A Review on Compost-Based Biostimulants: Production, Functional Mechanisms, and Current Challenges

Abstract

Compost-based biostimulants (CBB) have emerged as a promising tool in sustainable agriculture, offering an eco-friendly approach to improving soil health, crop productivity, and environmental resilience. Derived from the controlled biodegradation of organic waste, CBB contains a diverse array of beneficial microorganisms, humic substances, and bioactive compounds that act synergistically to stimulate plant growth and soil biological activity. Mechanistically, CBB enhances nutrient acquisition by increasing plant-available nitrogen and phosphate solubility, promoting root development through phytohormone synthesis, and improving stress tolerance by modulating plant defense pathways and antioxidant activity. Additionally, their application enhances soil structure, microbial diversity, and carbon sequestration, making them integral to climate-smart agriculture. Despite their growing relevance, several challenges impede the widespread adoption of CBB. Variability in compost quality, lack of standardized production protocols, limited field-scale validation, and inconsistent regulatory frameworks hinder reproducibility and commercialization. Addressing these gaps requires interdisciplinary research that integrates microbiology, biochemistry, agronomy, and data science to better understand how microbial metabolites interact and optimize formulation strategies. Future research should prioritize the standardization of composting methods, long-term multi-crop field evaluations, and integration with precision agriculture tools for real-time soil monitoring. Policy harmonization, quality assurance frameworks, and farmer education are also vital for ensuring safe and effective use of CBB.

1. Introduction

Soil health is the cornerstone of sustainable agriculture, directly influencing crop productivity and environmental quality. Biostimulants, derived from organic waste materials, represent an innovative approach to improving soil health, enhancing crop productivity, and addressing pressing environmental concerns, such as nutrient leaching, soil degradation, loss of soil organic matter and biodiversity, and greenhouse gas emissions [1,2,3]. The European Regulation (EU) 2019/1009 defines biostimulants as “products that stimulate plant nutrition processes independently of their nutrient content with the sole aim of improving one or more of the following characteristics of the plant or the plant rhizosphere: (a) nutrient use efficiency; (b) tolerance to abiotic stress; (c) quality traits; or (d) availability of confined nutrients in the soil or rhizosphere” [4]. Several different types of biostimulants have been reported in the literature over the last two decades. They are primarily categorized into five types: microbial inoculants, humic acids and fulvic acids, protein hydrolysates and amino acids, compost tea, and seaweed extracts; each offers unique contributions to plant vitality and soil function. These substances can be multifunctional, influencing various biochemical and physiological pathways of plants that contribute to improved stress resilience, enhanced nutrient uptake, optimized metabolism, and more robust plant growth [5]. Historically, natural substances such as seaweed, animal manure, and compost have been pivotal in enhancing plant vitality and soil fertility. The empirical knowledge of early farmers, who recognized and harnessed the growth-promoting effects of these substances, laid the groundwork for the scientific exploration of biostimulants [6,7,8]. Many studies have demonstrated the benefits of biostimulants in improving nutrient use efficiency, enhancing plant stress tolerance, boosting plant growth, and increasing end-use quality traits [6,9,10,11].

Microbial inoculants have long been recognized for their capacity to enhance plant growth and resilience through targeted biological functions. Plant growth-promoting rhizobacteria (PGPR), mycorrhizal fungi, and endophytic microorganisms improve nutrient acquisition, stimulate phytohormone production, enhance stress tolerance, and suppress pathogens through mechanisms such as biological nitrogen fixation, phosphate solubilization, and induced systemic resistance [12,13,14]. These inoculants can deliver consistent, mechanism-specific benefits; however, their performance is often constrained under field conditions by poor microbial persistence, limited carbon availability, and sensitivity to soil physicochemical stresses [9,15]. Composts, in contrast, provide broad improvements to soil physical, chemical, and biological properties through the addition of organic matter and diverse biochemical compounds. Mature composts enhance soil aggregation, water-holding capacity, nutrient buffering, and microbial habitat formation while supplying humic substances, amino acids, phenolics, and enzymatic activities that stimulate root growth and plant metabolism [11,16]. While composts improve long-term soil fertility and ecosystem stability, their effects on plant growth are often variable, depending on feedstock composition, maturity, and environmental conditions [17,18].

Integrating microbial inoculants with composts in the form of compost-based biostimulants (CBB) represents an ecologically coherent strategy. In these systems, compost serves not only as a source of nutrients and organic matter but also as a protective carrier and metabolic substrate that enhances microbial survival, activity, and functional expression. The co-occurrence of beneficial microbes with humic substances, organic acids, and amino acids enables synergistic interactions that amplify plant responses beyond what either component can achieve alone [11,16]. These interactions regulate key processes, including phytohormone signaling, nutrient mobilization, stress mitigation, and pathogen suppression, at both the rhizosphere and whole-plant levels [19,20].

Growing evidence indicates that CBB outperform standalone microbial inoculants or compost amendments by promoting more stable microbial consortia, sustained bioactivity, and context-dependent adaptability across soils and cropping systems [10,11,21]. This is particularly important in addressing the pressing global challenges of food security and environmental sustainability. Integrating biostimulants into agricultural practices can significantly mitigate the ecological footprint of farming, thus contributing to a more sustainable and resilient agricultural system worldwide [11]. By leveraging these natural and multifunctional compounds, researchers and farmers alike are exploring new strategies to improve plant health and productivity in an ecologically sound manner, promising a future of agricultural practices that are not only more efficient but also environmentally friendly [9].

