Biomaterial amendments improve nutrient use efficiency and plant growth

Abstract

The achievement of global food security faces exceptional challenges due to the rapid population growth, land degradation and climate change. Current farming practices, including mineral fertilizers and synthetic pesticides, alone are becoming insufficient to ensure long-term food security and ecosystem sustainability. The lack of robustness and reliability of conventional approaches warrants efforts to develop novel alternative strategies. Bio-based management strategies offer promising alternatives for improving soil health and food productivity. For example, microbial inoculants can enhance nutrient availability, crop production and stress resistance while also remediating contaminated soils. Nanobiotechnology is a promising strategy that has great potential for mitigating biotic and abiotic stresses on plant toward sustainable agriculture. Biochar (including modified biochar) serves as an effective microbial carrier, improving nutrient availability and plant growth. Also, biochar amendments have been demonstrated to have great potential facilitating soil organic carbon sequestration and mitigating greenhouse gas emissions and therefore contribute to climate change mitigation efforts. This review examines the integration of microbial inoculants, nano-fertilizers and biochar, which demonstrates as a promising strategy to enhance soil health, crop productivity and environmental sustainability. However, overcoming challenges related to their mass production, application and potential risks remains crucial. Future research should focus on optimizing these bio-amendment strategies, evaluating their economic viability and developing robust regulatory frameworks to ensure safe and effective agricultural implementation.

Graphical abstract

Keywords

Biochar / climate change / food security / microbial inoculant / nanomaterial / soil amendment

Highlight

	● Microbial inoculants boost nutrient availability, crop yield, stress resilience and soil remediation.
	● Nanomaterials enhance soil fertility, nutrient delivery and mitigate abiotic stresses.
	● Biochar acts as an efficient microbial carrier, boosting soil organic carbon.
	● Integrating bio-amendments enhances soil health and crop productivity and environmental sustainability.
	● Challenges persist in scaling up production, ensuring safe use and understanding long-term impacts.

1 Introduction

With climate change and environmental degradation, a global food crisis is affecting an increasing proportion of the global population. A primary challenge in global agriculture is the sustainable provision of nutrition for the rapidly growing population, which will likely reach 10 billion by 2050 according to a World Health Organization prediction. The increasing probability of adverse agroclimatic conditions exacerbates both biotic and abiotic stressors on agriculture, significantly impacting productivity and soil health^[1]. The agricultural sector currently faces substantial challenges, including the optimization of agrochemical usage, enhancement of crop adaptation and resilience, and improvement of soil productivity and fertility^[2]. Soil amendment management has long been used as a strategy in agriculture to enhance soil health and, subsequently, boost the production of food. The amendments comprise various substances such as microbial inoculants (biofertilizer, bioherbicides, and biopesticides) and nano-fertilizers (metal oxides nanoparticles and nano-scale natural minerals).

Biochar, derived from organic materials such as wheat straw, peanut shells, waste wood, and manure, is another significant soil amendment. This carbon-rich product offers unique properties for soil improvement. Microorganisms, including arbuscular mycorrhizal fungi (AMF), microalgae, and plant growth-promoting bacteria (PGPB), are crucial for sustainable agricultural systems. These beneficial microbes, isolated from plants, water, composted manures, or other organic materials, have been extensively studied^[3]. These beneficial microbes exist in the rhizosphere, where they establish relationships with plant roots. One of the primary functions of PGPB is to promote plant growth. Some PGPB can fix atmospheric nitrogen, converting it into a form that can be readily absorbed by plants, effectively acting as biofertilizers. Others can solubilize phosphates and essential nutrients from the soil, making them more available to the plants. Additionally, PGPB can produce plant growth regulators, such as auxins, cytokinins, and gibberellins, which stimulate plant growth, root development, and overall plant vigor.

Microbial inoculants, also known as biofertilizers, bioherbicides, biopesticides, and bioremediation agents^[4,^5], are composed of living microorganisms. Their multifaceted nature stems from diverse effects on plants, including growth promotion, enhanced seed germination, and pest and disease management^[6]. The application of these inoculants offers a promising approach to sustainable agriculture, potentially reducing reliance on synthetic inputs while enhancing crop productivity and resilience. Also, microbial inoculants contribute substantially to nutrient cycling by enhancing the decomposition of organic matter, mineralization of nutrients and improving nutrient availability to plants. These processes not only support plant growth but also contribute to soil health and sustainability by maintaining a balanced nutrient cycle. The multitude of beneficial impacts associated with microbial inoculants has gained increasing attention within the agricultural sector, leading to their growing prominence and expanding market scale.

Nanotechnology represents a transformative strategy revolutionizing various sectors, including agriculture. Recent advances in nanotechnology have enabled the formation of diverse nanomaterials (NMs), such as nanoparticles (NPs) and nanotubes, which could potentially impact plant rhizosphere microbiome^[7]. These NMs promote plant growth through optimized application procedures and enhance soil physicochemical properties^[8]. For example, the application of nano-fertilizers has been reported to increase crop yield by 10% compared to conventional fertilizers^[9]. Additionally, the use of NPs has led to increase plant biomass (33.3%) and improve soil nutrient availability^[10]. The NPs can enter the rhizosphere through natural processes, such as plant-mediated mineralization, or through the application of industrially coated nanoparticle products, such as nano-fertilizers^[8,^11]. Studies have also shown beneficial effects on soil microbial communities, plant growth and yield^[8].

Biochar is an environmentally persistent material characterized by high carbon content and low oxygen and hydrogen levels. It is produced through thermochemical transformation of organic matter in an oxygen-limited environment. Feedstocks for biochar production include woody residues, crop straw, animal manures, and municipal solid waste^[12]. Over the last two decades, biochar has found widespread application in agriculture, environmental management, and energy sectors due to its cost-effectiveness, sustainability, and associated benefits^[12]. In environmental context, extensive research has been conducted on the use of biochar in the adsorption, degradation, and removal of toxic pollutants. There has been a sustainable growth in related literature, particularly focusing on the sorption of various organic contaminants on biochar, to comprehensively elucidate the underlying mechanisms^[13].

