Formulation of biofertilizers from oil palm empty fruit bunches and plant growth-promoting microbes: A comprehensive and novel approach towards plant health

1 Introduction

The Malaysian oil palm industry is one of the most significant lignocellulosic waste producers for the past few decades. More than 90% of its total agricultural biomass waste is generated from 5.87 million hectares of oil palm plantations (MPOB, 2020; Loh, 2017). Among the oil palm biomass produced by the Malaysian oil palm industry, empty fruit bunches (EFB) comprise 20–22 percent of the solid by-products generated during palm oil production, making it the most notable waste produced by this colossal industry (Tahir et al., 2019; Han and Kim, 2018; Loh, 2017). Such oil palm waste production has nevertheless presented a massive problem of waste management and various environmental issues. Composting the biodegradable matter that can be recycled back to the environment, like biofertilizers, is an efficient way of utilizing EFB biomass (Kour et al., 2020). This alternative method can perhaps turn this large quantity of biomass into a profitable and sustainable resource such as biofertilizer that can be used on oil palm plantation areas or as a market product (Truckell et al., 2019). Biofertilizers are microbial formulations containing native plant growth-promoting microorganisms (PGPM) which have the potential to enhance plant growth either directly or indirectly by producing different types of phytohormones, iron-binding metabolites (siderophores), and solubilizing soil nutrients and minerals (Soumare et al., 2020; Kour et al., 2020). Microbial biofertilizer is made up of several different types of bacteria, fungus, and blue-green algae, all which have a symbiotic interaction with plants. According to research, the application of biofertilizers can enhance the yield of diverse crops by around 25% while reducing the usage of inorganic nitrogen and phosphorus fertilizers by approximately 25–50% and 25%, respectively (Aloo et al., 2020). Thus, biofertilizers offer enormous promise for sustainable agriculture, particularly in the face of climate change. Despite the growing interest in this technology, its entire potential remains untapped. Converting EFB biomass will be beneficial for the environment and economically viable for the oil palm industry.

On the contrary, according to the Food and Agriculture Organization (FAO), the world population will reach 9.8 billion by 2050, up from 7.6 billion now. (Soumare et al., 2020; Kumar et al., 2019). This rapid increase in the world population will increase the food demand somewhere between 59% and 98% by 2050. Another estimation from the FAO indicates rice, wheat, corn, and soybean production all need to increase by 60% by 2050 to meet the demand (FAO, 2018; FAO, 2017). This increasing food demand will shape agricultural practices in a way that our civilization has never witnessed before in its history (Gouel and Guimbard, 2019). Conventional farming practices play a significant role in fulfilling a rising population's nutritional needs. This has resulted in a growing dependence on chemical fertilizers and pesticides for higher productivity (Singh et al., 2019; Bhardwaj et al., 2014). Chemical fertilizers are inorganic substances of entirely or partially synthetic origin produced, consisting of a known amount of nitrogen, phosphorus, and potassium (Kumar et al., 2019; Vandana et al., 2017). In most established agriculture sectors, chemical fertilizers are usually applied regularly to supply nutrients to the planted crops. The application of these fertilizers at the recommended rates usually gives quick responses to the crops' physiological growth and yield. However, the use of chemical fertilizers has various adverse effects on the environment. Besides the acceleration of soil acidification, chemical fertilizers also contribute to the risk of contaminating groundwater and the surrounding environment. It also weakens plant roots, thereby putting them at risk of harmful diseases (Singh et al., 2019; Mahanty et al., 2017; Chun-Li et al., 2014). As the global climate is changing and the loss of natural resources, the future remains unclear as to if conventional agriculture will be able to sustain this increasing food demand in the coming years (Singh et al., 2019). We could produce more food by destroying even more forests to create new farmland, use more chemical fertilizers, or figure out how to do it on the land we have right now, using a more efficient and environment-friendly way (FAO, 2018). The approach to sustainable farming has been strongly sought as a result of these difficulties. The use of biofertilizers can help to achieve sustainable farming because they protect the soil ecosystem through a variety of mechanisms such as nitrogen fixation, potassium, and phosphate solubilization, and decomposition of organic materials in the soil (Arif et al., 2020). A further advantage of biofertilizers over chemical fertilisers is that they are more affordable for marginal and small-scale farms. In this regard, attempts have been made to develop a high-quality biofertilizer using EFB of oil palm, rich in nutrients, to assure biosafety.

