Natural Biostimulants From Organic Waste for Sustainable Agriculture

Natural Biostimulants From Organic Waste for Sustainable Agriculture: History

View Latest Version

Please note this is an old version of this entry, which may differ significantly from the current revision.

Subjects: Agronomy

Contributor:

Roberto Carlos Solano Porras

Golafarin Ghoreishi

Agriculture has been experiencing a difficult situation because of limiting factors in its production processes. Natural biostimulants (NBs) have emerged as a novel alternative to sustainable agriculture. In this document, a review on the use of solid-state fermentation organic waste for the production of NBs is analysed in detail.

natural biostimulant
solid-state fermentation
organic waste
sustainable agriculture
crop improvement

1. Introduction

One of the main challenges in agriculture is achieving global zero hunger [1]. Therefore, sustainable agriculture is a viable method to ensure food security. In this regard, the Food and Agriculture Organization of the United Nations (FAO) envisions providing nutritious and accessible food for all while preserving natural resources to meet current and future needs. Sustainable agriculture also aims to benefit producers in terms of economic development [1]. In conventional agriculture, reducing the intensive use of agrochemicals is a significant challenge that negatively impacts soil health, water scarcity, and biodiversity [2]. In this context, natural biostimulants (NBs) have emerged as alternatives to sustainable agriculture. NBs are derived from products such as microorganisms, plant extracts, and seaweed extracts and can be classified into three main groups based on their source and content: humic substances (HS), hormone-containing products (HCP), and amino-acid-containing products (AACP). HCP, such as seaweed extracts, contain various active substances for plant growth, including auxins, cytokinins, and their derivatives [3]. These products contain biologically active compounds that stimulate plant physiological processes and promote growth, development, and resistance to biotic and abiotic stresses [4,5,6,7]. NBs offer significant advantages because they are derived from natural sources, such as waste materials, plant extracts, and microorganisms [8,9], making them more environmentally sustainable than chemical products based on synthetic compounds. Furthermore, NBs are generally safer for the environment and human health than chemical products, which can be harmful [10]. NBs also have the potential to promote beneficial interactions with soil microorganisms, unlike chemical products that lack this capacity [11]. Additionally, NBs can serve as an easier alternative to chemicals in order to comply with regulations and restrictions in many countries [6,12,13]. Given these issues, NBs present themselves as a promising alternative in agriculture.

Various production methods exist, including solid-state fermentation (SSF), a technology conducted in the absence or near absence of free water, allowing the use of solid materials as substrates for enhanced biotransformation. SSF has been reported as a promising eco-technology for the production of bio-based products, and studies have demonstrated the successful pilot-scale production of NBs using plant biomass as a support and carbon source for different microorganisms. These production processes are performed under controlled conditions, including temperature, humidity, and airflow, to optimize NB synthesis [14,15]. Furthermore, the utilization of organic waste as a substrate in the SSF process has gained attention, primarily involving various solid biodegradable materials derived from agricultural and forestry byproducts and waste [16]. NBs obtained through SSF have shown biostimulant effects on crop development, including physical parameters such as germination, growth, stem length, leaf count, root dry weight, leaf area, biomass production, macronutrients, and micronutrients [17]. They have also demonstrated positive effects on root development in forest species [18]. Therefore, NBs produced through SSF represent an emerging alternative to the limitations of conventional biostimulants, including their negative impact on agricultural sustainability, the need to reduce the impact of waste on the environment, and the desire to limit the use of synthetic compounds in agriculture [19].

2. Definition and Types of Biostimulants

NBs are derived from natural sources such as microorganisms, plant residues, and seaweed, among others [20]. These products contain biologically active compounds that stimulate plant physiological processes, promoting plant growth, development, and resistance to biotic and abiotic stresses [10]. However, biostimulants include a wide range of compounds, as highlighted by the European Biostimulants Industry Council (EBIC) and the Biological Products Industry Alliance (BPIA) [14]. The EBIC defines plant biostimulants as substances or microorganisms that stimulate natural processes to enhance nutrient uptake, efficiency, stress tolerance, and crop quality. They do not a have direct pesticidal action and are not regulated by pesticide laws. BPIA defines biostimulants as diverse materials that improve crop vigour, quality, yield, and tolerance to abiotic stresses by facilitating nutrient uptake, enhancing soil microorganism development, and stimulating root growth to increase water-use efficiency [12,13]. This growth is in line with an increase in scientific support for the use of biostimulants as agricultural inputs for various plant species [21].

