Composting refers to the dynamic process of converting macromolecular organic substances in organic solid waste into carbon dioxide, water, NH₃, and humus substances through microbial metabolism in a high-temperature.³³ One of the main challenges for crop production in sandy soils is the limitations of available nutrients. These soils suffer from continuous and significant losses in nutrients, and this might negatively affect plant growth. To improve the physical and chemical characteristics of these soils, organic compost is recommended.³⁴

Moringa leaves were collected mixed with soil and fermented for about 3 months with watering and shaking until a black color and good smell were obtained. Compost’s chemical and microbial properties, including pH, electrical conductivity (EC), temperature, moisture, organic matter, and mineral content were summarized in Table 1. The color was brown and the compost temperature was 30-35°C, pH 7.5, EC was 4.16 dS m^-1 with 45% moisture content while mineral contents were summarized in Table 1. Compost contained the highest count of true bacteria (2.201×10⁷ CFU/g) followed by thermophilic bacteria (0.113 x10⁷ CFU/g), then actinomycetes (0.009 x10⁷ CFU/g) and fungi (0.006 x10⁷ spore/g). The lowest counts were recorded for P-solubilizing bacteria (2.111×10⁴ CFU/g) and Free-living N-fixing bacteria (0.003 x10⁴ CFU/g).

Table (1):
Compost chemical and microbial properties, including pH, electrical conductivity (EC), temperature, moisture, organic matter, mineral content and microbial counts

Data are means of three replicates ± standard deviation.

The pH level is an essential parameter during composting.³⁵ Microbial activities during the composting process are influenced by time. Typically, compost has a pH ranging from 6 to 9.5.³⁶ The EC of compost is a crucial salinity indicator and indicates its suitability for use. EC typically rises during composting as a result of organic matter degradation, which produces inorganic compounds. The EC values of the mature composts fell within the previously documented range of 2.8-10.1 dS m^-1 for composts derived from solid fraction wastes. One of the most important elements for monitoring the composting process is temperature. Composting is an exothermic process that is affected by the starting substrate, temperature and biodegradability of the microbe present.³⁷ Under suitable moisture content, Moringa leaves give a good bioavailability to composting by soil microorganisms with high metabolic activities.³⁵ It is reported that under high moisture content, bacteria and fungi convert wastes into humus.¹³ The ideal moisture content was 45%-60% is the most suitable and 53% gives the maximum results and the longest duration of the thermophilic period (15 days) which enhances nitrogen content.³⁸ Moringa wastes were degraded by microbial activity to compost with a high decomposition rate which was noticed by the reduction in the pile volume and change in color. The detected organic matter was 27%, while the high-quality compost is in the range of 25-80%.³⁹ Aeration and the produced high temperature are required to enhance compost formation.⁴⁰ Carbon, nitrogen, and carbon/nitrogen ratio were important measured for the compost and the C/N ratio in compost may vary depending on the bioavailability of carbon and nitrogen.⁴¹ The high carbon content may be due to the high presence of lignin, cellulose, and hemicelluloses complex structures which are difficult to degrade.⁴² Moreover, the detected total nitrogen content (2.7%) is lower than that estimated which was 3.2%.⁴³ It was documented that compost with a C/N ratio of 15–20 is satisfactory as a good nitrogen source while low C/N ratio are due to carbon release as CO₂.⁴⁴ Microorganisms need a C/N ratio of approximately 10, which is optimal for their metabolism. Microorganisms require carbon, nitrogen, phosphorus, phosphate, and potassium as primary nutrients for optimal microbial activities. Moringa wastes stayed only for three months to be composted but in contrast to our study, it may stay for a year or even more without complete decay and extending composting time is necessary to produce compost with high nitrogen content.