In Malaysia, biocomposting of EFB biomass would be the best practical solution since the end-product could be directly applied in oil palm plantations. This will lower greenhouse gas emissions and reduce the demand for chemical fertilizers, thus saving valuable resources (Abdullah et al., 2017). In addition, it will reduce the possibility of pests, weed seeds, or parasites spreading into the plantation area, while direct application of EFB increases these risks. Much attention has been drawn to the issue of sustainability, and technical advancements in solid waste management have been introduced in order to minimize the creation of needless material and waste disposal costs (Begum et al., 2019). Agricultural biomass consists of rich nutrients that, if handled correctly, may be put to good use in a variety of applications. As a source of high-quality organic matter, they may be processed to eliminate pathogens before being utilised to feed the soil (Dimkpa, 2016). In order to create useful organic matter that may be utilized as biofertilizers or additions to improve the soil structure, waste materials are being recycled and reprocessed (Kour et al., 2021). Modern agricultural industries have become more conscious of the need to embrace a much more environmentally friendly approach in providing plant nutrition and the conservation of soil fertility (Ai May et al., 2020; Ariana and Candra, 2017). In recent years, various studies were carried out to evaluate the most appropriate method for managing different types of solid waste in the oil palm industry, including the EFP, and turning this abundant biomass into biofertilizer is the most appropriate solution (Alizadeh et al., 2014). Microbial communities especially plant growth-promoting microbes (PGPM), produce various biomolecules, which enhance the breaking down of the lignocellulosic components in EFB biomass. These microbes also help convert all the macronutrients and micronutrients available in the EFB biomass from an inaccessible form to an accessible form for the plants (Mącik et al., 2020; Kuan et al., 2016). PGPM play an important role in regulating numerous biological processes, including the breakdown of organic matter, the availability of different plant nutrients such as iron and magnesium as well as phosphorus, potassium, and nitrogen (Mącik et al., 2020). Microbial inoculants have long been regarded as an essential element of unified nutrient management, which leads to sustainability.

Since the primary aim of biofertilizer is to improving overall soil quality and enhancing plant growth whether via direct or indirect mechanisms, there are several reviews on the influence of biofertilizers on soil and plant health. However, no comprehensive review has been reported which describe the potential of utilizing EFB biomass as a biofertilizer. It is not enough to emphasize just soil impact and plant health alone, and a thorough assessment must also be given on the utilization of agricultural biomass as biofertilizer for a futuristic sustainable agriculture. The review first collectively describes what is biofertilizer and way it is important for sustainable agriculture. It then critically evaluates the Malaysian Oil Palm Industry, its overall waste production during oil palm cultivation and the nutrient profile of EFB biomass. It also assesses the possibility of utilizing EFB biomass as biofertilizer. Then it characterizes the roles of PGPM in plant growth and development. Finally, it summarises significant findings and suggests areas of ambiguity regarding successful application of biofertilizer and proposes important areas for future research on improving PGPM traits of beneficial microbes that will potentially enhance the biological pathway of different biocompounds production.