The objective of this review is to evaluate the effects of various biomaterial amendments, specifically microbial inoculants, NMs, and biochar, on soil health and crop production. Additionally, the review aims to explore the potential of these amendments to enhance sustainable agricultural practices, improve food security, and mitigate climate change impacts.

2 Microbial inoculants

More than a century has passed since the production of the first microbial inoculant for plants occurred^[3]. Now the use of microbial inoculants in agriculture is widespread throughout the world for a variety of crops and transporting a variety of microorganisms. Concurrently, abiotic plant stress has increased as a result of global climate change, promoting the development of microbial technologies for conservation and agricultural applications^[14]. Environmental concerns have redirected focus from chemical inputs toward sustainable techniques, such as organic farming and microbial inoculants. Also, recent advances in sequencing and plant-microbiome science has increased investments into microbial inoculants, driven by their demonstrated potential to enhance crop yield^[15]. The subsequent discussion will examine the impact of microbial inoculants on agricultural productivity.

2.1 Enhancing nutrient availability and thus increasing crop yield

Microbial inoculants provide crop with a wide range of nutrients for growth and development, primarily through nitrogen fixation, phosphorus solubilization, and potassium mobilization. Plants mainly absorb nitrogen in the form of nitrate or ammonium ions from the soil to produce amino acids, proteins, nucleic acids, chlorophyll, phytohormones, and many vitamins needed for growth^[16]. However, only a limited number of plants possess the constitutive ability to assimilate atmospheric nitrogen and convert it into a bioavailable form. Nitrogen-fixing microbes (NFMs) with the nitrogenase metalloenzyme, such as iron-iron, vanadium-iron, and molybdenum-iron nitrogenases, have ability to converts inert gaseous dinitrogen into plant-available ammonia^[17], thereby supplementing nitrogen nutrition crucial for enhanced crop yield. For example, Huang et al.^[18] demonstrated that the application of bacterial inoculants enhanced growth and nitrogen use efficiency in Pyrus betulifolia under nitrogen-limited conditions by modifying the existing soil bacterial communities. In agricultural systems, approximately 80% of BNF contributed by symbiotic association made between leguminous plants and species of Allorhizobium, Azorhizobium, Bradyrhizobium, Mesorhizobium, Rhizobium, and Sinorhizobium^[19].

Also, microbial inoculants have been intensively explored as a sustainable and economical substitute for mineral fertilizers, with a focus on their effects on nutrient availability and crop productivity. Sun et al.^[20] showed that the strategy of substituting 50% urea with biofertilizer reduced the nitrogen loss from farmland soil by 54%, increased nitrogen use efficiency by 11.2% and achieved a 5.0% increase in crop yield. The application of the PSMs as biofertilizers has been proved to positively affect the leaf nutrient content (N, P, and K) of Chinese fir, and increase the total N, P, and K, as well as available P and K content in the soil^[21]. The addition of a Pseudomonas as KSB increased K concentration and content by more than 50% and 70% in tomato aerial tissue, respectively. In addition, numerous studies have shown that NFMs, PSM, or KSB can be used as microbial inoculants to decrease the use of mineral fertilizers and address nutrient deficiencies^[20,^21].

Phosphorus is a vital biogenic element involved in the biosynthesis of various plant compounds, including phytic acid, nucleic acids, and phospholipids. In soil, phosphorus exists primarily in insoluble inorganic and organic forms, but plants mainly absorb it in an anionic form (HPO₄^2– or H₂PO₄^–)^[22,^23]. The MPKV bacterial consortium improves the P solubilization by increasing maize plant production^[24]. Phosphate-solubilizing microorganisms (PSMs) have several mechanisms to increase phosphorus bioavailability in soil. They secrete organic acids that lower soil pH, aiding in a dissolution of insoluble phosphates. The PSMs also produce chelating agents that bind cations associated with phosphate compounds, releasing phosphate ions. Additionally, they secrete enzymes like phosphatases and phytases that mineralize organic phosphorus into inorganic forms assimilable by plants. Also, PSMs catalyze the enzymolysis of phosphoric esters, contributing to the liberation of phosphate ions from organic and inorganic sources. Collectively, these mechanisms by PSMs enhance soil phosphorus bioavailability, a crucial factor for optimizing plant growth and productivity^[22]. Potassium is essential for crop growth, involved in vital physiological and biochemical activities such as protein synthesis and nutrient cycling^[21]. In the natural soil ecosystems, potassium is mostly present in a non-exchangeable (fixed) form that is unavailable to plants and bacteria. Potassium solubilizing bacteria (KSB) enhance potassium availability by converting fixed forms into bioavailable forms through acidification and complexation with secreted EPS (e.g., organic and inorganic acids, polysaccharides, and proteins)^[21,^25]. Also, wheat inoculated with different bacterial combinations, the plant N content increased from 40.7% to 97.7% (P < 0.05). Additionally, the P content of wheat increased from 41.2% to 96.4%, and the K content increased from 2.3% to 42.1%^[26].

Numerous studies have demonstrated that microbial inoculants can significantly improve soil nutrient availability, particularly in the soil-root interface crucial for plant nutrient uptake and yield enhancement^[6]. Root exudates are important carbon sources for soil microorganisms, stimulating microbial colonization near the root surface, enhancing nutrient absorption rates by crops^[27]. Additionally, plants exude a diverse array of secondary metabolites that interact with the surrounding biota, influencing microbial communities and nutrient availability^[28]. The studies conducted by Zhang et al.^[28] observed that Hansschlegelia zhihuaiae S113 colonized cucumber roots for 20 days, highlighting its potential for sustained nutrient uptake enhancement. Also, research in oats has shown a positive correlation between the ability of a microbe to utilize root exudates and its abundance in underground tissues, underscoring their role in enhancing nutrient availability and supporting plant health^[29]. At that point, rhizosphere microorganisms have an impact on the plant host fitness in diverse manners, such as by aiding in the acquisition of essential nutrients^[29] and providing in biocontrol benefits^[30].

