Research Progress on the Regulation of Plant Rhizosphere Oxygen Environment by Micro-Nano Bubbles and Their Application Prospects in Alleviating Hypoxic Stress

Abstract

Rhizosphere hypoxia, caused by soil compaction and waterlogging, is a major constraint on agricultural productivity. It severely impairs crop growth and yield by inhibiting root aerobic respiration, disrupting energy metabolism, and altering the rhizosphere microecology. Micro-nano bubbles (MNBs) show significant potential for alleviating rhizosphere hypoxia due to their unique physicochemical properties, including large specific surface area, high oxygen dissolution efficiency, prolonged retention time, and negative surface charge. This paper systematically reviews the key characteristics of MNBs, particularly their enhanced mass transfer capacity and system stability, and outlines mainstream preparation methods such as cavitation, electrolysis, and membrane dispersion. And the multiple alleviation mechanisms of MNBs—including continuous oxygen release, improvement of soil pore structure, and regulation of rhizosphere microbial communities—are clarified. The combination of MNBs aeration and subsurface drip irrigation can increase soil aeration by 5%. When applied in soilless cultivation and conventional irrigation systems, MNBs enhance crop yield and nutrient use efficiency. For example, tomato yield can be increased by 12–44%. Furthermore, the integration of MNBs with water–fertilizer integration technology enables the synchronized supply of oxygen and nutrients, thereby optimizing the rhizosphere environment efficiently. This paper sorts out the empirical effects of MNBs in soilless cultivation and conventional irrigation, and provides directions for solving problems such as “insufficient oxygen supply to deep roots” and “reactive oxygen species (ROS) stress in sensitive crops”. Despite these significant advantages, the industrialization of MNBs still needs to overcome challenges including high equipment costs and insufficient precision in parameter control, so as to promote large-scale agricultural application and provide an innovative strategy for the management of rhizosphere hypoxia.

1. Introduction

Soil compaction poses a significant challenge in modern agricultural systems. Continuous cropping, along with improper irrigation and fertilization practices, has contributed to declining soil organic matter, altered soil pH, and the degradation of soil structure. As global food demand rises to meet the needs of an estimated population of 10 billion by 2050, modern agriculture increasingly relies on heavy machinery [1]. However, the widespread use of such equipment exacerbates soil compaction, with over 20% of cropland at risk of chronic subsoil compaction, leading to substantial declines in agricultural productivity [2]. Soil compaction adversely affects soil pore structure, morphology, and network connectivity. It also significantly influences root system development, changing root length, volume, surface area, and mean root diameter across soil profiles [3]. Furthermore, compacted soils hinder the infiltration of rainfall and irrigation water, promoting surface runoff and evaporation, thereby reducing water use efficiency. Concurrently, soil compaction limits gas diffusion, impairing soil permeability and decreasing oxygen (O₂) availability.

With global climate change, the frequency and intensity of flooding events are increasing due to more frequent and intense precipitation [4]. Soil is a dynamic three-phase system, and these three phases are solid, liquid and gas. To reach the root tissues, O₂ diffuses through gas-filled pores, water films, and the tissues themselves [5]. The diffusion of O₂ from the soil surface to the root zone is strongly influenced by soil water content and soil structure. Under waterlogged conditions, where excess water fills soil pores, soil air is displaced by water, drastically reducing O₂ diffusion rates. Since the diffusion rate of O₂ in water is approximately 10,000 times slower than in air, and O₂ flux in fully saturated soils can decrease by up to 320,000 times [6,7], high soil moisture severely restricts O₂ movement into deeper soil layers. This limitation disrupts the balance between O₂ supply and consumption, leading to soil hypoxia and imposing hypoxic stress on submerged plants due to impaired gas diffusion underwater [8]. Consequently, elevated soil water content significantly hinders O₂ diffusion to deeper soil horizons, resulting in a pronounced decline in soil O₂ availability.