Compost is a rich source of P and other nutrients e.g., Ca, Fe, and N (Table 1) which were essential elements for numerous physiological functions of the plants.⁴⁵ In this work, the most abundant bacteria that were noticed during compost preparation were selected and counted using different agar media. The counts of total mesophilic and thermophilic bacteria, Nitrogen fixing and phosphate solubilizing bacteria in addition to actinomycetes and fungi were recorded in the compost. Most microbes maintained their growth ability when the temperature was 25°C. Total counts of mesophilic ≥ phosphate solubilizing ≥ thermophilic bacteria ≥ actinomycetes ≥ fungi ≥Nitrogen-fixing bacteria. Various bacteria like the genera of Thermus, Bacillus, and Streptomyces were detected.⁴⁶ The type of raw materials in the compost leads to different microbiota compositions during composting. Simultaneously, bacteria might have different responses to nutrients in the raw material, and the bacteria that can degrade these materials rapidly increase in abundance and counts.⁴²

Table (2):
The tested morphological, physiological and biochemical characters of the isolate MB5 and MB11

+: positive results, -: negative results

The most abundant isolates MB5 and MB11 were selected and identified using morphological and physiological methods (Table 2). The isolate MB5 belongs to free-living bacteria that grow well in the nitrogen-free medium as gelatinous colonies with dark black color and the cells were cocci, motile Gram-negative, cyst forming (Figure 1A and B), oxidase, and catalase positive. They grew well at 25-32°C and generated indole, citrate, catalase, and oxidases according to,⁴⁷ and using several approaches, it was recognized as a species belonging to the genus Azotobacter.^40,48 Using the previous morphological and biochemical characters in addition to molecular methods using partial sequencing of 16S rDNA, the isolate MB5 was identified as A. chroococcum MB5 (Figure 2). The cells are abundant in soil, increasing soil nitrogen concentration and agricultural sustainability.

The most dominant actinomycete isolate MB11 was obtained from compost and showed leathery and powdery grey color colonies on Starch nitrate agar. After examination under light and electron microscopes, the cells were Gram-positive with well-developed filamentous aerial and substrate mycelia carrying straight long spore chains with smooth surfaces and colonies produced pale yellow soluble pigments on agar medium (Figure 1C and 1D). Cells were gram-positive and molecular analysis showed that these isolates belong to genus Streptomyces and identified as S. griseus MB11 with a 94% similarity level (Figure 3). Similar results was recorded by the previous species.⁴⁹

In this work, IAA was found in the culture filtrate of A. chroococcum MB5 and S. griseus MB11. It was confirmed that bacteria and actinomycetes produce IAA in growth media.^50,51 Streptomyces rochei, S. livaceoviridis, and S. rimosus were a producer of IAA, which increase plant growth, and other bacteria were found as efficient producers of IAA, siderophore, and phosphate-dissolving organisms.^52-54 Soil bacteria synthesize phytohormones that promote plant growth and alter the morphology and structure of roots.⁵⁵ These bacteria are considered eco-friendly biofertilizers since they are inexpensive and provide a sustainable source of nutrients to plants, reducing reliance on chemical fertilizers. They also play an important role in boosting nutrient availability, which promotes plant growth.⁵⁶ In this investigation, the filtrates of A. chroococcum MB5 or S. griseus MB11 or their mixture increased seed germination, which could be related to the presence of IAA and other secondary metabolites.

Soil pH, EC, and microbial counts of the detected bacteria and fungi in inoculated and inoculated soil after one month of plant growth were summarized in Table 3. After plant growth, soil pH was not affected by the inoculated microorganism except, in the case of using cells of AZ+ST, a significant increase was noticed (Table 3). Also, the addition of compost to the soil increases soil pH from 6.5 to 6.9. Moreover, soil EC was increased with soil inoculation compared to control (soil with 20% Moringa compost only). Total bacterial counts at 25°C were increased while the counts at 45°C were under detection limits but the counts of AZ or ST were increased in their inoculated soil. In contrast, there is non-significant increase in fungi counts compared to control.

Table (3):
Soil pH, EC and microbial counts of the detected bacteria and fungi in inoculated and inoculated soil after 2 months of plant growth

ND: under detection limits, *: significant differences at p > 0.05, #: acid washed sterile washed soil, MC: Moringa compost, AZ: Azotobacter, ST: Streptomyces.