2.2 Improving crops resistance to drought and salinity stress

Abiotic stresses, such as soil salinity, drought and toxic pollutants, are major constrains to agricultural production, influencing crop growth through physiological disorders, hormonal imbalances, and nutritional deficiencies^[5]. Microbial communities interact with crops through direct or indirect mechanisms, regulating crop growth and productivity^[3]. Therefore, the role of microbial inoculants in enhancing crop stress resistance has become increasingly prominent. Soil salinization poses a significant threat to food production and security, affecting arable croplands in over 100 countries. Projections indicate that by 2050, more than 50% of arable lands will be severely affected by salinization^[31]. High soil salinity leads to stunted plant growth, significantly reducing yield and quality. Excessive soluble salts and exchangeable sodium in soil disrupt ion activity, causing nutrient deficiencies and inhibiting overall plant growth and yield. Microbial inoculants ameliorate salt stress through enhanced nutrient uptake, induction of antioxidative defense systems, and reduction of ethylene levels^[31]. Notably, salt stress effects can be diminished by 1-aminocyclopropane-1-carboxylate (ACC) deaminase^[32]. For example, inoculation of pea plants with a strain of Variovorax paradoxus that produces ACC deaminase decreased stomatal resistance and xylem balancing pressure, improved photosynthetic efficiency and electron transport rate, balanced ion homeostasis through increased K⁺ flow to shoots and Na⁺ deposition on roots, and increased biomass under salt stress at 70 and 130 mmol·L⁻¹ NaCl^[33]. Additionally, microbial biofilms, particularly extracellular polymeric substances (EPS), maintain and retain high soil moisture, facilitating the availability of dissolved nutrients for plant growth and development, and enhancing plant growth under salinity-induced osmotic stress and nutrient imbalance^[34]. Also, EPS and biofilms can restrict Na⁺ uptake by binding and retaining Na⁺ in the soil^[34]. Haque et al.^[35] demonstrated that biofilmET-producing rhizobacteria enhanced tomato biomass accumulation through various plant growth-promoting activities, including production of IAA, ACC deaminase, and siderophores, improving nutrient bioavailability, inducing antioxidant defense systems, maintaining higher relative water content, reducing Na⁺ uptake and increasing the K⁺/Na⁺ ratio.

Due to global climate change, drought is becoming increasingly frequent and extreme in most parts of the world. Microbial inoculants can enhance plant drought tolerance through various mechanisms such as decreased stomata conductance, increased nutrient uptake, modulation of antioxidant enzymatic activities and increased hydric content^[6,^7]. Arbuscular mycorrhizal inoculants have been reported to improve tea plant drought tolerance by modulating root architecture such as increasing root volume, number of lateral roots, length of lateral roots and hormones (abscisic acid, brassinosteroids, gibberellins, and indole-3-acetic acid)^[36]. Under drought conditions, microbes can enhance plant physiological and biochemical parameters that aid adaptive plant response, root growths, water content, and plant nutrient content. For example, the combined use of AMF and Bacillus megaterium contributed to plant drought tolerance through enhancing plants K⁺ and hydric content, and affecting plant antioxidant activities^[37]. Pseudomonas putida ameliorated drought tolerance in chickpea plants by altering various physical, physiological, and biochemical parameters, as well as by reducing expression of stress responsive gene^[38]. The impact of biofertilizers on crop yield and soil health is summarized in Tab.1.

2.3 Decontaminating soil toxic pollutants

Soil toxic pollutants, such as heavy metals, petrochemicals, and agrochemicals, pose a key risk to the environment due to the irreversible environmental damage and the long half-live^[51]. Prolonged exposure and higher accumulation of toxic pollutants can have deleterious health effects on human life and soil biota^[51]. The role of microorganisms and plants in removing toxic contaminants from the ecosystem is well-documented, and it has attracted the attention of investigators over the centuries^[52]. Despite being natural components of the environment, heavy metals have emerged as significant threats, having several associated health risks. Microorganisms have different detoxifying mechanisms, such as biotransformation, biosorption, bioaccumulation, and phytoremediation, which allows them to grow in heavy metal-polluted habitats^[51]. These detoxifying mechanisms work primarily through EPS and cell wall properties. The EPS are macromolecules secreted by various organisms into their environment, and have been reported to be essential in bioremediation of heavy metals^[53]. The microbial cell walls, which mainly consist of polysaccharides, lipids and proteins, offer various functional groups such as that can bind heavy metal ions, and these include carboxylate, hydroxyl, amino and phosphate groups. Biotransformation refers to the conversion of heavy metals into low or non-toxic substances through oxidation/reduction, complexation, methylation and other processes, which changes the state of heavy metals and reduces ecological hazards. The biosorption of heavy metals is divided into intracellular adsorption and extracellular adsorption: Intracellular adsorption means that the heavy metals enter the cell and combine with intracellular molecules to form stable substances, thus accumulating in the cell; Extracellular adsorption refers to the adsorption of heavy metals on the cell surface relying on the cell wall, capsule and surface mucus layer via complexation, precipitation, chelation and ionic interactions^[54]. For example, multiple studies have shown that Cr(VI) can be adsorbed and accumulated near the cell surface, subsequently may act as a terminal electron acceptor getting reduced to Cr(III) and binds to cell wall, which is closely involved in the EPS and cell walls^[42]. Further, Maurya et al.^[55] found that bacterial biofilm (60%–99%) has a greater potential to remove Cr(VI) than planktonic cells (43%–94%).