The absence of O₂ in the rhizosphere, the soil region influenced by root system biochemistry, is one of the most significant factors affecting plant performance at the whole-plant level. Hypoxic conditions impair aerobic respiration in roots, thereby reducing hydraulic conductivity, limiting nutrient uptake, and inhibiting plant growth and development. At the cellular level, oxygen deficiency disrupts energy metabolism, resulting in a decline in intracellular pH and the onset of cytoplasmic acidosis. To maintain redox homeostasis, plants enhance nicotinamide adenine dinucleotide (NAD+) regeneration by suppressing ATP-consuming pathways and shifting toward glycolytic metabolism, a metabolic adjustment that substantially compromises crop yield [9,10]. In the rhizosphere environment, reduced O₂ availability represents a major constraint for plants and their associated microbiomes. The shift from oxic to hypoxic soil conditions drives a sequential transformation in microbial communities: from dominance by aerobic organisms, to increased prevalence of facultative anaerobes, and ultimately to dominance by obligate anaerobes [11]. These anaerobic microorganisms can contribute to soil nitrogen loss through pathogenic effects or by promoting denitrification, processes that may negatively affect plant performance. At the tissue level, hypoxia impairs root water uptake and transport, restricts mineral nutrient absorption and assimilation, and leads to hormonal imbalances resulting from both nutrient deficiencies and limited energy supply, collectively suppressing growth in roots and aeration tissues [11]. Furthermore, hypoxia disrupts meristem activity by inhibiting the degradation of key regulatory proteins [12]. At the whole-plant level, root hypoxia induces alterations in root architecture, which can impair plant water relations and potentially result in wilting [7,13]. It also suppresses root water transport, triggers stomatal closure, alters root morphology and aquaporin function, and consequently reduces both root hydraulic and electrical conductivity [5].

MNBs represent a promising technological approach for alleviating rhizosphere hypoxia, offering significant potential for practical application. Owing to their unique physicochemical properties, MNBs have been successfully applied in various fields, particularly in water treatment. However, their use in agricultural irrigation remains in the early stages of development, with considerable potential yet to be fully realized. In light of this, this paper presents a systematic review of the fundamental characteristics and mainstream generation methods of MNBs, with a particular focus on their potential applications in mitigating root-zone hypoxia under agricultural irrigation conditions. The aim is to provide theoretical insights and guidance for future research and practical implementation of this technology (Figure 1).

Figure 1. Application of micro-nano bubbles in agriculture and their effects on rhizosphere hypoxia.

2. Physical Properties and Preparation Methods of MNBs

2.1. Physical Properties of MNBs

MNBs are characterized by diameters of less than 100 μm and are further classified into micron bubbles (100–10 μm), submicron bubbles (10–1 μm), and nanobubbles (less than 1 μm) [14]. These MNBs represent small gaseous cavities in the micro to nanoscale range dispersed in liquid media, exhibiting high internal pressures due to their size [15]. MNBs in the root zone environment can penetrate soil pores and reach the rhizosphere directly. A key feature of MNBs is their large specific surface area, for instance, a microbubble with a radius of 1 μm has a specific surface area approximately 1000 times greater than that of a conventional bubble with a radius of 1 mm. Moreover, MNBs typically possess a negative surface charge. This combination of high specific surface area and surface charge enables efficient adsorption of oppositely charged molecules and fine particles [16,17]. Under near-neutral pH conditions, the zeta potential of air nanobubbles, oxygen nanobubbles (O₂ NBs), and nitrogen nanobubbles (N₂ NBs) generally ranges from −27 to −45 mV [18].