2 Biofertilizer: Why is it necessary for sustainable agriculture?

Biofertilizers can easily be referred to as living fertilizers because, unlike chemical fertilizers, biofertilizers have a combination of beneficial microbes such as bacteria and fungi (Thomas and Singh, 2019; Alori et al., 2017). In a broad sense, the term biofertilizer can also include every organic plant growth resource made accessible for plant development via microbial community associations or interactions (Mącik et al., 2020; Mahanty et al., 2017; Bhardwaj et al., 2014). Microbes are tiny but exceptionally useful (Soumare et al., 2020; Nath et al., 2018). Microorganisms can provide all the 16 essential elements plants need for healthy growth and development. Microbes present in biofertilizer can provide all these elements, especially nitrogen, phosphorus, and potassium. These elements are abundantly present in the soil or environment, but it is an inaccessible form. Microbial activities convert these unusable elements into a usable form for plants (Mącik et al., 2020; Kuan et al., 2016). Biofertilizer's introduction will enhance the biodiversity that makes up all sorts of beneficial bacteria and fungi (Prasad et al., 2017; Ritika and Utpal, 2014). These microbial communities benefit the plant's development (Kour et al., 2020; Yadav and Sarkar, 2019). Biofertilizers improve soil fertility and crop production in sustainable agriculture as an alternative to conventional fertilizers (Zaidi et al., 2017). Adding beneficial PGPM to agricultural activities began approximately 60 years ago. It is now evident that such beneficial microbes may improve plant tolerance to unfavorable environmental changes such as water and nutrient shortage and pollution of heavy metals (Mendoza-arroyo et al., 2020; Ritika and Utpal, 2014). Such potential biological fertilizers will play a very significant role in soil biodiversity and productivity. Producing chemical fertilizers is very costly and challenging to fulfil the current demand. This created a need for the development of biofertilizers as a potential replacement for chemical fertilizers. Biofertilizers will also protect the ecosystem in the farmers' community's interests as an environmentally sustainable and better economic output (Qu et al., 2019).

In the coming decades, PGPMs are expected to become widely utilized bioinoculants around the world and it will play a significant role in boosting the nutrient and yield status of agroecosystems by decreasing the reliance on pesticides and synthetic fertilizers in conventional agriculture (Mącik et al., 2020; Singh et al., 2019). PGPM stimulates the development of plants via a variety of methods, both direct and indirect. It is important to distinguish between direct and indirect mechanisms (Fig. 1). Direct mechanisms include processes such as P solubilization and N fixation, as well as the production of phytohormones, siderophores, and vitamins. Indirect mechanisms include those mechanisms that do not directly promoting growth but do so through the path of synthesis. For example, ACC deaminase activity, antibiotic synthesis, cell wall disintegrating enzymes, and increased systemic resistance are all examples of indirect methods (Khanna et al., 2021; Mącik et al., 2020). More detail explanation of mechanism of PGPMs both direct and indirect is given in section below.

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Fig. 1

A schematic descriptive mechanism of various traits shown by Plant Growth-Promoting Microbes present in biofertilizers that acts as bioinoculants and promote plant growth.

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3 The oil palm industry in Malaysia

The Oil palm (Elaeis guineensis) is a primary plantation crop in Malaysia. Malaysia currently ranks second at global palm oil production after Indonesia (Begum et al., 2019; Tahir et al., 2019; European Palm Oil Alliance, 2016). Malaysia has ideal climatic conditions for oil palm crop cultivation which fostered a rapid expansion of the oil palm industry (Begum et al., 2019: Tiemann et al., 2018). Taking a look at the most productive parts of Malaysia, it is clear that, the oil palm crop requires a relative humidity of at least 85%, an average of 5 h of sunlight each day, and at least 2000 mm annual rainfall spread uniformly throughout the year with little or no dry season in order to achieve optimal growth and production (Tiemann et al., 2018). In addition, steady average temperatures between 24C and 28C appear to have optimal conditions, with seasonal fluctuations of less than 6C. Average temperatures below 17C will reduce growth by more than half, and no growth will occur anymore at maximum daily temperatures of 15C. The crop usually thrives at altitudes below 400 m in tropical lowlands (Tiemann et al., 2018: Alam et al., 2015). In Malaysia, oil palm is primarily cultivated, which is a hybrid between the dura and pisifera. The oil palm tree can grow 7–15 m high depending on various conditions. The oil palm leaves look like feathers, emerging from both sides of the frond. The oil palm fronds can grow up to 5 m. Within three years of planting, the oil palm tree can begin to produce fruits, and after 5–6 months, fruits are mature enough for harvesting. The fruits are in bunches or known as fresh fruit bunches (FFB), weigh 15–30 kg, and depending on the condition, can reach up to 50 kg (Awalludin et al., 2015). FFB consisted of the oily pericarp, shell, and edible oil contenting palm kernel. The average economic lifespan of an oil palm tree is 25–30 years. Therefore, oil palm cultivation is the best option for many countries since it can produce a high volume and quality of edible oil in a limited area (Foong et al., 2019).