To fully appreciate the role of compost-based inoculants, it is essential to situate them within the broader framework of biostimulants and their growing potential to transform current agricultural systems. This review, therefore, examines the composition, mechanisms, and agronomic benefits of CBB, with particular attention to their interactions with soil microbial communities and their capacity to address persistent challenges such as nutrient inefficiency, soil degradation, and climate-related stress. Current research is hindered by the absence of standardized production and characterization protocols, a scarcity of long-term and multi-crop field evaluations, and an incomplete mechanistic understanding of how the microbial and biochemical constituents of compost-derived products influence plant physiology and soil functions [17,22]. Addressing these gaps through integrative, mechanistic, and large-scale research is critical for unlocking the full potential of CBB in advancing sustainable and resilient agricultural systems.

2. Compost-Based Biostimulants (CBB)

2.1. Research Evolution and Trends

CBB represent an emerging class of biologically active materials derived from organic waste streams, including crop residues, animal manure, and food waste. Unlike conventional composts, which are primarily used as organic fertilizers, CBB are specifically processed and applied to stimulate plant growth and soil biological activity through microbial and biochemical mechanisms rather than nutrient supply alone [10,11]. Although compost and compost tea have long been recognized as soil amendments, their explicit classification and study as biostimulants is relatively recent. Research interest in CBB began to increase markedly after 2015, coinciding with growing attention to circular bioeconomy strategies and regulatory definitions of biostimulants [4,10,23]. Early studies (2015–2018) focused mainly on characterizing compost teas and vermicompost extracts, examining their microbial composition, humic substances, and enzymatic activity [16,18]. From 2019 onwards, the literature expanded to include detailed evaluations of CBB effects on soil physicochemical and biological properties, such as microbial biomass, enzymatic activity, and nutrient availability [20,24]. More recent studies (2021–2025) have shifted toward multi-omics approaches to profile microbial and metabolic constituents of compost teas and to link these with plant performance and stress resilience [21,25].

2.2. Composition and Production

The composition and functionality of CBB are largely determined by the production process, in which feedstock type, composting parameters, and microbial inputs play central roles. The process typically involves feedstock selection, followed by pre-processing, composting, and product formulation that yield concentrated bioactive liquid or solid formulations. These processes enhance the availability of humic substances, plant growth-promoting enzymes, phytohormones, and beneficial microbes, all of which contribute to plant growth-promotion and enhanced soil functionality [16,17]. The most common CBB products include compost teas, vermicompost extracts, and microbially enriched composts. Figure 1 illustrates the production process of CBB.

Aerobic composting involves the controlled microbial oxidation of organic matter under thermophilic conditions (50–70 °C), producing stable humic material and diverse bioactive metabolites that improve soil structure, nutrient availability, and overall fertility [16,26]. Vermicomposting, in contrast, relies on earthworms and associated microbes to decompose organic residues at lower temperatures, resulting in end-products with higher microbial biomass, elevated enzymatic activity, and plant growth regulators such as indole-3-acetic acid (IAA) and gibberellins [18]. Compost teas represent another widely used category of CBB. These aqueous extracts brewed under aerated or non-aerated conditions contain soluble nutrients, microbial metabolites, and enzyme-rich fractions that can be applied as foliar sprays or soil drenches to stimulate plant growth and suppress diseases [21,26].

Across these production pathways, the choice of feedstock, composting temperature, aeration, pH, moisture, C:N ratio, and maturation time critically shape the structure and function of microbial communities, particularly those involving PGPR, actinomycetes, and mycorrhizal fungi [12,25]. Optimizing these parameters is therefore essential for maximizing biostimulant bioactivity. Typical composting conditions include temperatures of 50–70 °C, moisture content of 50–60%, a C:N ratio of 25–35:1, pH range of 6.5–8.0, and maturation periods of 8–16 weeks [27,28,29]. Upon maturation, solid compost is typically applied at 2–10 t ha⁻¹, while compost teas are diluted at 1:10–1:20 (compost: water) and applied at 2–4 times per season to optimize nutrient and microbial delivery [30,31]. Recent advances further include microbial inoculation and co-fermentation strategies, in which strains such as Bacillus, Trichoderma, and Pseudomonas are introduced during pre-processing to enhance nutrient mineralization, promote plant growth, and increase pathogen suppression [22,24].

Table 1. Plant growth-promoting functions performed by microbes present in compost-based biostimulants.