In addition, organic pollutants including pesticides, polycyclic aromatic hydrocarbons (PAHs) and dyes used in textile and other industries are of significant concern due to their toxic, persistent, carcinogenic and lipophilic nature, as well as bioaccumulation in the food chain^[56]. Zhang et al.^[57] reported that inoculation with endophytic bacterial strains effectively increases the abundance of PAH catabolic genes, thereby reducing PAH concentrations in vegetated soils and plants. In bioremediation processes, microorganisms mineralize organic contaminants to end products such as carbon dioxide and water, or to metabolic intermediates that are used as primary substrates for cell growth. For example, Candidatus Methanoperedens utilized methane as the sole carbon source to degrade methyl orange via direct interspecies electron transfer or the syntrophy pathway^[58]. The bioremediation of organic waste through microbial inoculants mainly depend on microbial specific enzymatic systems, which can assimilate, adsorb and biodegrade organic wastes in soil^[59]. Liu et al.^[59] demonstrated that the degradation rate of PAHs positively correlates with soil laccase activity. Notably, the adsorption of organic pollutants by microbial aggregates, attributed to hydrophobic regions in EPS, may contribute to the removal of organic pollutants^[53,^60]. The primary mechanism for the removal of Microcystin-RR from aquatic solutions has been reported to be biosorption by microorganisms and microbial aggregates^[60].

2.4 Restoring degraded soils

Degraded soils have been abandoned worldwide due to both geogenic factors and anthropogenic factors, particularly in high- and middle-income countries^[61]. For example, one-third of the grasslands on the Qinghai-Tibet Plateau have become degraded under the interference of regional and persistent natural destruction, including wind and water erosion, freezing, and anthropogenic activities, such as overgrazing and unsuitable reclamation^[62]. Microbial inoculants can be used to restore abandoned lands because of their positive effects on plant growth and soil nutrients^[6]. Also, it is crucial to recognize the positive impact of microbial inoculants on the recovery of microbial community structure and function in abandoned soil. Li et al.^[63] conducted a 4-year field experiment at an abandoned mining site and examined the changes in microbial characteristics, soil nutrients, enzyme activities and functional genes to investigate the efficacy of mineral-solubilizing microbial inoculants in restoring derelict mine ecosystems. The study concluded that the application of mineral-solubilizing microbial inoculant significantly enhanced soil multifunctionality, including soil nutrients and enzyme activities, while decreasing microbial network complexity but increasing stability.

3 Nano-fertilizer for enhanced agricultural productivity

3.1 Nanomaterials for soil improvement

The NMs have emerged as promising tools for soil improvement, providing innovative solutions to enhance soil fertility, nutrient availability, and overall agricultural productivity^[64,^65]. Due to their nanoscale size, these materials exhibit unique physical and chemical properties that make them particularly effective in addressing various soil-related challenges^[11]. The increased surface area and improved solubility of NMs facilitate better nutrient absorption by crop roots, leading to enhanced growth and yield. Additionally, NMs contribute to improving soil structure by promoting better aggregation of soil particles, which enhances porosity and aeration^[66]. They also improve water retention in soil, reducing water runoff and evaporation, thereby making water more available to plants. These properties make NMs valuable for sustainable agricultural practices, helping to address issues related to soil degradation and poor nutrient availability.

3.2 Nano-fertilizers

Nano-fertilizers might have the potential to revolutionize agriculture and help to meet the growing food demand for future generations^[65]. It offers innovative solutions to boost crop quantity and quality, particularly through fertilizer application and genetic improvement^[8]. A key advantage of nano-fertilizers over traditional fertilizers is their ability to release nutrients gradually over time due to their high surface area and nanoscale particle size, which can efficiently penetrate plant pores and improve nutrient use efficiency^[60]. For instance, the enhanced bioavailability of nutrients leads to improved root system development, increased leaf surface area and higher biomass production. These physiological and morphological improvements contribute to a substantial increase in crop yields and overall plant health. NM-based slow releasing fertilizer enhances nutrient bioavailability and absorption by crops^[67]. Zeolite-based nano-fertilizers can gradually release nutrients to crops, improving nutrient availability throughout the growing season and reducing losses from volatilization, leaching, denitrification, and fixation^[68]. Also, nano-fertilizers have been shown to reduce nutrient leaching, minimize environmental pollution, and improve soil fertility^[69]. A study by Chhowalla^[70] found that hydroxyapatite-urea nanohybrids released urea 12 times slower than urea fertilizer. Additionally, the incorporation of metal-based engineered NMs such as Cu, Fe, and Zn into nano-fertilizers addresses micronutrient deficiencies in soil^[71] promoting healthier and more resilient crops. De Souza-Torres et al.^[72] demonstrated that Fe₃O₄ NPs, significantly enhances biological nitrogen fixation, leading to improved soil nitrogen levels and better crop performance. According to Kumar et al.^[73] nano-fertilizers can increase crop yields by up to 50% while using 50% less nitrogen than mineral fertilizers. Existing research on nanotechnology-based crop fertilizer application encompasses wide range of NMs, with a special focus on metal-based NMs comprising Cu, Fe, and Zn as micronutrients^[11,^74]. Also, molybdenum disulfide NMs have shown great potential for enhancing biological nitrogen fixation and soybean growth, with a 30% increase in yield compared to the standard molybdenum fertilizer^[75]. Apart from their characteristics of gradual nutrient release, nano-fertilizers can be tailored to release nutrients selectively under various conditions, such as in the presence of water or at specific pH or temperature levels^[76]. Researchers assess the function of nano-fertilizers by examining various parameters, including morphological aspects such as plant growth, leaf surface area, root system size, and wet and dry matter production^[74,^77], physiological^[78], biochemical^[60], and changes at molecular level^[77].