Palm oil has evolved to be the world's most significant oil produced and consumed. Palm oil is everywhere in our daily life, as food palm oil is used for frying, and it is also an ingredient in many processed foods. In addition, it is used in dietary supplements, medicine, cosmetics, dye, soaps, waxes, lubricants, and ink (MPOB, 2021). The world population has increased, leading to a further increase in the production of palm oil. The use of palm oil has risen exponentially in the last ten years, leading to the use of palm oil from 6% to 28%. To meet this increasing global demand, more farming land and rain forest in Malaysia are converted to oil palm plantations (Begum et al., 2019). Oil palm cultivation began in Malaysia nearly a century ago. In 1871, the British planted the first oil palm tree in Malaysia, and oil palm was used in those days as ornamental plants (Onoja et al., 2019). In 1917 in Tenamaran Estate, Selangor, Malaysia, began its first commercial palm oil production (Alam et al., 2015; Basiron, 2007). Oil palm planting statistics reveal that Malaysia only had 54,000 ha of oil palm plantation area back in the 1960s. The cultivated area of oil palm has risen exponentially since then. The oil palm plantations have occupied 5.87 million hectares of Malaysian soil in 2020 (MPOB, 2020). Abdullah et al. (2017) stated that the demand for Malaysian palm oil commodities is exponentially increasing in India, China, and Europe. One of the major agricultural products exported from Malaysia is refined and crude palm oil (Abdul-Hamid et al., 2020; Truckell et al., 2019). According to the Malaysia Palm Oil Board (MPOB), in the year 2020, total Malaysian exports of palm oil and other oil palm products amounted to 26.73 million tonnes, contributing to the total export revenue of approximately USD 17.88 billion. With the rapid advancement of the palm oil industry, sustainability development throughout the sector is extremely difficult. Therefore, there is an interest in incorporating sustainable development task forces within the frameworks of palm oil companies which include key elements, such as economic, environmental and social factors (Tiemann et al., 2018). In Malaysia, for example, a Roundtable on Sustainable Palm Oil (RSPO) has been formed to regulate the sustainability of the palm oil sector, describing key elements of legal, economic viability, environmental and public policy via policies known as RSPO Principles and Criteria (Begum et al., 2019). Although this sector is regarded as financially secure, the oil palm industry may enhance its sustainability by improving various aspects of oil palm cultivation and mill processing.

4 The oil palm biomass

Oil palm biomass is an inevitable by-product of the oil palm industries during trimming, harvesting, replanting, and milling activities (Basiron, 2007). Being the world's second-largest palm oil producer, Malaysia has a diverse range of palm biomass generated as by-products from its vast oil palm industry (Fig. 2). Oil palm trunks (OPT), oil palm fronds (OPF) generally produce at plantations area, empty fruit bunches (EFB), palm kernel shells (PKS), mesocarp fibers (MF), and palm oil mill effluent (POME) is usually produced during mill operation of the FFB (Abdul-Hamid et al., 2020; Onoja et al., 2019). Nevertheless, utilization of oil palm biomass as value-added products is minimal due to the increased cost of labor, transportation, distribution, and storage. Thus, in most cases, these abundant resources are left to decompose in the plantation area. It is estimated that by 2020, Malaysian oil palm industries would produce 100 million tonnes of solid oil palm biomass. Such oil palm waste production has nevertheless presented a massive problem of waste management and various environmental issues (Awalludin et al. 2015).