Microbial Source	Microbial Category	Functions	Ref.
Compost:
Solid compost/compost tea	Plant growth-promoting rhizobacteria (e.g., Azospirillum, Bacillus subtilis)	Improves root architecture, nitrogen fixation, and nutrient uptake.	[12]
Anaerobic compost	Anammox bacteria (e.g., Brocadia, Kuenenia)	Anaerobically oxidizes ammonium (NH4+) to nitrogen gas (N2) using nitrite as an electron acceptor, thereby reducing nitrous oxide (N2O) production.	[11]
Solid compost/Bioactive compost	Actinomycetes (e.g., Streptomyces, Micromonospora)	Produces antibiotics, suppresses pathogens, and decomposes complex organic materials.	[11]
Legume-compost blends/Vermicompost	Nitrogen-fixing bacteria (e.g., Rhizobium leguminosarum, Azotobacter)	Converts atmospheric nitrogen into ammonium for plant absorption.	[32]
Mature compost	Decomposing microbes (e.g., Trichoderma, Aspergillus niger)	Breaks down organic matter into humus, releasing nutrients for plant use.	[11]
Fermented compost extract	Antagonistic fungi (e.g., Trichoderma harzianum, Gliocladium)	Competes with pathogens and produces enzymes that degrade cell walls of harmful microbes.	[33]
Compost extract	Endophytic fungi and bacteria (e.g., Piriformospora indica, Bacillus amyloliquefaciens)	Enhances systemic resistance, produces phytohormones, and aids in stress adaptation.	[12]
Fermented compost	Lactic acid bacteria (e.g., Lactobacillus plantarum)	Contributes to the fermentation process, reduces pH, and inhibits the growth of spoilage microorganisms.	[11]
Mature compost/ Vermicompost	Ammonifying bacteria (e.g., Bacillus, Pseudomonas, Clostridium)	Decomposes organic nitrogen compounds into ammonia or ammonium (NH4+).	[32]
Mature compost	Nitrite-oxidizing bacteria (e.g., Nitrobacter, Nitrospira, Nitrococcus)	Oxidizes nitrite (NO2−) to nitrate (NO3−), which plants can readily absorb.	[4]
Microbially enriched compost:
Sulfur-amended compost	Sulfur-oxidizing bacteria (e.g., Thiobacillus, Acidithiobacillus)	Oxidizes sulfur compounds to sulfate, improving sulfur nutrition and acidifying alkaline soils.	[33]
Compost tea/Phosphate-solubilizing bacteria-enriched compost	Phosphate-solubilizing bacteria (e.g., Pseudomonas fluorescens, Bacillus megaterium)	Solubilizes bound phosphate and mobilizes trace elements to enhance bioavailability.	[4]
Solid compost/mycorrhizal-enriched compost	Endophytic fungi and bacteria (e.g., Glomus, Rhizophagus Irregularis)	Enhances phosphorus uptake, drought tolerance, and soil aggregation.	[33]

provides a detailed description of various plant growth-promoting functions performed by microbes either native to the compost source or enriched in the CBB. Importantly, these production-stage microbial and biochemical transformations establish the mechanistic foundation for the nutrient mobilization, rhizosphere enzymatic activity, and plant physiological responses.

3. Functional Mechanisms of Compost-Based Biostimulants

CBB exert their beneficial effects through the synergistic interactions between microbes, humic substances, and organic molecules, which foster a dynamic soil–plant ecosystem supporting sustainable agricultural productivity [14]. Figure 2 illustrates the major functional mechanisms by which CBB influences plant and soil systems: (i) nutrient acquisition and mobilization, (ii) root development, (iii) stress tolerance and resilience, (iv) pathogen suppression and plant defense activation, and (v) reduction in chemical input. These mechanisms are primarily mediated by the diverse microbial consortia and bioactive compounds present in composts and compost-derived extracts (Table 1).

3.1. Nutrient Acquisition and Mobilization

CBB play a significant role in nutrient cycling by enhancing the availability and uptake of macro- and micronutrients. Because composts naturally harbor diverse microbial consortia, CBB application introduces a wide range of PGPR, mycorrhizal fungi, actinomycetes, and nutrient-transforming bacteria that collectively improve nutrient availability and uptake (Table 1). Microorganisms within composts produce a range of hydrolytic enzymes, such as cellulases, proteases, and phosphatases, that decompose complex organic matter into simpler, bioavailable nutrients [34]. This enzymatic activity supports nutrient mineralization and organic carbon stabilization, ensuring a sustained nutrient supply to plants. Nitrogen-fixing bacteria such as Rhizobium, Azotobacter, and Azospirillum, as mentioned in Table 1, are frequently enriched in vermicompost and legume-residue composts, where their proliferation is supported by high organic nitrogen and enhanced microbial activity [12,32]. These microorganisms fix atmospheric nitrogen (N₂) into ammonium (NH₄⁺), increasing the pool of plant-available nitrogen for crops [35,36]. For instance, Azospirillum introduced through compost tea has been shown to enhance nitrogen uptake and root proliferation in cereals [12], while Azotobacter enriched in vermicompost improves nitrogen nutrition in maize and tomato (Table 1) [37]. In legumes, symbiotic nitrogen fixers such as Rhizobium further boost plant nitrogen status by forming nodules that increase biological nitrogen fixation efficiency [38]. Ammonifying bacteria, including Bacillus, Pseudomonas, and Clostridium, are abundant in mature compost and vermicompost [32]. These organisms decompose organic nitrogenous compounds into ammonium, facilitating a steady release of plant-available nitrogen [32]. In nutrient-poor soils, this ammonification pathway becomes essential for improving baseline soil fertility. Animal-manure composts often contain ammonia-oxidizing bacteria like Nitrosomonas and Nitrosococcus [33], which oxidize ammonium to nitrite (NO₂⁻). Mature composts also host nitrite-oxidizing bacteria such as Nitrobacter and Nitrospira, which convert nitrite into nitrate (NO₃⁻), facilitating plant uptake of available nitrogen [39]. Together, these sequential transformations maintain nitrogen continuity in the soil nitrogen cycle. Anaerobic composts may additionally support anammox bacteria (e.g., Brocadia, Kuenenia) [11], which convert ammonium and nitrite directly into nitrogen gas. Although slower, this pathway contributes to more stable nitrogen cycling under low-oxygen conditions [11]. Conversely, denitrifying bacteria such as Pseudomonas denitrificans and Paracoccus denitrificans commonly appear in well-matured composts with micro-anaerobic pockets. While denitrification can lead to nitrogen loss, controlled activity reduces nitrate accumulation and limits environmental risks such as nitrate leaching [12].