3.3 Impact on soil microbial communities

Soil microbes are crucial for sustaining the fertility and productivity of the soil^[79]. Nanoparticle treatments have the potential to substantially modify the soil microbiome, affecting the abundance and diversity of microbes. This, in turn, can influence crucial microbial processes such as mineralization, nitrogen fixation, and activities that promote plant growth^[8,^77]. In addition, nitrogen and carbon cycles are often positively affected by nano-fertilizers. The stabilization of NPs in soil by natural organic matter can modify their surface chemistry, potentially affecting microorganisms and plants^[64]. Also, there is notable variability in the responses of plants and their associated rhizosphere bacteria to NPs. The overall influence of NPs on rhizosphere microbiome is contingent upon factors such as the properties and concentration of the NPs, the types of microbes in the environment and prevailing soil conditions^[80]. Although certain studies indicate that NPs may have detrimental effects on rhizosphere microbiome, NPs can actually stimulate it. This implies the potential to leverage these beneficial root-microbe relationships^[7,^81]. The presence of NPs in the plant rhizosphere establishes a distinctive environment conducive to the interaction between plants and NPs^[82]. While silver NPs had a differential impact on rhizosphere of maize and overall microbial community structure, with a more notable alteration in bacterial community compared to the fungal community^[83]. Despite changes to rhizosphere microbial community, maize growth was positively affected, suggesting either the suppression of pathogenic bacteria or an expansion in valuable bacteria^[83]. Cerium dioxide NPs at concentrations of 300–2000 mg·L⁻¹ significantly improved strawberry plant growth, however reduced the diversity of the rhizosphere microbial community^[84]. The change in microbial diversity corresponded with an increased prevalence of microbes that promoting plant growth, affecting nutrient cycling, and providing protection against diseases^[83].

It has been reported that the some species in the phylum Bacteroidota have silver resistance genes, enabling them to withstand the presence of silver NPs and, in fact, thrive in their presence^[8]. The disintegration of silver-sensitive microbial cells liberates a readily accessible pool of carbon sources, which Bacteroidota efficiently metabolize, fostering their proliferation and enhancing their relative abundance within the microbial community^[85]. Application of 1 mg·g⁻¹ iron(III)-silver NMs decreased the abundance of Verrucomicrobia and Chloroflexi, likely due to their lack of an outer membrane and high sensitivity to silver^[86]. Further, Lin et al.^[87] illustrated that green synthesized iron oxide NPs improved the composition of the soil microflora in cadmium-contaminated soil. The NMs have a neutral to positive impact on native microflora and biodiversity at low concentrations.

3.4 Nanomaterials for disease management

Plant pathogens, ranging from simple viroid to complex organisms like bacteria, fungi, nematodes, and oomycetes, cause a variety of plant diseases, resulting in significant agricultural losses worldwide. NPs not only act as nutrient carriers for plants but also demonstrate effectiveness against a broad spectrum of pathogenic microorganisms^[74]. Their application, therefore, holds the potential to serve a dual purpose by bolstering agricultural productivity through enhanced disease resistance. Rexlin et al.^[88] addressed the challenge of declining crop production by using biocompatible, relatively non-toxic and biodegradable zinc oxide NPs to improve both quality and productivity of cluster bean seeds and pods. Applying zinc oxide NPs effectively addressed zinc deficiency in plants and showed remarkable and diverse antibacterial and antifungal properties, likely due to the release of reactive oxygen species (ROS) and zinc ions, which deactivate bacterial cell walls and exhibit strong antimicrobial activity^[89]. Additionally, research on carrots has revealed that silica enhances plant development and its resistance^[90]. Silica NPs interact with cell wall to form an additional cuticle layer, which acts as a barrier to inhibit the entry of pathogens into plant cells and contribute to disease resistance in various crops^[91]. El-Shetehy et al.^[92] demonstrated that silica NPs induced resistance in Arabidopsis in a dose-dependent manner, while copper NPs effectively treated Fusarium wilt and improved tomato plant growth. Copper NPs served as a micronutrient source in copper-deficient soils, primarily enhancing plant growth and elevating total chlorophyll levels^[93]. Nano-Cu and -Si are widely used in the contemporary management of plant diseases. Similarly, the presence of 30 nm silica NPs reduced the growth of the pathogen Fusarium oxysporum in tomato stems, highlighting the potential of silica NPs as a contributor in a sustainable crop protection strategy^[94]. Abdelkhalek and Al-Askar^[89] used an aqueous leaf extract of Mentha spicata to biologically synthesize zinc oxide NPs and then evaluated their antiviral efficacy against the tobacco mosaic virus. Recent research has demonstrated that diverse NPs can be effectively used to manage plant diseases, by reducing the incidence of diseases they can improve overall plant health^[81]. The NMs have broad-spectrum antimicrobial properties, making them inherently suitable for direct use as pesticides for plants and edible crops to manage pests and disease. Various NMs are also used as nano-carriers to deliver pesticides precisely to the intended site of action^[95].

4 Role of nanotechnology in mitigating abiotic stress

In addition to impacting growth and development, NPs can alleviate harmful effects of various significant abiotic stresses affecting plants. Plant productivity and yield are severely limited by ever-increasing abiotic conditions such as drought, salt, waterlogging, heat stress, and heavy-metal toxicity^[96]. Abiotic stresses inevitably induce production of ROS in response to restricted oxygen supply within plant cells^[96]. Although plants acquire enzymatic machinery to mitigate oxidative stresses from environment, severe stress can overwhelm their defense mechanisms, leading to significant physiological and molecular consequences^[97]. The NPs alleviate abiotic stresses by activating specific genes, accumulating osmolytes, enhancing phytonutrient levels and producing photopigments^[98]. Their particularly small size gives them unique properties that distinguish them from their bulk counterparts and open new possibilities in various fields of application^[99].

4.1 Nanomaterials for drought mitigation

Drought stress in plants causes several changes in growth traits and biochemical reactions, including stunted growth and slowed photosynthesis^[100]. The application of phytogenic NPs is an emerging strategy to mitigate these adverse effects. Available reports suggest that NPs enhance drought resistance in a variety of crops, including wheat^[101], eggplant^[102], strawberries^[103], and barley^[104]. The NPs increase photosynthetic pigment and crucial compound levels, such as proline and carbohydrates, in strawberry leaves under drought stress^[103]. Excessive production of malondialdehyde (MDA) under drought stress causes oxidative damage in plants^[103]. The application of NPs to drought-stressed plants elevates levels of enzymatic antioxidants and reduces MDA content. The NPs enhance drought resilience of hawthorn by increasing photosynthetic rates and stomatal conductance^[105]. Similarly, applying selenium NPs (30 mg·L⁻¹) to wheat under drought conditions promotes plant growth while decreasing ionic leakage and lipid peroxidation, mitigating the toxicity of drought-induced cellular damage^[106]. By regulating ion balance and ROS metabolism, silica NPs reduce cellular damage and promote photosynthesis in stressed plants^[107]. Silica NPs promoted post-drought plant recovery in barley by altering morphophysiological properties. Chitosan NPs improved relative water content, photosynthetic rate, CAT and SOD activities, as well as yield and biomass in drought-stressed wheat^[108].