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Fig. 2

Simplified representation of biomass and product produced from an oil palm tree (Wan Daud and Law, 2011). The fresh fruit bunches (FFB) are collected from the plantation area during harvesting season. In the oil palm mill, the FFB is sterilized, and fruits are removed from the bunches, so the empty fruit bunches (EFB) become residues. While oil palm trunks (OPT) and oil palm fronds (OPF) generally produce at plantations during maintenance pruning and replanting. After various milling operations, crude palm oil is extracted from the oil palm fruits.

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The necessary waste management steps are reducing waste generation to a minimal level, recycling, and safe disposal. In Malaysia, the oil palm industry implements environment-friendly waste management strategies recommended by the Department of Environment Malaysia and the MPOB. EFB is now utilized in the plantation areas for better weed control, preventing soil erosion, and retaining soil moisture (Begum et al., 2019; Awalludin et al., 2015). Oil palm biomass can be converted into high-value bioproducts such as biofuels, biofertilizers, bioplastics, biosugars, biochemicals, and biomaterials. When various economically valuable alternatives that will benefit both the environment and the oil palm industries are available, steps should be taken to utilize the oil palm biomass for a sustainable bioeconomy.

5 Empty fruit bunches (EFB)

EFB is lignocellulosic biomass formed once oil palm fruits are separated from the FFB during palm oil production. EFB comprises 20–22 percent of the solid by-products generated during palm oil production, making it the most notable waste produced by the oil palm industries (Tahir et al., 2019; Han and Kim, 2018; Samiran et al., 2015). Previously, EFB was used in oil palm mills as a fuel source to create steam, which leads to greenhouse gas emissions. Although using EFB as boiler fuel is less feasible due to its high moisture content (Ahmad et al., 2019). Sometimes it returned to the plantation area to decay naturally and prevent soil erosion. Considering that EFB is generated in large quantities in oil palm mills, it is an ideal residue for recycling as a biofertilizer.

6 Conversion of empty fruit bunches into biofertilizer

One of the most effective ways to address all of the difficulties associated with traditional EFB biomass management is a biological-based composting process that has been introduced for a very long time to create valuable end-products (Wei et al., 2016). Biocomposting is an aerobic process that relies on the presence of a diverse microbial population to break down the organic components of EFB and release nutrients available in EFB and destroy pathogens. During biocomposting, lignocellulosic components are broken down by the aerobic thermophilic microbes naturally present in biomass to produce nutrient-rich humus-like materials (biofertilizer). The biocomposting process can be divided into three stages: high-rate composting, stabilization, and maturation (Meyer-Kohlstock et al., 2013). Long formation time and low nitrogen content are some of the common issues faced while composting EFB biomass. Several factors can lead to these issues, including temperature, pH, moisture content, agitation, conductivity, and EFB characteristics such as size, initial C:N ratio, and nutrient contents.

It is worth noting that most of the literature reported that the EFB biomass takes about 2–3 months to complete the composting process. However, Siddiquee et al. (2017) reported a 30-day composting process which significantly reduces the EFB composting time. With the assistance of two strains of Trichoderma to composting the EFB biomass. The same researchers also found that the EFB compost showed a high amount of macronutrients (N, P, and K), which improves soil acidity and electrical conductivity. In a separate report, Lim et al. (2015a) reported the effects of microbial inoculation additive to enhance the efficiency of EFB decomposition and at the end of the composting process lower C:N ratio, increase pH value (∼8), microbial population, and degradation of organic acids. Another study by Lim et al. (2015b) investigated the physicochemical properties of EFB compost while inoculated with effective microorganisms. The EFB composting process took approximately two months. The microbial-treated EFB compost experienced an increase in the mineral content (Mg, Ca, B, and K), pH (∼8), and loss of total organic carbon (10.8%). They concluded that the addition of effective microorganisms speeds up the EFB composting process.

