1. Brief introduction
Onions are one of the most important commercial crops and are ranked as the second most economically valuable after some grown vegetables. This plant has a unique flavor and is used in dishes, soups, sandwiches, and salads, and can also be cooked as a vegetable on its own.
Onions are eaten in the form of dried bulbs or at the young green stage when ripe. Ripe dried bulbs contain protein, starch, sugar, and some vitamins.
In addition to its nutritional value, onion has shown some medical applications, as it contains several anti-cancer drugs that have been shown to prevent cancer in animals.
In general, the edible part of the onion plant contains volatile sulfur-containing compounds that have a unique flavor and are responsible for the pungent odor.
Bioactive organosulfur compound, S-alkyl-L-cysteine sulfonate oxide dominates onion flavor. In addition, sulfoxide is found in the cytoplasm and in the vacuum. These compounds are responsible for the characteristic smell and taste of onions and most of their biological properties.
Currently, chemical fertilizers are used to increase the yield of most crops and to meet the growing demand for food due to population growth. Unfortunately, these fertilizers are expensive to produce and exhibit harmful effects on the environment.
As a result, interest in finding alternative green sources such as biofertilizers as environmentally friendly systems that can increase crop yields and reduce production costs is growing and will continue to rise.
Recently, the use of microbial populations as biofertilizers has emerged as a promising alternative to these chemicals for low-cost production, enhanced environmental sustainability, and increased yields.
Several reports have shown that clumped mycorrhizal fungi increase water and nutrient uptake and increase phosphorus bioavailability to host and improve plant growth.
AMF has synergistic effects with other microorganisms and has increased yields in several crops. In general, plant-AMF interactions are important associations in the rhizosphere with improvements in soil physical, chemical, and biological properties.
AMF operates in rhizosphere soils using a variety of different mechanisms. This nutrient dissolves phosphorus by producing organic acids. The use of AMF as phosphate-solubilizing microorganisms has been proposed as a low-energy and low-cost mechanism to improve the effectiveness of phosphate fertilizers.
Nitrogen fertilizers are also an important part of agriculture-related pollution through nitrate leaching, ammonia volatilization, and nitrous oxide emissions;
This paper is the first to evaluate the role of AMF in reducing phosphorus fertilization, improving nitrogen use efficiency, and increasing onion plant yield in sandy soils under drip irrigation systems. We also evaluated root colonization rates and their potential to improve rhizosphere P levels over two consecutive growing seasons.
2. Materials and methods
Onion seeds were purchased from the Giza Agricultural Research Center in Egypt. In this study, the field soil was recovered from sandy soil. It underwent chemical, physical, and mechanical analysis.
The spores of AMF were extracted from the rhizosphere of fertile soil grown with onion plants at the Nuclear Research Center of the Egyptian Atomic Energy Authority in Insath, Egypt.
Gerdemann and Nicolson believe that mycorrhizal spores are extracted by wet sieving and pouring techniques. Extracted AMF spores are stored at 4 °C until use.
The morphological characteristics of attached hyphae, hyphae spores, chlamydia and sporozoida of the collected AMFs were extensively studied. Identification and characterization were carried out based on the key of Schenck and Perez.
AMF inoculation is carried out after each season as follows: 100 ml of spore suspension is added to 1 kg of sterilized soil, evenly distributed over the entire plot, mixed with the soil by tillage at the time of planting.
During the two growing seasons of 2018/2019 and 2019/2020, two field experiments were conducted at the experimental farm of the Nuclear Research Center to study the effect of clumping mycorrhizal fungal inoculation interacting with mineral P fertilizers on onion plant growth and yield.
The onion seeds were planted in the nursery for 55 days and then transplanted into the field. The transplant settles on both sides of the irrigation line and includes a 25 cm dropper. The experimental area was divided into plots of equal size, with a plot size of 3 square meters.
The experiment consisted of three levels of monocalcium phosphate fertilizer (15.5% P2O5) as follows, 40, 80 and 120 kg ha-1 with or without AMF inoculation. In the form of ammonium sulphate (21.2% N), nitrogen fertilizer was added at a rate of 140 kg per hectare in two equal doses.
The first started 21 days after the transplant and the second 21 days after the first transplant. SO4 enriched with a surplus of 5% N atoms was mixed with ordinary fertilizer at 0.30 square meters to measure the efficiency of nitrogen utilization by nitrogen gas.
Root samples were collected during the two seasons of the experiment at 30, 60 and 90 days post-transplant. The collected samples were stained according to the method described by Phillips and Heyman.