Plant growth enhancement by soil bacteria and compost
The percentage of seed germination and Germination Index were increased by soaking in MC or AZ or ST extracts and the significant increase was clear in the case of using MC+AZ or MC+AZ+ST extracts (Table 4). The effect of Zn and Mn on seed quality and germination in common beans (P. vulgaris L.) was studied.⁵⁷ The bacterial biofertilizer Azotobacter, and of gibberellic acid affect the germination of guava seeds and the single effect of each agent was superior in all studied characteristics and the control treatment recorded the lowest rate.⁵⁸

Table (4):
Effect of Moringa compost and inoculation with bioagents on Fresh and dry weights, Leaf number, plant height, and root length of Phaseolus vulgaris grown under greenhouse conditions for one month

Data are means values of 10 replicates ± standard deviations. *: statistically significant results compared to control at p < 0.05. The results with different letters in the same row were statistically significant (p < 0.05), MC: moringa compost, AZ: Azotobacter, ST: Streptomyces.

The bacterial inoculum can be applied to the soil containing organic fertilizers, such as compost or manure, or by spraying it onto the soil surface during the seedling stage and the efficacy of Azotobacter or Streptomyces as a biofertilizer depends on several factors, including the soil type, crop species, and environmental conditions.^12,59,60 In this experiment, soil containing compost was inoculated with the bacterial inoculum before seedlings were transferred to the pots. After two months of growth, inoculation of soil with AZ, ST and a mixture of AZ+ST had a beneficial effect on plant growth and chemical analysis. Microbial inoculation or the presence of compost enhances the availability of nitrogen and other nutrients to plants, promotes nutrient cycling, suppresses plant pathogens, and enhances plant growth-promoting activities.

In the present study, plant height, root depth, and leaf number, as well as fresh and dry weight, were significantly affected after applying compost treatments or bioagents to the soil (Table 4). It was clearly shown that P. vulgaris shoot height, root depth, fresh weight, and dry weight were significantly increased in all treatments compared to the control (compost only). Moringa compost increased all tested plant parameters compared to plants grown in soil without compost (Figure 4). In agreement with the present study, previous reported that compost addition to sandy soil led to higher plants,⁶¹ while the application of different levels of termite mound compost material to sandy soil significantly improved plant height and leaf number.⁶² Adding compost and dry Azolla to the soil improved both the quality and growth of squash plants.⁶³ Thus, enhancing soil quality facilitates nutrient absorption by the plant and promotes optimal root development and growth, increasing the total number of roots.^34,64 The increase may be related to N fixation and production of IAA.

Many researchers demonstrated that the enhanced mineralization of nutrients and decomposition of organic matter by bacteria could be ascribed to improved plant growth and productivity.^65,66 Furthermore, it is hypothesized that the compost with growth-promoting bacteria exerts a direct influence on plants through hormonal mechanisms (cytokinins and auxin-like activity) and an indirect influence via metabolic soil microorganisms in response to changes in soil nutrient uptake and physical properties.⁶⁷

Chlorophyll and carotenoids are pigments widely distributed in the chloroplasts in plant leaves and play an important role in plant photosynthesis and there is a close correlation between the leaf chlorophyll content and nitrogen availability. As shown in Table 5, there were significant differences in chlorophyll contents of the leaves in all treatments. However, C+AZ+ST showed significant improvement in chlorophyll a and b, with 1.86±0.02 and 9.69±0.04 mg/g, respectively. Meanwhile, carotenoid content slightly increased in plants inoculated with C+AZ+ST. It was reported that chlorophyll contents were positively and significantly correlated to organic fertilizers. There is a correlation between nutrient uptake by plant tissues and leaf content of chlorophyll. Therefore, the continuous nutrient supply from the soil by compost addition or inoculation with plant growth-promoting bacteria increases the adsorption of essential elements needed for effective plant photosynthesis.