In another study, Wei et al. (2016) compared how the final quality of EFB compost was affected by the different temperatures, aeration rates, and reaction time during the composting process. The team mixed urea as a nitrogen source and fresh compost as inoculum with EFB samples and carried out the composting process for 42 days. Temperature and reaction time were observed to have a significant effect on moisture content. All three factors have a major impact on carbon loss. Nitrogen content was influenced by aeration intensity, reaction time, and the temperature-reaction-time relationship. Total ion changes over time revealed a strong association with conductivity (Pearson correlation coefficient of 0.853), with the most significant decrease in C/N ratio (from 30.2:1 to 17.6:1) obtained at a temperature of 40 °C and aeration rate of 0.4 L/min kg. The findings of this study will help the oil palm industry improve the quality of EFB composts produced in a short period of maturation and with a low C:N ratio. Tahir et al. (2019) experimented with isolating and identifying competent fungi that can naturally compost EFB biomass. The results showed that the changes in the physical characteristic of EFB were correlated to fungal growth. They concluded that one of the strategies for rapid composting is the inclusion of competent lignocellulolytic fungi in the composting process combined with an appropriate EFB composting method. In a similar study, Ichriani et al. (2018) reported the growth and yield of maize while applying EFB biochar mixed with phosphate-solubilizing fungi. They stated that the positive effects on maize growth and yield were recorded.

Trisakti et al. (2017) studied the effect of frequency on the rate of composting of EFB mixed with activated liquid fertilizer (ALOF) in a basket composter for 40 days, with the 3rd day showed the best results. The best EFB + ALOF compost characteristic was pH 9.0; moisture content (MC) 57.24%; water holding capacity (WHC) 76%; C:N ratio 12.15%; P 0.58%; and K 0. 95%. In an extensive study, Trisakti et al. (2018) experimented with producing EFB compost mixed with ALOF in a basket composter for ornamental plant cultivation. After 40 days data revealed that the characteristic of EFB + ALOF compost were pH 9.0; MC 52.59%; WHC 76%; C:N ratio 12.15; N 1.96%; P 0.56%; and K 0.95%. They also reported that the best-growing medium was for cactus, sansevieria, and anthurium was EFB + ALOF compost-sand-husk rice mixture at 1:3:1; 1:1:1; and 1:0:1; respectively. Hau et al. (2020) conducted a study to examine the effects of EFB + POME based biofertilizer mixing with various organic wastes to enhance biofertilizer nutrient content. They mixed EFB + POME with fishmeal, bonemeal, and bunch ash in a 1:1 wt ratio. It took approximately 40 days to achieve maturation of the EFB mixed composts with a pH of 6.3. It was reported that all batches of EFB mixed compost experienced an increase in N, P, and K. They concluded that EFB based biofertilizers could be mixed with other organic wastes to increase their quality and nutrient availability.

In another study, Yahya et al. (2010) examined the composting of EFB and decanter cake (DC) slurry by incorporating POME in an ordered turning process. They observed that applying a DC slurry improved the rate of the EFB composting process. The compost (EFB and DC slurry) matured after 51 days of composting and included 46.4% N, 17.9% P, 17.7% P, and Ca 23.1% compared to compost without a DC. Finally, the physicochemical modifications of an oil palm biomass during vermicomposting were investigated by Nahrul Hayawin et al. (2010). According to the results, vermicomposting of EFB is a suitable method for recycling oil palm wastes into value-added vermicompost. The overall organic carbon, C/N ratio, and pH value decreased during the phase, while the N, P, and K proportions improved. Furthermore, the heavy metal amount rose but not above the nutrient range in the vermicompost.