Briefly, the roots are washed in sterile distilled water, softened in KOH (10%) for 24 h, and washed in sterile distilled water. After that, the roots are acidified with lactic acid (5%) for 1 h at room temperature.
It is then stained with aniline blue (0.01%) in pure lactic acid for 24 h. Finally, the roots are cut into 1 cm sections and mounted on a slide of glycerol.
3. Effect of phosphorus and AMF inoculation on root colonization
The effects of AMF inoculation on root colonization were recorded 30 days, 60 days, and 90 days after transplantation. The data showed a microscopic visualization of AMF, showing the clumps, spores, and hyphae in the roots of the onion. The results obtained clearly show that the roots are not colonized after 30 days.
The results confirmed that after 60 days of planting, the colonization of AMF in the roots of the onion reached 70% at the level of 80 kg P fertilizer. Similarly, the results showed that after 90 days, after two growing seasons, the AMF colonization rate increased the fastest, reaching 81 percent.
Inoculation of AMF and application of P fertilizer together are more effective in enhancing root colonization. During the two growing seasons, the highest rate of AMF infection was recorded for onion plants using 80 kg of fertilizer. When the 60-day or 90-day increase exceeds 80 kg, mycorrhizal colonization in the roots decreases.
IV. Effect of phosphorus-fertilized AMF inoculation on P availability in the rhizosphere
The data list the effects of different P levels of AMF on P availability in the soil rhizosphere after 90 days of planting. The data obtained suggest that inoculation with AMF increases the concentration of P in the soil compared to the control treatment.
The data obtained further suggest that an increase in phosphorus fertilization is accompanied by an increase in P recorded in the rhizosphere. The highest concentration of available P in the rhizosphere (54.39% in the 2018/2019 season and 76.13% in the 2019/2020 season) was 120 in both growing seasons
The maximum usable P in 120 kg of soil was 76.13%, while the control group was at the same level of 20.20% in the second season. Similar results were noted in the first season to a low degree.
The data clearly show that the fresh and dried weight of onion plants responds positively to AMF inoculation at each mineral phosphate level compared to the control group.
The recorded fresh and dry weight values give a maximum of 120 kg inoculated mycorrhizae as the recommended dose. With the increase in phosphate fertilizer levels between 80 kg of grade 2 and 120 kg of grade 3, there is no significant difference in this observation.
Comparatively, mycorrhizal inoculation resulted in an increase in fresh weight (34.8, 22.7 and 11.7% in Season 1 and 23.1, 22.4 and 12.6% in Season 2), and mycorrhizal inoculation exceeded the control of the concentrations of the three respective P fertilizers.
During the 2018/2019 and 2019/2020 growing seasons, the dry weight per plant was increased by increasing the phosphate fertilizer rate from 40 to 120 kg. The highest dry weights were 33.7 and 32.6 in the first quarter and 41.0 and 35.3 in the second quarter.
With or without mycorrhizal inoculation, all different levels of phosphorus fertilization affect the total chlorophyll content. The results obtained further indicate that the chlorophyll content in the inoculated plants is slightly higher than in the control group.
With up to 120 kg of phosphate fertilizer applied in mycorrhizal inoculation, the maximum average of total chlorophyll values in the first and second seasons was equal to 56.2 and 56.0, respectively.
5. The impact of phosphorus and AMF inoculation on economic output
The data results clearly show that AMF inoculation and P fertilization have a significant effect on onion bulb yield. Over the two growing seasons, onion bulb yield increased linearly with increasing levels of phosphate fertilizer inoculated with AMF.
The increase in phosphate fertilizer levels gradually increased the economic yield of onion plants throughout AMF inoculation. The maximum yields of AMF inoculation at the 120 kg level were 32.69 and 32.12 tonnes in the two seasons, respectively, but there was no significant difference in the 80 kg and 120 kg applications in the two growing seasons.
6. Conclusion: AMF can increase onion yield under drip irrigation
This study showed that in two consecutive seasons, AMF could increase onion yield with P fertilizer by secondary and tertiary stages under drip irrigation. Since the fertility root zone can improve nutrient uptake, then planting onion plants in the field may be effective for onion plant production before inoculating the soil with AMF.
With an increase of 120 kg of AMF inoculated with P fertilizer, it is recommended to increase the yield (dry bulb weight) significantly more. These findings suggest that in the near future, AMF will be an emerging and promising candidate country in sustainable agriculture as an alternative green technology that minimizes the use of synthetic fertilizers.