Phosphate-solubilizing bacteria (PSB) include species such as Pseudomonas fluorescens and Bacillus megaterium, which are commonly detected in compost teas and PSB-enriched composts. In contrast, certain fungi, such as Aspergillus niger, also exhibit strong phosphate-solubilizing capacity. These microorganisms enhance phosphorus availability by secreting organic acids (e.g., gluconic, citric, and oxalic acids) and phosphatases that mobilize phosphorus from insoluble mineral complexes [40,41,42]. Mycorrhizal fungi, commonly enriched in solid composts and mycorrhizal-inoculated compost products such as Glomus and Rhizophagus irregularis [43], extend hyphal networks into the soil, dramatically improving phosphorus acquisition and enhancing water and nutrient uptake under drought stress [33]. Potassium-solubilizing bacteria such as Bacillus mucilaginosus and Bacillus megaterium thrive in microbially active composts [44]. They release potassium ions from silicate minerals, improving plant osmotic regulation, enzyme activation, and photosynthetic performance [45]. Sulphur-oxidizing bacteria such as Thiobacillus and Acidithiobacillus [33] occur naturally in Sulphur-amended composts. They convert elemental Sulphur into sulphate (SO₄²⁻), improving Sulphur nutrition and acidifying alkaline soils to increase micronutrient solubility [33].

Composts, particularly those rich in actinomycetes like Streptomyces and Micromonospora, also accelerate the mineralization of micronutrients such as iron, zinc, and manganese by producing siderophores, organic acids, and enzymes [11,12]. Mature compost is dominated by decomposers such as Trichoderma and Aspergillus niger, which break down cellulose, lignin, and other complex polymers into simpler forms that plants and microbes can readily use [11]. This decomposition releases humic substances, organic acids, and enzymes that improve soil structure, water retention, and cation exchange capacity. Fermented compost extracts often contain lactic acid bacteria such as Lactobacillus plantarum, which reduce pH and inhibit spoilage microorganisms while enhancing nutrient solubility [11]. Composts are also rich in bioactive metabolites, including humic and fulvic acids, amino acids, organic acids, and phenolics, which directly enhance soil functionality [11,33]. Humification reactions, including polymerization and condensation of phenolic compounds, generate stable humic substances that strongly influence nutrient cycling and soil organic carbon dynamics. Humic substances improve soil cation exchange capacity and root membrane permeability, facilitating nutrient absorption and stimulating root morphogenesis [12,46]. By mobilizing nutrients from organic and mineral sources, CBB improves nutrient use efficiency, reduces dependence on synthetic fertilizers, and supports sustainable productivity [47,48]. Collectively, CBB-mediated nutrient mobilization ensures both short-term growth stimulation and long-term soil fertility restoration.

Figure 2. Schematic representation of the functional mechanisms underlying plant growth promotion by compost-based biostimulants (CBB). CBB enhance crop growth and production, improve root development and plant tolerance to abiotic stresses, by enhancing cation exchange, nitrogen fixation, and water-use efficiency. They also suppress soil-borne pathogens and induce plant defense responses through organic and inorganic compounds. At the soil level, CBB enhances nutrient acquisition and mobilization, microbial diversity, and enzymatic activity. Together, these mechanisms contribute to improved crop growth, yield stability, reduced reliance on synthetic inputs, and enhanced soil fertility.

Figure 2. Schematic representation of the functional mechanisms underlying plant growth promotion by compost-based biostimulants (CBB). CBB enhance crop growth and production, improve root development and plant tolerance to abiotic stresses, by enhancing cation exchange, nitrogen fixation, and water-use efficiency. They also suppress soil-borne pathogens and induce plant defense responses through organic and inorganic compounds. At the soil level, CBB enhances nutrient acquisition and mobilization, microbial diversity, and enzymatic activity. Together, these mechanisms contribute to improved crop growth, yield stability, reduced reliance on synthetic inputs, and enhanced soil fertility.