7 Characterization of plant growth-promoting microbes present in EFB biofertilizer

Once applied to the plants or soil, the PGPM present in the biofertilizer colonizes the plants' roots largely and encourages plant development by accumulating nutrients for the host plants (Kour et al., 2020; Mishra et al., 2017). All these different microorganisms flourish in the soil, particularly in the plants' rhizosphere, and a very significant number of such microbes have a functional link. The rhizosphere is a tiny layer of soil that adheres to the root surface of plants. (Soumare et al., 2020; Naik et al., 2019). Plant roots release various organic compounds that attract the microbial community, and they colonize the plant's rhizosphere. The soil dwelling microorganisms present in the rhizosphere comprise of different taxa such as bacteria, arbuscular mycorrhizal fungi (AMF), actinomycetes, and cyanobacteria (Singh et al., 2019; Kaushal and Wani, 2016). Furthermore, high lignocellulosic compounds, which can be used as a microbial energy source for the biosynthesis process, make EFB a natural habitation for different beneficial microbes (Dermiyati et al., 2020; Rosli et al., 2017). For example, Ariana and Candra (2017) isolated 430 microbes from EFB as part of their study. In another study, Lai et al. (2017) reported that 34 bacterial isolates were extracted from EFB compost. Besides, Harith et al. (2014) obtained 24 bacterial isolates from EFB biomass. These microbial communities directly or indirectly help plant growth, preventing diseases or stress (Singh et al., 2019; Babalola, 2010). Important characteristics of PGPM include the production of organic acids, phytohormones, antibiotics, siderophores, volatile bacterial compounds, solubilization of phosphorus, and nitrogen-fixing, as illustrated in Table 5.

As a result of the constant communication that occurs between the plant and its microbiota, it makes feasible to manipulate the microbes present, which in turn can influence the development of the plants defence against pests and diseases, thereby increasing total yield (Arif et al., 2020; Aloo et al., 2020). Several direct and indirect methods by which PGPM contribute to the promotion and development of plants have been identified (Fig. 3). Plant endogenous hormone levels are modulated by certain PGPM, which produce phytohormones such as auxins, gibberellins, and cytokinins, among other things. In addition, several PGPM species, such as Pseudomonas spp. and Arthrobacter spp., and Bacillus spp. secrete ACC deaminase, which breaks down ACC, an intermediate in the production of the plant stress hormone ethylene, resulting in reduced stress levels in plants growing in less-than-optimal conditions (e.g., salinity, drought, or heavy metal toxicity) (Arif et al 2020; Khanna et al., 2019a). Plant growth-promoting effects by the PGPM, such as N fixing and phosphorus solubilization, as well as ACC deaminase and auxin synthesis, have been shown in wheat and soybean. On 32 different plant species, including maize, tomato, melon, and pepper, Xia et al. (2015) examined the impact of isolated bacterial endophytes on their growth and development. Of the isolates examined, 61% were found to increase tomato growth, while 50–64% were shown to increase biomass production.

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Fig. 3

Simplified representation of beneficial plant-microbial interactions. Microbial activities benefit plants in terms of physical growth, disease resistance, and stress tolerance, resulting in up to a fourfold increase in biomass output. microbe-associated molecular patterns (MAMPs) generated from pathogens or beneficial PGPM can be used to achieve both systemic acquired resistance and induced systemic resistance to plant diseases., and resistance can also be induced through the use of MAMPs from beneficial PGPM (Arif et al., 2020).

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8 Plant responses to biofertilizers

The technology that goes into the manufacturing of biofertilizers is very economical and straightforward when compared to chemical fertilizers (Alori et al., 2017). In the past several decades, significant advancements have been achieved in the study and use of various biofertilizers. The scope of this study includes the variety of PGPM, as well as the commercial issues and technology for the manufacture of biofertilizers for agricultural and environmental sustainability, among other topics. Newer technologies, including as coating, co-encapsulation, fermentation, lyophilization, and inoculation, are now being developed for this purpose (Arif et al., 2020). Examples include: The development of fertilizers derived from chemical, organic, and microbial sources (biofertilizers) is expected to gain widespread adoption in the future for environmentally benign and economically successful agriculture. A lot of assumptions are imposed on biofertilizers. Biofertilizers consist of living organisms like bacteria and fungi; therefore, their performance varies depending on the surrounding environment. Furthermore, the influence on the plants is slow in comparison with chemical fertilizers. Consequently, the results are supposed to be inconsistent.