4.2 Nanomaterials for heat stress alleviation

Heat stress occurs when plants are exposed to elevated temperatures exceeding their optimal growth range for durations ranging from minutes to hours, significantly impacting plant growth and development^[109]. The ROS, particularly hydrogen peroxide, are believed to be crucial for signaling in plant response to heat stress^[110]. Regulating levels of ROS is essential for effective signaling and reducing heat stress damage. While heat stress can alter plants at all stages of development, it has the most significant impact during preliminary establishment, flowering, gametogenesis and floral meristem growth^[111]. The application of biocompatible NPs has demonstrated high efficacy in enhancing the survival of wheat plants under heat stress conditions^[111].

4.3 Nanomaterials for cold stress mitigation

Plants frequently encounter two distinct types of low-temperature stress, namely chilling and freezing. The optimal temperature range for plant growth and development is influenced by various factors, including the specific plant species and its individual tolerance level. In general, chilling temperatures range from 0 to 15 °C. Additional variables, such as ambient air temperature and wind velocity throughout the period of exposure, also exert an impact on chilling temperatures. In contrast to their ability to withstand cold temperatures, plants have to enduring freezing temperatures, which fall below 0 °C^[112]. Low and non-freezing temperatures can harm or kill crops, affecting their productivity, survival and ecological distribution^[113]. Nanoparticle-mediated approaches have been used to mitigate cold stress through various mechanisms. For example, titanium dioxide NPs mitigate the harmful effects of cold stress by enhancing antioxidant enzyme activities, reducing oxidative damage, and increasing glycyrrhizin content in licorice plants^[114]. Chitosan NPs mitigate ROS levels and promote the accumulation of osmoprotectants, such as proline and soluble sugars, which help stabilize cellular structures and maintain osmotic balance in banana plants under cold stress^[115]. Additionally, the foliar application of zinc oxide NPs alleviates chilling stress in rice plants by modulating the antioxidative system, enhancing the activity of enzymes like superoxide dismutase and catalase, and upregulating transcription factors related to the chilling response^[91]. Similarly, silica NPs enhance the photosynthetic capacity of sugarcane plants experiencing chilling stress by improving chlorophyll content and maintaining the integrity of the photosynthetic apparatus^[116].

4.4 Nanomaterials for salinity stress reduction

In the field of agriculture, NPs have been recognized as an effective way to improve crop productivity under salt stress condition^[98,^117] (Fig.1). Several studies have demonstrated that NPs enhance the ability of different plant species to tolerate high levels of saline^[98,⁹⁹,^102]. Additionally, zinc oxide NPs enhance the salinity tolerance of Oryza sativa by preserving cell membrane stability. This is achieved through the reduction of lipid peroxidation and the maintenance of membrane integrity, preventing the loss of essential ions and cellular components under salt stress^[118]. Iron oxide NPs have been shown to increase the biomass, antioxidant concentrations, and photosynthetic pigments in wheat under salinity stress. This improvement in photosynthetic efficiency leads to better energy capture and utilization, supporting plant growth and productivity even in saline conditions^[117]. The NPs, including Cu, K₂SO₄, Se, SiO₂, TiO₂, and ZnO, significantly enhance the activity of several antioxidant enzymes such as APX, CAT, GPX, GR, POD, POX, and SOD. This enhancement mitigates oxidative stress by scavenging reactive oxygen species, thereby protecting cellular structures and metabolic functions from oxidative damage caused by excessive salt stress^[119]. This ability to enhance antioxidant enzyme activity allows NPs to alleviate complications of oxidative stress induced by salt stress.

4.5 Nanoremediation of heavy metals

Soil contamination by various pollutants stands as one of the most critical environmental challenges, adversely impacting both terrestrial and aquatic ecosystems. Heavy-metal pollution is perturbing the environment and posing severe health risks to human beings^[120]. Exposure to cadmium, lead, arsenic, and fluoride through contaminated food and drinking water can cause significant health problems, including skin, lung, kidney, and brain damage^[121]. Nanotechnology offers significant benefits for sustainable agriculture by enhancing nutrient use, mitigating climate change impacts and remediating heavy metals in soil^[122]. Researchers have reported that NPs can immobilize heavy metals in soil^[123]. The NPs can immobilize heavy metals, reducing their mobility and bioavailability, thus benefiting soil microbes and plants^[121,^123]. Applying NPs reduces the mobility and bioavailability of metal contaminants in the soil. For example, selenium and iron(III) oxide NPs stabilize Cd, limiting its movement and biological activity in plants^[124]. The NPs form complexes with heavy metals that become immobile after absorption, preventing heavy metals from moving within plants and reducing their biological activity^[98]. Additionally, NPs enhance apoplastic barriers in roots, controlling heavy-metal uptake^[125]. This results in improved plant growth and yield under metal stress conditions due to the stress-alleviating role of NPs^[126,^127], as shown in Fig.2. Reactive NMs can also detoxify and transform specific pollutants^[76].

5 Nanotechnology for enhancing seed germination, growth and crop productivity

Nanotechnology has recently gained increasing prominence in enhancing agricultural productivity. The NMs have demonstrated efficacy in improving seed germination, leading to enhanced plant growth and fruiting rates. Numerous studies have consistently corroborated the high efficacy of NMs for both germination and plant growth^[98,^124]. Montanha et al.^[128] found that nanocoating soybean seed surfaces with zinc (as ZnSO₄ and ZnO) improved germination. Some NPs can stimulate seed metabolism, seedling vigor and overall plant growth by modulating cellular signaling pathways^[65,^90]. For example, graphene and its derivatives can serve as efficient water transporters in the soil, accelerating water absorption by seeds, promoting seed germination, and stimulating the growth and development of plants^[129]. In another study, Ag NPs were absorbed by roots and localized in the cell wall and intercellular spaces. Their presence influenced cell division, thereby regulating root elongation^[124]. Zinc NPs promote germination in various plants, including beans^[130], chili^[131], maize^[132], and wheat^[133].