3.2. Root Development and Plant–Microbe Symbiosis

Root development is one of the primary pathways through which CBB enhances plant growth, owing largely to the diverse microbial communities and bioactive compounds they supply. Compost types such as vermicompost, compost teas, microbially enriched composts, and mature solid composts are especially rich in PGPR, mycorrhizal fungi, and endophytes that directly influence root architecture and symbiotic functioning [12,33]. PGPR, such as Azospirillum, Bacillus subtilis, and Pseudomonas fluorescens, are commonly found in solid composts and compost teas. They synthesize plant growth regulators, including auxins (especially IAA), gibberellins, and cytokinins [7,49]. Microbial production of gibberellins and cytokinins regulates cell elongation and division, promoting shoot and leaf development [50,51]. Auxin (IAA) produced by Bacillus, Pseudomonas, and Azospirillum stimulates root elongation, lateral root initiation, and root hair formation, increasing root surface area and enhancing soil exploration [49,52]. Vermicompost extracts, which are particularly rich in auxin-producing microbes, consistently improve early root establishment in cereals and vegetables [53]. Certain Azospirillum and Bacillus strains in compost teas produce gibberellins that enhance cell elongation, accelerating root and shoot expansion [50]. Cytokinin-producing microbes, including Pseudomonas species enriched in mature compost, promote cell division and shoot–root balance, contributing to vigorous seedling development [51]. AMF, including Glomus and Rhizophagus species, are commonly found in solid composts and mycorrhiza-enriched compost amendments [33]. These fungi establish mutualistic associations with plant roots, resulting in several key benefits. AMF hyphal networks not only extend the effective root surface area and enhance nutrient and water uptake, but also improve soil–root hydraulic conductivity, enabling plants to extract water from deeper or drier soil layers, an especially valuable trait under drought or water-limited conditions [54]. Studies show that dual application of compost and AMF increases plant hydraulic conductance and promotes better water status under stress, likely through enhanced root fungal uptake area and osmotic adjustment compared with compost alone or inorganic fertilizer application [54,55]. Beyond physical soil improvements, AMF and PGPR exert synergistic effects on root morphology and overall plant growth. In combined inoculation studies, both AMF and PGPR increased total root length, surface area, and branch number, while enhancing soil nutrient availability and soil enzyme activities associated with nutrient cycling, relative to uninoculated controls [56]. The interaction between PGPR and AMF can also affect secondary plant metabolites that influence root–microbe signaling. For instance, PGPR may promote AMF colonization by upregulating flavonoid biosynthesis in the rhizosphere, which acts as a molecular signal facilitating mycorrhiza establishment and improved root penetration into the soil [57]. Collectively, through hormone modulation, enhanced water and nutrient uptake, soil structural improvements, and synergistic plant–microbe interactions, CBB fosters the development of robust root systems that underpin greater plant performance and resilience.

3.3. Improvement in Soil Structure

Beyond biological activity, the agronomic performance of CBB is strongly influenced by its physical properties. Soil structure is characterized by a heterogeneous distribution of pore sizes categorized as macropores, mesopores, and micropores. Macroporosity facilitates aeration and gas exchange, supporting aerobic microbial processes and root growth, whereas micropores enhance water-holding capacity and nutrient retention, particularly the retention of carbon [58,59,60]. The type of compost incorporated can change the soil porosity. Mature compost amendments can substantially alter soil physical qualities by increasing total porosity and modifying pore size distribution, which in turn affects oxygen diffusion, water movement, and microbial habitat formation in amended soils [61,62,63]. For example, with increasing compost application rates, total porosity and the volume of water-retaining pore fractions increase significantly, resulting in improved soil structure and hydraulic characteristics [62]. Furthermore, compost application alters soil bulk density, where organic amendments generally decrease the bulk density, leading to reduced compaction and improved root penetration and gas exchange [64,65].

Particle size distribution and morphology further influence mass transfer dynamics. Finer particles tend to increase surface area and water retention but may reduce aeration if the fine fractions dominate the pore network, whereas coarser structural fractions improve macroporosity and gas exchange [62,65]. CBB derived from coarser feedstocks such as vermicomposts with larger structural particles (straw, wood chips) can increase mesopore volume of amended soils, enhancing overall porosity and aeration [66]. In contrast, finely sieved compost introduces fine organic fractions that increase total surface area and water retention in the soil matrix [62]. These structural attributes directly influence effective diffusion of oxygen and soluble nutrients, thereby affecting microbial activity, decomposition rates, and plant nutrient availability.

Moisture retention characteristics and hydraulic conductivity also play critical roles in regulating nutrient mobility and microbial metabolism in soil. Compost incorporation enhances water retention capacity and plant-available water content, reflecting shifts in pore size distribution toward fractions that retain water at less negative matric potentials [62]. Collectively, porosity, bulk density, particle geometry, and mass transfer parameters determine the efficiency of nutrient transformation processes and ultimately modulate the agronomic effectiveness of CBB.

3.4. Stress Tolerance and Resilience

CBB enhance plant resilience to abiotic stresses, including drought, salinity, and heavy metal toxicity, through a combination of microbial activity, compost-derived biochemical compounds, and improved soil physical properties. These mechanisms act synergistically to protect plants under adverse environmental conditions. Under drought stress, CBB improves plant water-use efficiency primarily by stimulating root system expansion and promoting osmotic adjustments. PGPR such as Azospirillum, Bacillus, and Pseudomonas spp. synthesize auxins and cytokinin-like compounds that intensify root branching and root hair formation, increasing the plant’s ability to access water under dry conditions [12]. AMF found in solid composts or mycorrhiza-enriched composts extend the effective root absorptive area through their hyphal networks, improving hydration and facilitating the accumulation of compatible osmolytes such as proline and glycine betaine that maintain cellular osmotic balance during water deficit [67]. These improvements are often more pronounced in mature composts rich in humic and fulvic acids, which act as signalling molecules influencing aquaporin expression and drought-responsive hormonal pathways [68].

Under salinity stress, PGPR-mediated regulation of plant ethylene levels represents a key protective mechanism. Solid composts and vermicomposts are rich in microbial strains such as Pseudomonas syringae, Pseudomonas fluorescens, and Rhizobium that produce aminocyclopropane-1-carboxylate (ACC) deaminase, which reduces stress-induced ethylene accumulation that would otherwise inhibit root elongation [32,42,69]. For example, ACC deaminase-producing PGPR enhanced primary and lateral root growth in salt-stressed mung bean plants [70]. AMF further support salinity tolerance by maintaining ionic homeostasis; their extraradical hyphae modulate nutrient uptake processes, helping plants sustain optimal K⁺/Na⁺ ratios that prevent sodium toxicity and protect metabolic functions [71,72,73].