Biofertilizers are often portrayed as even more expensive as synthetic fertilizers due to the shortage of skills and technology to produce biofertilizers by utilizing abundant palm oil waste (Chun-Li et al., 2014). Recent ground-breaking advancements in molecular research on plant-pathogen interactions and genomics can help understand the necessary protocols to overcome these shortages. Nevertheless, many researchers have reported positive outcomes while using a variety of PGPR microbes with biofertilizers. The usage of different biofertilizers is an essential part of agricultural activity in recent days. The successful introduction of the biofertilizer depends on several important factors such as inoculums size, types, in which state biofertilizer is applied to field (solid, liquid, or powder), the competition between microbes in biofertilizers with the native niche for survival. The farmer's knowledge about the proper usages of biofertilizers is also an essential factor for a positive outcome.

9 Commercialization of biofertilizers

A century ago, biofertilizers were first commercialized, when “Nitragin” was registered for plant bioinoculation with Rhizobium sp. Biofertilizers account for around 5% of the overall fertilizer industry, and over 150 products based on microbial strains agricultural use were registered. Table 6 lists some of the most popular biofertilizers used throughout the globe. Rhizobial bioinoculants have become the most common microbial inoculants in recent years, accounting for about 79 percent of global demand. P-solubilizing biofertilizers account for 15% of the worldwide market, whereas other bioinoculants, such as mycorrhizal fungi, account for 7% (Mącik et al., 2020). In 2020, the global biofertilizers market reached a value of $2.3 billion dollars and is predicted to hit $3.28 billion by 2027 (Research and Markets, 2021; Fortune Business Insights, 2021). As the global demand for organic food increases, so does the market for biological fertilizers. On the basis of geography, the global biofertilizers market is segmented into the following regions: North and South America; Europe; Asia-Pacific; Latin Biofertilizers have the fastest expanding market in Asia-Pacific.

10 Conclusion and future perspectives

This review updates our understanding of microbial activities to convert biodegradable waste into valuable biofertilizer, how microbes can improve biofertilizer quality by converting inaccessible nutrients in EFB into an accessible form for plants and critically propounds on their future prospects for sustainable crop production. Chemical fertilizers are proven effective and convenient for agricultural production and disease management, but they are a severe threat to public health and the environment. Currently, there is a big gap between chemical fertilizer's demand and production. Therefore, converting EFB biomass into biofertilizer and using it for agricultural practices will be a more sustainable approach for modern agriculture in the context of their cost, environment-friendly nature, and profitable conversion of oil palm biomass. There has been a substantial increase in the health, growth, and yield of plants due to the application of biofertilizer. PGPM stimulates in a variety of ways, both direct and indirect. The Malaysian government should give more support and encourage the usage of EFB biomass from the palm oil industry for biofertilizer production. Developing biofertilizers using EFB biomass will have a significant impact on the country's stainable agricultural economic development. It will also contribute to the protection of the environment and public health. The government should encourage more organic farming by offering special incentives to the local farmers and private sector and educate them about the benefits of using biofertilizers. Government and privately funded research and development (R&D) activities need to increase to find new opportunities and solutions to current issues related to converting EFB into biofertilizers. Molecular biotechnological tools can improve traits of beneficial microbes that can enhance the biological pathway of various biomolecules such as phytohormones, antibiotics, enzymes, Volatile compounds productions, and many more. A better knowledge of the whole process of PGPM may aid in the development of more specialized strains capable of working in more unfavourable and variable circumstances.