Table (5):
Chlorophyll a, b, and carotenoid and macro-nutrient and mineral contents of common bean grown in the presence of Moringa compost and bioagents

The results with different letters in the same column were statistically significant (p < 0.05), MC: Moringa compost, AZ: Azotobacter, ST: Streptomyces

Following inoculation with AZ, ST, or a combination of the two, soil microbiota boosted root and shoot growth, mineral, and protein content which may be due to nitrogen fixation, auxins, and the synthesis of unidentified chemicals. Similarly, plants inoculated with A. chroococcum, Azospirillum brasilense, and S. mutabilis showed a significant increase in growth, mineral contents like P, Mg, and N, and total soluble sugars due to IAA release and/or nitrogen fixation by soil microbiota.^68,69

Due to the presence of nutrient elements, particularly potassium, which aids in photosynthesis, carbohydrate transport, regulation of stomatal opening, and respiration, the physiological properties of plants treated with final compost are enhanced.⁷⁰ These results are consistent with prior research that demonstrated the effectiveness of chlorophyll as well as confirmed the final compost’s maturity and high quality.⁷¹ Furthermore, compost increased the levels of chlorophyll and carotenoids in comparison to control, as demonstrated.⁷²

In soil, plant residues or compost are changed to value-added products by microorganisms and this increases plant growth and nutrient contents, namely protein, carbohydrates, and lipids. In addition, compost is a good bio-fertilizer that contains nutrients that have the potential to be utilized in agriculture.⁷³ However, during composting, microorganisms degraded the nutrients to simple forms at the same time, N contents were increased in plants grown in inoculated soil with AZ compared to Control, ST, and AZ+ST, respectively. In this study, addition of compost to soil increased plant protein and carbohydrate and P, N, Ca, and K contents (Table 4) compared to control (soil only) and inoculation of soil with AZ, ST, or AZ+ST improved all the previous parameters compared to plants grown in soil with compost only, therefore, addition of Moringa compost and inoculation with benefit microorganisms can be utilized to enhance plant growth and productivity and may lead to distinct functional evolutions of microorganisms. In the present study, compost slightly increased the protein and carbohydrate content of the plant compared to control. This result is in close agreement with other results⁷⁴ that reported carbohydrates and protein mainly were increased due to increased microbial activity during composting and plant growth. The inoculum of three strains of nitrogen-fixing bacteria, two concentrations of Moringa leaf extract, and their combinations significantly increased fennel plant growth, yield, and oil components in addition to plant height, branch number per plant, herb fresh weight, fruit weight, umbel number per plant, and fruit yield. Photosynthetic pigments, total phenols, and oil components were also improved.⁷⁵ It was reported that bacteria might have been capable of absorbing nutrients to endure the composting process. Nitrogen-fixing bacteria Azotobacter and plant growth-promoting Streptomyces have many biological activities like nitrogen fixation and production of IAA, phosphate solubilization, and other eco-friendly activities that increase plant growth. When N was easily accessible, plants would predominantly produce compounds with high N content (e.g., proteins for growth). When N availability was scarce, metabolism shifted toward carbon-containing compounds like starch and cellulose, phenolics, and terpenoids. A balanced proportion of nitrogen in soil enhances protein production, hence increasing microbial activity resulting in a faster degradation rate of unsoluble compounds and good compost which increases plant growth. Moreover, K⁺ uptake is extremely selective and is intimately linked to plant metabolic activity. Potassium acts as an activator or cofactor for numerous enzymes involved in metabolizing carbohydrates and proteins. Meanwhile, potassium ions are necessary for the activity of over fifty enzymes and plants prefer it in soluble form.⁷⁵ Soil N content affects K⁺ uptake consequently influencing the levels of carbohydrates and proteins. The nitrogen concentration in the compost was relatively low, thus, the presence of bioagents increased the nitrogen content of the soil, which impacted the carbohydrate and protein content of the plant.

The nutrient content in the soil significantly increased after adding compost which is rich in nutrients, P, N, K, and Ca contents. For plant development, phosphorus is always an essential nutrient and microbial P solubilization seems to be an effective process to release the precipitated P in soil. In the present work, the two selected isolates belong to P-solubilizing bacteria which has beneficial effects on plant yields. Similarly, P solubilizers Pseudomonas, Bacillus, and Paenibacillus were isolated from Jujube rhizosphere.⁹

In the results of this work, the total P content was shown to be higher in all treatments compared to compost as the control. The gradual increase in P content during the composting process occurred due to the increase in the P water solubility by bacteria during the decomposition of plant wastes. The Ca, P, K and N content in plant shoots increased in microbial-treated plants compared to the control. Organic fertilizers improve soil’s organic matter content and nutrient availability which balance plant nutrition and improve soil structure, organic matter, and microbial activity.