CBB also improve plant tolerance to heavy metal stress, both through microbial transformations and sorption processes driven by compost organic matter. Compost-derived microbial communities, including Pseudomonas, Bacillus, and actinomycetes, immobilize or oxidize heavy metals, reducing their bioavailability and preventing translocation to shoots [74]. Fungal species like Trichoderma and AMF can sequester cadmium and lead within their cell walls, mitigating phytotoxicity and preserving photosynthetic performance [75,76]. Vermicompost and mature composts rich in humic substances further enhance metal chelation due to their high organic ligand content (

Table 2. Key functions of microbes and bioactive components present in compost-based biostimulants.

Functional Category	Microbes/Bioactive Components	Source	Key Functions and Mechanisms	Refs.
Nutrient acquisition & mobilization	N-fixers (Rhizobium, Azotobacter)	Compost tea/legume–compost blends/mature compost	Biological N fixation	[32]
	PSB (Pseudomonas, Bacillus), decomposers	Legume–compost blends/mature compost	P solubilization, enzymatic mineralization	[4]
Root development and plant–microbe symbiosis	Humic acids, fulvic acids, auxin-like compounds	Vermicompost/ Compost extract	Stimulates lateral root formation, root elongation, and root hair density via hormone-like activity and signaling modulation	[11,78]
	Azospirillum, Bacillus, Pseudomonas, ACC deaminase-producing bacteria	Compost + PGPR-enriched formulations	Enhances root architecture, root surface area, and rhizosphere competence through phytohormone production and ethylene regulation	[12,79]
	Arbuscular mycorrhizal fungi	Mycorrhizal-enriched compost	Promotes root branching, increases absorptive surface area, and strengthens symbiotic nutrient exchange networks	[33]
Stress tolerance and resilience	Azospirillum, Pseudomonas fluorescens, Bacillus subtilis	Solid compost/ Vermicompost	Produces ACC deaminase, auxins, and osmolytes that reduce stress-induced ethylene, enhance root growth, and improve drought and salinity tolerance	[12,67]
	Glomus, Rhizophagus irregularis	Mycorrhizal-enriched compost	Improves water uptake, osmotic adjustment, nutrient acquisition, and antioxidant activity under drought and salinity stress	[33,67]
	Humic substances, amino acids, phenolics	Vermicompost/ Compost tea	Enhances antioxidant enzyme activity (catalase, peroxidase), regulates hormone signaling, and improves plant resilience to abiotic stress	[37,77]
Pathogen suppression and plant defense activation	Bacillus subtilis, Pseudomonas fluorescens	Bioactive compost/Compost tea	Produces antibiotics, lipopeptides, siderophores, and volatile organic compounds that suppress soil-borne pathogens and compete for nutrients and niches	[12,13]
	Trichoderma harzianum, Gliocladium	Fermented compost extract	Mycoparasitism, production of cell-wall-degrading enzymes, and induction of systemic resistance	[33,79]
	Endophytic bacteria and fungi (Bacillus amyloliquefaciens, Piriformospora indica)	Compost extract/ Microbial consortia	Activates jasmonic acid, salicylic acid, and ethylene mediated defense signaling, primes immune responses, and enhances systemic resistance	[15,80]
Reduction in chemical fertilizer inputs	Diverse decomposers and nutrient-cycling microbes	Mature compost	Improves soil structure, increases nutrient retention, and enhances mineralization, reducing the need for synthetic fertilizers	[11,81]
	Azotobacter, Rhizobium, Bacillus, PSB	Compost + PGPR-enriched formulations	Enhances biological nitrogen fixation, phosphorus solubilization, and nutrient use efficiency, enabling partial replacement of mineral fertilizers	[32]
	Antagonistic microbes, phenolics, organic acids	Compost-based systems	Suppresses pathogens and strengthens plant immunity, reducing reliance on chemical pesticides	[16,82]

PSB: phosphorus solubilizing bacteria; PGPR: plant growth-promoting rhizobacteria; ACC: aminocyclopropane-1-carboxylate, N: nitrogen; P: phosphorus.

). CBB application also strengthens plant antioxidant defences by upregulating enzymes such as catalase and peroxidase, thereby mitigating reactive oxygen species accumulation and protecting cellular integrity during stress exposure [17,77]. Collectively, CBB function as biological stress modulators, combining microbial activity, organic matter chemistry, and soil structural improvements to enhance plant adaptability and resilience across diverse environmental stressors.

3.5. Pathogen Suppression and Plant Defence Activation

A key function of CBB lies in their ability to suppress soil-borne pathogens and induce plant immune responses. A central mechanism of pathogen suppression involves beneficial microorganisms commonly enriched in mature composts, vermicomposts, and compost teas. Beneficial bacterial and fungal microbes, such as Bacillus, Pseudomonas, and Trichoderma species, respectively, inhibit pathogen proliferation through competitive exclusion, antibiosis, and resource depletion [13,82]. For instance, Bacillus subtilis, frequently isolated from solid and bioactive composts, produces lipopeptides (e.g., urfactins, iturins, fengycins) and antibiotics that disrupt fungal pathogen cell membranes and inhibit pathogen growth. Similarly, Pseudomonas fluorescens, commonly enriched in compost teas and aerated extracts, releases volatile organic compounds that inhibit the growth of pathogens such as Fusarium oxysporum [83,84]. Fungal biocontrol agents present in compost-based products, particularly Trichoderma species, further enhance disease suppression through mycoparasitism and enzymatic degradation of pathogen cell walls. These fungi secrete chitinases, glucanases, and proteases that weaken or lyse pathogenic fungi, while also rapidly colonizing root surfaces to prevent pathogen establishment. Vermicompost and fermented compost extracts are especially rich in Trichoderma and actinomycetes, which contribute to sustained pathogen suppression in soil systems. Beyond direct antagonism, CBB also promotes plant defense activation by stimulating induced systemic resistance and systemic acquired resistance in plants. Compost-associated microbes trigger plant immune pathways through the release of signalling molecules such as salicylic acid, jasmonic acid, ethylene, and microbe-associated molecular patterns, which prime plants for faster and stronger defense responses upon pathogen attack [15,80]. This dual action-pathogen suppression and immune activation reduce disease incidence while decreasing reliance on chemical pesticides, supporting more sustainable pest management.