Foliar application of silver and zinc NPs enhanced the growth and productivity of mung beans, increasing the number of branches per plant, chlorophyll content, grain yield and number of pods per plant^[101]. Foliar spraying of K₂SiO₃ on maize leaves improved water use efficiency and yield under low irrigation conditions, while also enhancing grain protein and oil contents^[134]. Applying Si fertilizer increased concentrations of nitrogen, phosphorus, potassium, calcium, iron, manganese, copper and zinc in sugarcane plants. Sugarcane parameters correlated positively with Si levels, with height, stalk diameter and dry leaf biomass being 50%, 58%, and 71% higher, respectively, in treated plants than in controls^[135]. The impact of NPs on crop growth under various stress conditions is summarized in Tab.2.

Green nanotechnology holds the potential to enhance crop yields through multiple mechanisms, such as developing crops resistant to extreme temperatures, formulating targeted insecticides for specific pests, addressing global warming issues, and creating nanotubes to retain soil moisture^[142]. Both iron(III) oxide and iron(II) hydroxide NPs can be used as nano-fertilizers to enhance iron nutrition^[143]. The impact of NPs on biological systems is largely determined by their physicochemical properties, including size, zeta potential, and concentration^[77,^98].

6 Biochar as an additive toward improving soil quality and functionality

6.1 Biochar as a microbial carrier

Biochar significantly influence the microbial community diversity and composition within charosphere environment^[12], as illustrated in Fig.3. Specific bacteria immobilized on biochar have demonstrated excellent capabilities in adsorbing organic compounds such as nonylphenol^[144] and phenanthrene^[145], as well as heavy metals^[146]. Notably, the removal efficiencies of these contaminants appear to be higher when using biochar-immobilized bacteria compared to either biochar or bacteria alone. Liu et al.^[146] demonstrated that the combined application of biochar and manganese-oxidizing bacteria was effective for soil contaminated with Pb and As, while also showing greater potential for addressing Pb and Cd pollution in water. Numerous studies have established biochar as a viable carrier for beneficial bacteria, demonstrating its potential to sustain the viability of these microorganisms over extended periods. Thies and Rillig^[147] observed that bacteria attach to biochar particles through multiple methods, including flocculation, adsorption on surfaces and covalent bonding. They noted that the high porosity, sorption capacity and water holding capacity of biochar created suitable habitats for microorganisms, promoting these activities. A study by Husna et al.^[148] focused on agricultural waste materials as biochar, demonstrating that certain biochars maintained microbial viability for up to 6 months. For instance, coconut shell biochar exhibited high phosphate-solubilizing microorganism viability. In another study, Xiang et al.^[149] compared pine bark-derived biochar and sewage sludge-derived biochar with standard carriers like perlite and poultry litter. They discovered that pine bark-derived biochar extended the shelf life of bacteria by up to a year. These findings highlight the promise of biochar as a microbial carrier, offering a cost-effective, eco-friendly solution to improving crop yield and soil quality. As research progresses, our understanding of the potential applications for biochar continues to expand. The increasing evidence suggests that this versatile chemical has substantial potential for addressing current challenges in agricultural and ecological oversight. A global meta-analysis revealed that biochar could increase soil microbial biomass and affect the activities of extracellular enzymes, such as those involved in N cycling (urease) and P cycling (alkaline phosphatase), and intracellular enzyme, such as dehydrogenase, with no significant negative effects on any of the enzymes analyzed in this investigation^[150]. Similarly, Yan et al.^[151], demonstrated that biochar-based fertilizers (combined with compost or NPK fertilizers) improved soil properties and contributed to the recovery of degraded karst soils. This was achieved by stimulating microbial activities and growth and enhancing soil nutrient (N, P, and K) availability. These findings further emphasize the multifaceted benefits of biochar in soil amendment and microbial ecology^[150,^151]. Recent research has also investigated the use of biochar as a carrier material for microbial inoculants to promote early colonization of the rhizosphere with microbial incubations^[32,^152]. Compared to direct soil application of Enterobacter cloacae, using pinewood biochar as a carrier resulted in increased survival of the inoculum.

Also, biochar applied in conjunction with microbial inoculants has been reported to provide, in most cases, apparent benefits in the form of increased yields, improved soil properties, reduced greenhouse gas emissions and improved nutrient circulation^[150]. The study of Rafique et al.^[23] investigated the effects of biochar, P fertilizer, PSB, and AMF on maize growth, and found that the addition of biochar significantly improved nutrient absorption and plant biomass production when combined with P fertilizer, AMF, and PSB. Also, the combined use of biochar and microbial inoculants to control plant diseases has been well-reported in the literature^[106]. Combined amendment of biochar and Bacillus synthetic community effectively controlled soilborne disease and improved plant physiological parameters through remodeling the plant and rhizosphere microbiome^[153].

6.2 Biochar improve the nutrient cycling in soil ecosystems

Biochar serves as an excellent carrier of essential nutrients such as N, P, and K, attributing to the large load ability of the porous structure and strong adsorption ability toward nitrate, ammonium and phosphate ions. Biochar amendments enhance soil porosity, water retention capacity, nitrogen retention time and microbial activity. Research has shown that soil organic carbon can increase by an average of 39% with the application of appropriate biochar types^[154,^155]. A long-term microcosm experiment demonstrated that the interaction between biochar and plants led to increased fungal biomass, soil macroaggregate formation, and iron-bound non-biochar carbon content^[156]. Experiments and meta-analysis conducted on different types of soils and biochars consistently indicate that the applications of biochar help to improve the soil carbon sequestration and reduce greenhouse gas emissions^[154,¹⁵⁶,^157]. While the effects of biochar on N cycling depend on both biochar characteristics and soil properties, meta-analyses suggest that biochar amendments can significantly alter nitrogen dynamics^[157]. Field and laboratory experiments showed that biochar application significantly enhances soil ammonium and nitrate content, N mineralization, N₂ fixation, and plant N uptake, while inhibiting nitrate leaching and gaseous nitrogen emissions^[158] These effects contribute to improved crop productivity. Also, biochar can effectively be used to enhance soil properties and crop yields (Tab.3).