Currently, there are various types of NBs, including those produced by SSF, which can serve as a starting point for future research (Table 1).

Table 1. Types of NBs, mode of action, and effects produced by SSF.

Natural Product	Type of NB	Molecules Present	Action Mode	Biostimulant Effect	SSF-Relevant Origin	Refs.
Hormone-Containing Products (HCP)	Auxins	3-indoleacetic Acid (IAA)	Promotes cell elongation	Stimulates cell elongation and rooting	Produced by SSF	[22,23]
		Indole Propionic Acid (AIP)	Promotes vegetative growth and cell division	Stimulates growth, flowering, and rooting in plants	Not produced by SSF	[24,25]
	Cytokinins	Zeatin	Stimulates cell division and vegetative growth	Promotes growth and development of plants	Not produced by SSF	[26,27,28]
	Cytokinins	Kinetin	Stimulates cell division and vegetative growth	Improves the quality of the crops, increasing the size and weight of the fruits	Produced by SSF and vermicompost	[29,30,31]
	Abscisic Acid (ABA)	ABA	Regulates stress responses and plant development	Improves stress tolerance and fruit ripening	Produced by SSF	[32,33]
	Gibberellins	Gibberellin A3 (GA3)	Stimulates growth and vigor in plants	Inducts germination and flowering	Produced by SSF	[34,35,36]
	Gibberellins	Gibberellin A4 (GA4)	Promotes plant growth and development	Stimulates germination, development of lateral shoots, and flowering	Produced by SSF	[37,38]
Seaweed Extract (AM)		Alginic Acids	Improves nutrient absorption and stimulates enzyme activity	Increases growth and resistance to abiotic stress	Produced by SSF	[39,40,41]
	AM	Fucoidan	Improves the defense mechanisms of plants	Increases resistance to abiotic stress	Produced by SSF	[42,43,44,45]
		Oligosaccharides	Stimulates physiological responses in plants	Improves immune response and growth	Produced by SSF	[46,47,48,49]
Humic Substances	Humic and Fulvic Acids (AHF)	Humic Acids	Improves soil structure and nutrient availability	Stimulates root growth and nutrient absorption	Produced by SSF	[50,51,52,53]
Humic Substances	Humic and Fulvic Acids (AHF)	Humic Acids	Stimulates plant growth and development	Improves nutrient uptake and stress resistance.	Produced by SSF	[54,55]
Amino-Acid-Containing Products (AACP)	Amino Acids	L-proline	Regulates plant stress and development	Enhances stress tolerance and resistance	Produced by SSF	[56,57,58]
Amino-Acid-Containing Products (AACP)	Peptides	Low Molecular Weight Peptides	Stimulates plant growth and development	Improves plant nutrition and growth	Produced by SSF	[59,60,61]
Other NBs	Siderophores	Siderophores	Binds to Fe and is solubilized	Improves absorption and mobilization of Fe	Produced by SSF	[62,63,64]
Other NBs	Chitosan Fungal	Chitosan Fungal	Promotes plant growth, cell division, increases enzyme activity, and improves nutrient transport	Presents biostimulant activity in seed germination	Produced by SSF	[65,66]

3. Advantages of Natural Biostimulants over Conventional Ones

In this regard, NBs obtained through SSF have emerged as an alternative to conventional biostimulants, primarily because of their positive impact on agricultural sustainability, reduced environmental waste, and limited use of synthetic compounds in agriculture [46].