3.6. Reduction in Chemical Inputs

In addition to improving soil physical, chemical, and biological properties, CBB can substantially reduce the need for synthetic fertilizers by enhancing nutrient use efficiency. Physically, compost applications enhance soil aggregation, porosity, water retention, aggregate stability, and pore continuity, which facilitate root growth, water infiltration, and gas exchange [24,85]. Chemically, they increase cation exchange capacity, nutrient buffering, and pH stability while reducing nutrient leaching losses [81]. Nutrient cycling facilitated by compost-derived microbes and organic matter increases the availability of nitrogen, phosphorus, and other essential elements, allowing crops to achieve similar or higher yields with reduced external inputs [86]. Quantitative studies have demonstrated that integrating compost with chemical fertilizers can significantly reduce fertilizer requirements while maintaining or even increasing crop productivity. For example, integrated nutrient management trials with vermicompost showed that yields were 12–140% higher when vermicompost was combined with chemical fertilizers compared to using 100% recommended doses of mineral fertilizer alone in various crops such as cowpea, wheat, pepper, tomato, lentil, and sunflower [87,88,89,90,91,92,93]. Incorporating organic amendments like compost into cropping systems can increase yields by 10–29% and simultaneously reduce fertilizer input, leading to higher profitability for farmers over multiple seasons [85]. These findings illustrate that CBB not only improves soil nutrient reservoirs but also allows substantial reductions in chemical fertilizer use (often 25–67% or more) without compromising, and frequently enhancing, crop yield and quality. Over time, these processes contribute to the accumulation of stable soil organic carbon, improved soil aggregation, and increased microbial biomass, thereby reinforcing long-term soil fertility [94].

4. Economic Feasibility of Compost-Based Biostimulants

The adoption of CBB is influenced not only by agronomic performance but also by techno-economic feasibility at both the production and application stages. CBB are typically produced from agricultural residues, municipal green waste, and agro-industrial by-products, converting low-value organic streams into marketable soil amendments. Production economics primarily involve feedstock collection and transportation, composting infrastructure (windrow or in-vessel systems), labour and energy inputs for turning and aeration, maturation time, post-processing (screening, extraction, filtration for liquid products), packaging, and quality control to meet regulatory standards. A recent assessment by Su et al. [95] reported that industrial-scale food waste composting facilities achieved positive net present value under multiple market scenarios, with compost selling prices ranging from USD 61-75/MT of compost and a 28–67% return on investment within a 4-year margin. Another study showed that decentralized composting systems reduce transport and disposal costs while achieving economic feasibility at the community scale [96]. When compared to synthetic plant growth regulators and commercial microbial inoculants, compost-derived products generally require low capital-intensive fermentation systems, sterilization, downstream purification, and cold-chain storage. A technoeconomic comparison of microbial inoculant production showed that fermentation-based inoculant production incurs substantially higher capital and operational expenditure due to bioreactor infrastructure and quality control requirements.

At the application stage, the economic advantage of CBB arises from improved yield performance, reduced synthetic fertilizer inputs, enhanced soil health, and long-term sustainability [85]. Economic modeling studies further indicate that partial fertilizer substitution combined with soil health improvements increases long-term profitability by reducing input costs and buffering against yield variability [97,98]. Tang et al. [99] reported that a comprehensive economic cost–benefit evaluation of substituting synthetic fertilizer with 50% pig manure increased economic benefit per unit area by 37–46%, and reduced agricultural inputs and environmental impacts per unit product by 22–44%. In certain cases, the partial substitution of mineral fertilizers with compost and biochar combinations has been shown to enhance monetary returns, with benefit–cost ratios exceeding 1.3 in several cropping systems [100]. Collectively, these findings demonstrate that CBB offers a technically viable and economically competitive strategy that integrates waste valorization, input cost reduction, and long-term soil health enhancement within sustainable agricultural systems.

5. Challenges and Limitations in Compost-Based Biostimulant Research

Despite their proven potential to improve soil health and crop productivity, CBB face several scientific, technical, and socioeconomic challenges that constrain their scalability and consistent performance in agricultural systems. These challenges are interconnected and span from methodological variability in production and composition to gaps in mechanistic understanding, field validation, and policy frameworks.