Biochar can also act as electron mediators (donor, shuttle, and conductor) in the redox reactions in soils, including denitrification, dissimilatory nitrate reduction to ammonium, ammonia oxidation, and iron oxidation/reduction^[170]. The function of biochar as the electron shuttle is proved to decrease the N₂O emission and N₂O/(N₂O + N₂) emission ratio by controlling the electrochemical properties of biochar^[171], and enhance anaerobic ammonium oxidation coupled to iron reduction and long-distance extracellular electron transport^[172,^173]. The Fe(III) reduction in clay minerals was found to be affected by the biochar concentration due to its electron shuttling and buffering capacity^[174]. Villada et al.^[175] demonstrated that biochar-based fertilizers, created by impregnating biochars from various agricultural residues with biogas slurry, show promising results in controlled nutrient release and enhanced plant nutrition. This approach offers a sustainable method for NPK recovery and fertilizer production in agriculture. Biochar application generally increases plant-available P in agricultural soils. A meta-analysis of 108 studies found that biochar addition increased soil P availability by a factor of 4.6 on average^[176]. Ye et al.^[177] found that biochar prepared under CO₂ or H₂O (gaseous state) atmosphere contained more surface groups for K adsorption, and active groups on biochar surface had enhanced N and P adsorption^[178]. Also, biochar modified with nutrient elements, such as nitrogen and phosphorous, or biochar-based controlled-release fertilizer is used to improve the microbial community function and food production^[179]. The combined adsorption and long-distance electron transport capabilities of biochar enhance its potential applications in the agricultural industry.

7 Challenges and perspectives

7.1 Difficulties with microbial inoculants application

Despite decades of research into soil microbial inoculants, significant obstacles remain to their widespread application. The introduction of microbial cells into soil aims to promote rapid colonization of the host rhizosphere. However, this approach faces several limitations. Effective inoculants, selected for their strong competitive and root-colonizing abilities, can substantially impact plant health^[180]. Plants and soil microbiomes possess sophisticated communication networks^[108]. The introduction of microbial inoculants could disturb the coevolved plant-microbe interactions and the equilibrium of soil microbial communities^[7,^109]. These inoculant-induced changes can result from direct trophic competition or from antagonistic or synergistic interactions between introduced and resident microbes. Notably, one of the most critical factors to consider when introducing microbial inoculants is their capacity to disperse within the recipient habitat, as this will influence their potential access to locations and resources following application^[111].

7.2 Mass production, application, and health risk of nano-fertilizer

Nano-fertilizers have substantial potential for enhancing agricultural productivity and sustainability by improving nutrient delivery to plants^[181] (Fig.4). However, their production, application and potential health risks also present significant challenges and complexities. The production of nano-fertilizers requires precise control over particle size, composition and surface properties. Achieving this level of precision can be technically demanding and expensive. While laboratory-scale production may yield promising results, scaling up the production process to meet agricultural demand while maintaining consistent quality is challenging. Some nano-fertilizer production methods rely on rare or expensive materials, energy-intensive processes or complex synthesis techniques, which can limit scalability and increase production costs. The environmental impact of nano-fertilizer production, including energy consumption, waste generation and potential pollution from byproducts, requires careful consideration. Ensuring uniform distribution of nano-fertilizers in soil or on plants is crucial for their effectiveness. Achieving this uniform dispersion can be challenging, especially in large-scale agricultural settings. Nano-fertilizers must integrate seamlessly with existing agricultural practices, including machinery, irrigation systems, and crop management techniques. Nano-fertilizers must integrate seamlessly with existing agricultural practices, including machinery, irrigation systems, and crop management techniques. Additionally, they should maintain stability over time and under various environmental conditions to provide sustained nutrient release and prevent leaching or degradation. Also, NPs from nano-fertilizers could potentially enter water bodies, soil and the food chain, posing risks to ecosystems and human health through bioaccumulation and biomagnification. Current regulations may not adequately address the unique risks associated with nano-fertilizers, necessitating robust safety assessments and regulatory frameworks. While nano-fertilizers offer exciting possibilities for enhancing agricultural productivity and sustainability, addressing the challenges of production, application and health risks is essential to ensure their safe and effective implementation. Close collaboration between researchers, industry stakeholders, policymakers and regulatory bodies is crucial to overcome these challenges and realize the full potential of nano-fertilizers while safeguarding human health and the environment.

7.3 Effective application of biochar and its contribution to carbon neutrality

Biochar has substantial potential for mitigating climate change and advancing carbon neutrality. However, its widespread adoption faces several challenges that require careful consideration and further research. Large-scale production and cost management remain major hurdles in the widespread adoption of biochar. There is a pressing need to develop efficient and environmentally-friendly pyrolysis technologies that can be scaled up economically. The economic feasibility of large-scale biochar production, including a comprehensive assessment of costs and benefits across various sectors, must be thoroughly evaluated. Despite these challenges, biochar has clear potential for addressing environmental issues. It has been shown to improve soil health, enhance microbial function, and increase food production. Importantly, biochar acts as a stable carbon sink, effectively reducing atmospheric CO₂ levels. This carbon sequestration potential makes biochar a valuable tool in mitigating greenhouse gas emissions and combating climate change.

Effective biochar application offers a multifaceted approach to achieving carbon neutrality by sequestering carbon, improving soil health and promoting sustainable agricultural practices. Integrating biochar into climate change mitigation requires collaboration among policymakers, researchers, farmers and industry stakeholders. Economic feasibility of large-scale biochar production, including costs and benefits for various sectors, must be assessed. Supportive policies and incentives are needed to promote biochar adoption. Further research and policy support are crucial for realizing the full potential of biochar in addressing global environmental challenges.

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