The importance of bioagents formula with compost of Moringa to enhance plant growth was noticed (Figure 4), whereas the free-living A. chroococcum and Streptomyces were used as bioagents, which are known to have many biological activities and the presence of plant-based compost like Moringa compost plays a vital role in improving soil structure and fertility and enhance plant growth. The results also emphasize the importance of sustainable agriculture and the use of eco-friendly dry Azolla increased the nutrient efficiency in soil compared to other fertilization methods.⁶³ Microorganisms from the genera Streptomyces, Bacillus, Trichoderma, Pseudomonas, and nitrogen-fixing bacteria in addition to their enzymes have a role in biological process and plant infection resistance.⁷⁶ The use of compost and some plant growth-promoting (PGR) bacteria for enhancing plant growth is more practical, easy to handle, less expensive, and low-cost due to their presence in the soil as agricultural wastes or soil microflora.⁷⁷ The use of Moringa compost increases plant growth helps in recycling organic matter, contributes to sustainable agriculture, and enhances soil fertility.⁷⁸ The incorporation of compost into the soil enhances water retention, aeration, and nutrient availability, thereby mitigating the adverse effects of stress conditions and promoting beneficial microbial activity, which aids in nutrient cycling and the suppression of soil-borne pathogens.⁷⁹ Nitrogen-fixing Azotobacter introduces an additional source of nitrogen, reducing the plants’ reliance on external nitrogen inputs and potentially mitigating nutrient deficiencies,⁵⁹ thus enhancing plant growth and development, especially when nitrogen availability is limited. There are synergistic effects of compost and A. vinelandii inoculation when used in combination and increase tomato growth due to many benefits, the first through enhanced stress tolerance to drought and salinity, improved soil structure, and increased nutrient availability resulting from compost application and the additional nitrogen supply from A. vinelandii.⁸⁰ There is increased nutrient uptake and utilization by plants due to compost solubilization and nutrient release by the action of the soil microbes⁸¹ and there is disease suppression by promoting the healthy soil microbiome, which can help suppress soil-borne pathogens.⁸⁰ Plant-based compost reduces environmental impacts and makes it a highly desirable option for sustainable and productive plants. With the evidence from various scientific studies supporting its effectiveness, the use of plant-based compost should be encouraged as a natural and eco-friendly approach to maximizing plant yield, improved soil structure also reduces the risk of soil erosion and compaction, which are critical factors for successful plant growth.^81,82 Compost is a valuable source of essential nutrients for plant growth and contained a variety of macronutrients (nitrogen, phosphorus, and potassium) and micronutrients (such as calcium, magnesium, and iron) that are slowly released into the soil as the compost decomposes.⁸³ This gradual nutrient release provides a consistent supply of essential elements to plants, promoting steady growth and healthy fruit production.

The Saudi Arabia Standard Organization has released guidelines for organic fertilizers and compost to promote sustainability, environmental protection, and public health. According to the guidelines, compost should be produced from organic materials that are free from harmful substances, weed seeds, and contaminants. Additionally, the compost must have a balanced nutrient composition of nitrogen, phosphorus, and potassium to qualify as an effective fertilizer.^84,85 A quality control system should also be in place to ensure that the compost meets the required standards. Common metals that are tested for in compost include P, N, Ca, and P. A compost sample analysis results were compared to regulatory limits or guidelines to determine if the compost is safe for use in agriculture or other applications. Furthermore, Actinobacteria has shown a biosynthetic potential to produce large amounts of bioactive secondary metabolites with novel structures and remarkable biological activity in agriculture.⁸⁶

Plant-based composts also contribute to developing a diverse and thriving soil microbial community. A healthy soil microbiome is particularly crucial for plant growth. Research has shown that the incorporation of composts and bioagents in the soil can lead to a reduction in the population of harmful pathogens, creating a more disease-resistant environment for plant growth.⁸⁷ Utilizing compost for plants reduces the environmental footprint of agriculture and supports eco-friendly farming which aligns with sustainable agricultural practices, reduces the need for chemical fertilizers, helps divert organic waste from landfills, and reduces greenhouse gas emissions, which can have detrimental effects on the environment.⁸⁸