5.1. Lack of Standardization in Formulation, Production, and Regulation

The most significant barrier to CBB advancement is the absence of standardized methodologies for compost production, extraction, and microbial enrichment. Current research employs highly heterogeneous feedstocks ranging from green waste and manure to food residues processed under diverse composting durations, aeration regimes, and temperature conditions [16,22]. This variability results in wide fluctuations in the biochemical and microbial composition of compost, leading to inconsistent agronomic outcomes [17,18]. For example, thermophilic composts often yield higher humic and fulvic acid concentrations, whereas vermicomposts are richer in microbial biomass and phytohormones [101]. This compositional variability complicates the establishment of consistent quality benchmarks, underscoring the need for globally accepted standards for compost maturity indices, microbial load, nutrient profiles, and bioactive compound content. Without such benchmarks, meaningful comparisons across studies remain difficult, and farmer confidence in product performance is undermined. These technical challenges are further exacerbated by regulatory fragmentation, particularly in North America, where CBB lack clear and harmonized classification. Unlike the European Union, which has implemented a unified regulatory framework under Regulation (EU) 2019/1009, the United States and Canada currently classify CBB ambiguously across fertilizer, soil amendment, and microbial inoculant categories, creating uncertainty in product registration, labelling, and commercialization [4]. Establishing globally accepted benchmarks for compost maturity indices, microbial load, and nutrient composition is essential to ensure product consistency and quality assurance. This will allow for comparisons across studies to be conclusive and increase farmer confidence.

5.2. Incomplete Mechanistic Understanding

Although numerous studies have demonstrated the beneficial effects of CBB on plant growth and soil health, the mechanistic basis underlying these responses remains only partially understood. Much of the existing research examines either microbial inoculants or compost-derived organic compounds in isolation, often overlooking the complex and synergistic interactions among microbial metabolism, humic substances, amino acids, phenolics, and enzymatic activities that jointly regulate plant and soil responses [19,20]. Furthermore, the persistence, stability, and functional redundancy of compost-derived microbial consortia under field conditions remain poorly characterized, as do their dynamic interactions with plant roots and soil physicochemical properties. Furthermore, the influence of environmental variables such as temperature, pH, and moisture on microbial functionality during composting and after soil application remains underexplored, leading to variability in efficacy and reproducibility [102]. Advanced molecular tools such as metagenomics and metabolomics should be integrated into compost quality evaluation to identify the core functional microbiome and its associated metabolites that confer biostimulant activity [21,103].

5.3. Limited Crop- and Soil-Specific Field Validation

Evidence supporting CBB efficacy is primarily derived from short-term greenhouse trials on model crops like tomato, lettuce, wheat, or maize [24,25,104]. Such studies seldom capture the environmental variability inherent to real-world agricultural systems. Field-scale validation remains scarce, especially in diverse agroecological zones where soil type, climate, and management practices influence microbial survival and performance. To ensure reliability, multi-crop, multi-year, and multi-site field experiments are urgently needed. These trials should evaluate CBB performance across varying soil textures, organic matter levels, and climatic gradients, incorporating physiological, agronomic, and microbial parameters. Long-term datasets would allow assessment of CBB contributions to soil carbon sequestration, nutrient cycling, and yield stability [38].

5.4. Socioeconomic, Environmental, and Adoption Barriers

Despite their environmental benefits, the adoption of CBB is often hindered by limited farmer awareness, high production costs, and uncertain short-term returns [8]. Producing high-quality compost requires specialized infrastructure, controlled conditions, and microbial inoculation expertise, factors that increase costs compared with synthetic fertilizers. Additionally, over-application or improper use can cause nutrient imbalances, leaching, or greenhouse gas emissions, counteracting environmental goals [10,105]. Bridging this gap requires economic incentives, public–private partnerships, and farmer training programs. Participatory field demonstrations and cooperative composting initiatives could enhance farmer confidence and highlight long-term profitability through improved soil fertility and reduced chemical input dependency [3].

6. Future Perspectives and Research Directions

To fully exploit the potential of CBB, future research must adopt holistic and standardized frameworks that bridge laboratory innovation with field-scale application. A global priority is the development of unified guidelines for CBB production, including feedstock selection, microbial enrichment, extraction, and storage to ensure reproducibility, quality assurance, and consistency across regions. Advances in precision composting, aerobic fermentation, and biochar integration offer promising avenues to improve microbial stability and nutrient retention, while the establishment of compost maturity indices and microbial efficacy benchmarks would support both scientific comparability and commercial certification [11].

Critically, CBB should be investigated as integrated biological systems rather than as isolated components of compost chemistry or microbial inoculants. Traditional compost studies have largely focused on nutrient supply and soil organic matter dynamics, while PGPR research has emphasized strain-specific plant responses. In contrast, the efficacy of CBB emerges from synergistic interactions among diverse microbial consortia, organic substrates, and bioactive metabolites that collectively regulate nutrient mobilization, hormone signalling, stress tolerance, and disease resistance. These system-level effects cannot be fully captured through simple component-focused approaches, underscoring the need for experimental designs that explicitly account for microbial–organic matter–plant feedback.

Equally important is the implementation of coordinated multi-crop and multi-year field trials across diverse soil types and climates to validate agronomic, environmental, and economic outcomes. In parallel, the integration of multi-omics approaches with precision agriculture tools offers a transformative opportunity to elucidate CBB–plant–soil interactions and enable real-time monitoring of soil biological health and nutrient dynamics [32,106,107]. Climate resilience and carbon-smart agriculture should further guide future innovation, as CBB can enhance nutrient use efficiency, reduce reliance on synthetic fertilizers, and contribute to long-term soil carbon sequestration [81].

Finally, supportive policy frameworks, harmonized regulations, and stakeholder engagement, including farmer education and incentive programs, will be essential for translating scientific advances into widespread adoption [8]. By aligning mechanistic understanding with practical implementation and recognizing CBB as functionally distinct, systems-based inputs, CBB can move beyond experimental tools to become foundational components of resilient, productive, and sustainable agricultural systems.