Soil science
N. Khalili; R. Ghorbani Nasrabadi; M. Barani Motlagh; R. Khodadadi
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 ...
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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 MethodsIn 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. ResultsBased 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.
R. Khodadadi; Reza Ghorbani nasrabadi; M. Olamaee; S.A. Movahedi Naini
Abstract
Introduction: Worldwide studies have shown that inappropriate land uses over the past 45 years have resulted in salinization of 6% of the world's land. Salinity has negative effects on soil physicochemical properties and microbial activities. The imbalance in nutrient uptake, ion toxicity and ...
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Introduction: Worldwide studies have shown that inappropriate land uses over the past 45 years have resulted in salinization of 6% of the world's land. Salinity has negative effects on soil physicochemical properties and microbial activities. The imbalance in nutrient uptake, ion toxicity and decreasing water consumption due to high osmotic pressure are resulted from high accumulation of solutes in soil solution. One of the strategies to mitigate soil salinity is the inoculation of crops with different types of beneficial soil bacteria and fungi. Plant growth promoting bacteria (PGPB) are a diverse group of bacteria capable of promoting growth and yield of many crops. The most important growth promoting mechanisms of bacteria are the ability to produce plant hormones, non-symbiotic nitrogen fixation, solubilization of insoluble phosphate and potassium, biocontrol of plants pathogens through producing hydrogen cyanide and siderophore production. Plant inoculation with growth promoting bacteria causes an increase in several indices such as shoot fresh and dry weight, root dry weight and volume as well as chlorophyll content. The synergetic effect of Azotobacter and Azospirillum on the plant has been documented by increasing the absorption of nutrients, production of hormones that stimulate plant growth such as auxin, and influencing the root morphology. Due to the wide area of saline soils, appropriate methods to reduce the negative effects of salinity are of great significance. Given the importance of using bacteria adapted with climatic conditions and soil ecosystems in each region, as well as the efficiency of the combined application of growth promoting bacteria, this study was conducted to investigate the effect of growth promoting bacteria as a single and combined application at two levels of salinity calculated based on the threshold of barley yield reduction (Karoon cultivar) and 50 % reduction in barley yield.
Materials and Methods: In order to record the Azotobacter isolates, 15 soil samples were collected from salt affected lands of Golestan province. Thirty two Azotobacter isolates were isolated by physiological and biochemical tests and cyst production in old culture. Then, their ability to grow in different concentrations of salinity, drought stress tolerance, polysaccharide production, auxin production, phosphorus and potassium solubilization, hydrogen cyanide synthesis and biological fixation of molecular nitrogen were investigated. Based on physiological and growth stimulation tests, Az13 isolate was selected as the superior isolate of Azotobacter for greenhouse test. Azospirillum superior isolate was then prepared from the microbial bank of Soil Science Department, Gorgan University of Agricultural Sciences and Natural Resources. A soil with 16 dS/m salinity was selected to determine the effects of experimental treatments at two threshold salinity levels of yield reduction and 50 % reduction of barley yield. Then, soil salinity was reduced to 8 dS/m (yield reduction threshold) by leaching. After reaching to the desired salinity, the soil was removed from the pots and air dried. The sample was sifted through a 2 - mm sieve and again transferred to the pots. The barley seeds, Karoon cultivar, were used. To prepare the inoculum, firstly the bacterial isolates were grown in the pre-culture nutrient broth medium, and then incubated at 120 rpm in a shaking incubator at 28°C for 48 hours. Afterwards, each seed was inoculated with one milliliter of the bacterial inoculant with a population of 109 CFU/ml. This experiment was conducted as factorial in a completely randomized design with three replications in the greenhouse at Gorgan University of Agricultural Sciences and Natural Resources. The treatments included four levels of bacteria (without inoculation, Azotobacter inoculation, Azospirillum inoculation, combined inoculation of Azotobacter and Azospirillum) and two levels of salinity (8 and 16 dS/m). After 70 days (late vegetative growth period), some growth and physiological indices and concentration of nutrients uptake were measured.
Results and Discussion: The results showed that salinity stress had a significant (p < 0.01) negative effect on growth and physiological traits and nutrient uptake of the plant. The combined application of Azotobacter and Azospirillum bacteria showed a positive significant influence (p < 0.01) on growth, dry weight, and root dry weight in the plant under salinity stress. The combined application of bacteria increased the chlorophyll a, b and a + b content at a salinity level of 16 dS/m by 136.49, 117.86 and 127.97 %, respectively. The combined application of bacteria resulted in a 65.39 and 55.94 % increase in proline amino acid content at salinity levels of 8 and 16 dS/m, respectively. The results revealed that nitrogen, phosphorus and potassium levels increased by 81.97, 80 and 66.67%, respectively, at 16 dS/m salinity level in combined application of both bacteria. Sodium ion accumulation in all bacterial treatments decreased in both salinity levels compared to control treatment and the highest reduction was observed in combined bacterial inoculation. These findings underline the positive effect of bacterial inoculation, particularly their combined application, on the growth and nutrients uptake of barley under salt stress.
Conclusion: Our results indicate that increasing salinity level significantly decreased shoot dry weight, root dry weight, plant height, chlorophyll content and nutrient concentrations of barley. Inoculation of salt-resistant bacteria, including Azotobacter and Azospirillum, reduced the adverse effects of salinity on growth and physiological traits, which was more pronounced in Azotobacter than Azospirillum. The combined application of Azotobacter and Azospirillum had a significant effect on root dry weight, plant height, chlorophyll content, increasing nutrient concentration efficiency (nitrogen, phosphorus, and potassium) and decreased sodium concentration at both salinity levels (8 and 16 dS/m) compared with the individually inoculated bacteria. Hence, the application of Azotobacter and Azospirillum isolates is an appropriate method for pot experiments with saline soils. To apply these results, field experiments in saline soils must be carried out to evaluate the effect of these bacterial isolates on the crop growth, yield and physiological characteristics.
