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran

10.22067/jsw.2025.90227.1442

Abstract

Introduction
 Actinobacteria are one of the most abundant microbial groups in soil and play a crucial role in preserving ecosystems. They are among the soil microbial groups capable of releasing phosphorus from low-soluble or insoluble phosphorus sources, which enhances plant growth. Their application in agricultural systems is recognized as an environmentally friendly strategy to limit the negative effects of chemical inputs and improve the availability of nutrients, especially phosphorus, in the rhizosphere. Additionally, humic acid, as an organic growth stimulant, plays an important role in improving soil fertility and biological communities, and its combined use with actinobacteria increases the efficiency of fertilizer use, particularly phosphorus-based fertilizers. Therefore, the aim of this research was: (i) to screen the phosphorus solubilization potential of actinobacteria isolates at different incubation times, (ii) to investigate the effect of adding humic acid on the phosphorus solubilization capacity actinobacteria isolates under laboratory conditions, and (iii) to monitor the impact of selected actinobacteriun isolate and humic acid, at various phosphorus fertilizer levels, on soil phosphorus content, plant phosphorus uptake, and some biochemical properties of the soil.
 
Materials and Methods
In this study, five actinobacteria isolates, collected and purified from various agricultural, orchard, and rangeland ecosystems of Golestan Province, were screened based on their morphological characteristics. These strains were utilized for screening purposes. To prepare fresh cultures of the actinobacteria isolates, they were subcultured on solid yeast extract-malt extract agar medium. The effects of incubation time and the application of humic acid on the phosphate solubilization ability of the actinobacteria isolates were then investigated. This experiment was conducted in a factorial arrangement within a completely randomized design, with the following factors. To examine the effect of the selected superior actinobacterium isolate and its interaction with different phosphorus levels and humic acid application, a factorial pot experiment was conducted in a completely randomized design. The experimental factors included a mineral phosphorus source at three levels (control, 20 kg, and 40 kg of phosphorus per hectare from monoammonium phosphate), Streptomyces inoculation at two levels (control and inoculation with the selected isolate), and humic acid application at two levels (control and 2 mg per kg). The experiment was carried out on maize (Single Cross 704) with three replications. For seed preparation, a sufficient number of healthy maize seeds were selected and surface sterilized by immersing them in alcohol for 30 seconds. They were then exposed to 5% sodium hypochlorite for 2 to 3 minutes, followed by rinsing eight times with sterile distilled water. To prepare the microbial inoculum, the selected superior isolate was grown in yeast extract-malt extract medium at an appropriate (107 CFU/mL). The seeds were then placed in pots, and one milliliter of the Streptomyces suspension was applied to the seeds for inoculation. At the end of the experiment, the phosphorus content in the soil and plant, as well as the soil biochemical responses were measured.
 
Results
Based on the results obtained from this study, the application of humic acid led to an increase in microbial biomass and enhanced phosphorus release by actinobacteria isolates under laboratory conditions. As the incubation period extended from 7 to 14 days, the solubility of phosphate showed an increasing trend. The results showed that the highest phosphorus content in the soil was associated with the combined application of a high phosphorus level (40 mg per kg) along with humic acid and Streptomyces inoculation. Analysis of microbial biomass phosphorus revealed that the highest level was related to the treatment combining the highest level of phosphorus fertilizer and humic acid. According to the findings related to phosphatase enzymes, the combined application of the Streptomyces treatment, humic acid, and phosphorus resulted in an increase in the levels of these enzymes. Additionally, the results of microbial respiration in the soil indicated that the combined treatment of Streptomyces and the highest level of phosphorus fertilizer enhanced microbial respiration in the soil. The phosphorus content in the plants under the combined treatments of Streptomyces, humic acid, and phosphorus showed that the integration of Streptomyces inoculation and humic acid was effective in improving soil phosphorus availability and led to an increase in the phosphorus content of the plants. The results of this study showed that inoculation with the selected Streptomyces isolate, along with the combined application of humic acid, enhanced the efficiency of phosphorus fertilizer utilization, making it more readily available to the plant.
 
Conclusion
 In general, the results of current study revealed that the simultaneous application of humic acid and Streptomyces inoculation led to an increase in the availability of phosphorus in the soil and the phosphorus content in the plants, as well as an improvement in the biochemical responses of the soil. However, field experiments are necessary to confirm its effectiveness.

Keywords

• Actinobacterium
• Microbial biomass phosphorus
• Microbial respiration
• Phosphatase

Main Subjects

• Soil science

©2025 The author(s). This is an open access article distributed under Creative Commons Attribution 4.0 International License (CC BY 4.0).

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