NBs obtained by SSF from organic waste are gaining interest because of their numerous advantages over conventionally synthesized biostimulants [47]. This article reviews and compares the advantages of NBs in terms of effectiveness, safety, sustainability, and environmental benefits. Among these advantages, the following can be highlighted.

3.1. Sustainability and Environmental Impact

The importance of NBs as a sustainable option in agriculture lies in their renewable origin and lower environmental impact than chemical biostimulants [21].

Generally, the use of NBs has a positive environmental impact [19,48,49]. They can help to reduce or rationalize the amount of synthetic fertilizers and pesticides needed to grow plants [67,68,69]. For example, some NBs can have a positive effect on microbial communities in the soil and can be beneficial for agricultural practices [11]. In terms of environmental impact, NBs extracted from microorganisms are non-toxic and do not pollute the environment [70,71]. In addition, because they are obtained from natural sources, their production is more sustainable than that of chemical biostimulants.

3.2. Security

In contrast to the risks associated with the chemicals used in chemical biostimulants, NBs tend to be safer for both the environment and human health [72].

3.3. Broad Spectrum of Activity

NBs have a wide spectrum of activities, which implies multiple benefits for plants in terms of growth, nutrient absorption, stress resistance, flowering, and fruiting quality [20,73].

3.4. Positive Interactions

NBs promote beneficial interactions with soil microorganisms, improving soil health and favoring more balanced and productive agricultural systems [49,74].

3.5. Regulatory Compliance

NBs offer an easier option for complying with government regulations and restrictions on the use of chemicals in agriculture, which has become more relevant in many countries [6].

4. Production Processes of NBs by SSF

Thus, SSF is a promising method for NBs production. SSF produces a variety of bioactive products that promote plant growth, development, and responses to abiotic and biotic stress conditions [75,76]. In this chapter, the processes used to obtain natural biostimulants through SSF were explored, highlighting their importance and efficacy in sustainable agriculture.

4.1. Substrate Selection in NB Production by SSF

The appropriate choice of substrates is a crucial step in the production of NBs by SSF [77]. Substrates provide a source of nutrients, energy, and bioactive compounds for microorganisms during fermentation. [78]. The most commonly used substrates in SSF include agricultural residues, agro-industrial waste, food industry by-products, and lignocellulosic materials [16]. These substrates are rich in nutrients and can be degraded by microorganisms, allowing the production of beneficial metabolites [79].

4.2. Substrate Pretreatment

Pretreatment of substrates is necessary to improve their composition and nutrient availability. Pretreatment may involve steps such as crushing, grinding, sieving, pH adjustment, sterilization, and addition of nutritional agents [75,80,81]. These steps aim to optimize the conditions for microbial growth and production of desired metabolites [82]. Pretreatment can also facilitate the degradation of substrates and increase fermentation efficiency [83].

4.3. Microorganisms for NB Production by SSF and Inoculation

Microorganisms play a fundamental role in the production of NBs by SSF, as they are responsible for substrate degradation and synthesis of bioactive metabolites [84]. This section will focus on the different microorganisms used in this process and their relevance to NB production.

Examples of microorganisms used in SSF for NB production include bacteria, fungi, and yeasts. Each type of microorganism possesses specific characteristics that can influence biostimulant production.

The inoculation of microorganisms is a crucial step in the production of NBs by SSF [18]. Beneficial microorganism strains such as bacteria, fungi, and yeast are selected for their ability to degrade substrates and produce bioactive metabolites. These microorganisms were pre-cultivated under optimal conditions and then inoculated into substrates to initiate SSF [78,84]. The choice of suitable microorganisms and their interactions during SSF influence the composition and final quality of the biostimulant [18].

4.4. Control of SSF Conditions

Control of SSF conditions is essential for obtaining high-quality biostimulants through SSF. Parameters such as the temperature, humidity, pH, C/N ratio, moisture content, and process duration must be monitored and adjusted accordingly. These conditions affect the growth and metabolism of microorganisms [15,18]. The precise control of SSF conditions ensures the optimization and quality of the biostimulant.

The production of natural biostimulants through SSF involves the selection of suitable substrates, pretreatment of substrates, inoculation of microorganisms, and control of SSF conditions. These processes are crucial for obtaining high-quality NBs that can promote plant growth.

4.5. SSF Bioreactors in NB Production

The use of SSF bioreactors has proven to be a promising technique for improving NB production. These systems allow for better control of fermentation conditions and higher efficiency in obtaining high-quality biostimulants [15].

SSF bioreactors can be designed to maintain optimal cultivation conditions, including temperature, humidity, aeration, and water content [85]. The appropriate selection of the bioreactor depends on various factors, such as the type of microorganism, substrate used, and desired production scale [86]. Common types of SSF bioreactors include fixed-bed, fluidized-bed, and packed-bed bioreactors [79]. The implementation of SSF bioreactors in NB production represents a significant improvement in the efficiency and quality of biostimulants, contributing to a more sustainable and productive agriculture [87,88]. Table 2 presents examples of substrates commonly used in the production of NBs by SSF, together with their characteristics and advantages, microorganism selection, production mode, and bioreactor type.

Table 2. Comparison of substrates, microorganism selection, production mode, and ioreactor type in NB production by SSF.

Substrate	Characteristics and Advantages of Substrate	Microorganism Selection	Production Mode	Bioreactor Type	Refs.
Crop Residues	Abundant local availability, nutrient source, and microorganism support	Bacteria, Fungi	Batch, Continuous, Fed-Batch	Fixed-Bed, Packed-Bed	[89,90,91]
Agroindustrial Waste	Waste valorization and reduced environmental impact	Filamentous Fungi	Batch, Continuous	Fluidized-Bed, Packed-Bed	[47,92]
Food Residues	Rich in nutrients and organic matter, avoids food waste	Bacteria, Filamentous Fungi	Batch, Fed-Batch	Fixed-Bed, Packed-Bed	[37,93]
Plant Residues	High content of bioactive compounds and phytohormones	Bacteria, Filamentous Fungi	Batch, Continuous	Fluidized-Bed, Packed-Bed	[94,95]
Algal Biomass	Rich in bioactive compounds and auxins	Microalgae	Batch, Fed-Batch	Bubble-Column	[96,97]
Wood Residues	Sustainable source with lignocellulosic content	Filamentous Fungi	Fed-Batch, Continuous	Fluidized-Bed, Packed-Bed	[98,99]
Residual Sludge	Reduces waste volume and provides rich source of nutrients	Bacteria, Filamentous Fungi	Batch, Continuous	Plug- Flow, Packed-Bed	[100,101]
Fishery Waste	Utilization of waste from the fishing industry	Filamentous Fungi	Batch, Continuous	Packed-Bed	[102,103]
Brewery Waste	Valorization of waste from brewing processes	Filamentous Fungi	Continuous	Packed-Bed	[104,105]
Citrus Waste	Abundant source of bioactive compounds and antioxidants	Filamentous Fungi	Batch, Fed-Batch	Fixed-Bed, Packed-Bed	[33,106]
Coffee Residues	Rich in bioactive compounds and promotes soil health	Filamentous Fungi	Batch, Continuous	Packed-Bed	[107]
Rice Husk	Rich in organic matter and bioactive substances	Filamentous Fungi	Fed-Batch, Continuous	Packed-Bed	[108]

Full details of the References used in this review can be found at:

Solano Porras, R.C.; Artola, A.; Barrena, R.; Ghoreishi, G.; Ballardo Matos, C.; Sánchez, A. Breaking New Ground: Exploring the Promising Role of Solid-State Fermentation in Harnessing Natural Biostimulants for Sustainable Agriculture. Processes 2023, 11, 2300. https://doi.org/10.3390/pr11082300

Open access at:

https://www.mdpi.com/2227-9717/11/8/2300

This entry is adapted from the peer-reviewed paper 10.3390/pr11082300

© Text is available under the terms and conditions of the Creative Commons Attribution (CC BY) license; additional terms may apply. By using this site, you agree to the Terms and Conditions and Privacy Policy.