Compared with ordinary bubbles, MNBs possess a significantly larger specific surface area. Given that the gas–liquid mass transfer rate is directly influenced by the interfacial area between the two phases, MNBs can substantially enhance mass transfer efficiency and promote the dissolution rate of gases such as oxygen in water. Moreover, due to their slow buoyant rise velocity, MNBs remain suspended in aqueous solutions for extended periods, thereby increasing gas concentration and enabling sustained gas release [19,20]. In the root zone environment, the role of this large specific surface area is not only to enhance gas–liquid mass transfer efficiency but also to maximize the contact area with root surfaces and soil colloids, enabling efficient direct release of oxygen to root hair regions with the highest oxygen demand. As indicated by experimental findings, the oxygen transfer coefficient, oxygen transfer rate (OTR) and oxygen transfer efficiency (OTE) associated with micro-nano aeration are roughly 15 times higher compared to traditional aeration reactors [21]. MNBs also exhibit higher mechanical strength and thermal stability. The internal pressure within MNBs is considerably higher than the surrounding environment, which further accelerates gas dissolution into the liquid phase [22,23]. At the same time, in the root zone environment, this high internal pressure ensures that MNBs can release oxygen even in compacted soil—where ordinary bubbles often rupture or fail to dissolve. Their relatively high mechanical strength also prevents MNBs from breaking under the pressure of soil particles, ensuring they maintain structural stability and have sufficient time to deliver oxygen to the deep root zone.

2.2. Preparation Methods of Micro-Nano Bubbles

There are three primary methods for generating MNBs: cavitation encompassing hydrodynamic cavitation (HC) and acoustic cavitation (AC), the electrochemical method, commonly known as electrolysis; and the membrane filtration method [15,24].

2.2.1. Cavitation

Cavitation arises from ultrasonic compression and hydrodynamic flow, resulting in a rapid decrease in local transient pressure within the liquid phase to levels below the saturated vapor pressure. This phenomenon leads to the formation of cavitation bubbles, which may contain vapor from the liquid medium or dissolved volatile gases. Cavitation is consistently associated with the generation of numerous MNBs [25]. These bubbles possess high internal energy, and their collapse induces intense local turbulence and heat release, thereby causing a substantial increase in local temperature and pressure [26].

• Ultrasonic cavitation

During the ultrasonic cavitation process, gas cavities or microbubbles are generated in a liquid medium under the influence of ultrasonic wave energy radiation. The intensity of the ultrasound, which is determined by the applied driving voltage, directly influences both bubble size and cavitation efficiency [27]. To enhance the production efficiency of microbubbles (MNBs), several studies have proposed acoustic chemical reactors that employ microfabrication techniques to precisely control microbubble nucleation sites. Specifically, artificial nucleation sites, engineered as micropits, are fabricated on a silicon substrate and positioned at the base of a microchamber. These micropits effectively trap gas and, upon ultrasonic excitation, release a continuous stream of microbubbles. Importantly, the trapped gas can be replenished via diffusion from the surrounding solution, preventing depletion and enabling sustained microbubble generation over extended periods-up to several hours [28].

2.

Hydrodynamic cavitation

Hydrodynamic cavitation is a phenomenon in which fluid velocity increases and local pressure simultaneously decreases as the fluid passes through a constricted section, such as orifice plates, Venturi tubes, or throttle valves. This pressure reduction facilitates cavity formation, thereby promoting the generation of MNBs [29]. The Venturi-type MNB generator is a representative device that exploits hydrodynamic cavitation to produce MNBs. As water flows through the convergent nozzle and enters the throat section, its velocity rises while the local pressure drops, creating a negative pressure zone. Ambient air is subsequently drawn into the system via an air inlet, mixes with the water stream, and undergoes intense hydraulic shear. Upon exiting through the divergent section, the entrained air is fragmented into MNBs dispersed within the liquid phase [30]. A research team experimentally evaluated a Venturi tube and a Venturi-vortex microbubble aeration system, revealing that increasing the air flow rate enhances both the standard O₂ transfer rate (SOTR) and the standard aeration efficiency (SAE) [31].

Based on a venturi-type bubble generator, the integration of a vortex field enables the design of a swirling-type venturi bubble generator (SVBG), which enhances the efficiency of microbubble generation. By installing the optimized SVBG downstream of a conventional venturi, a novel cavitation reactor was developed, capable of producing bubbles as small as 780 nm, with significantly improved dissolved oxygen levels, representing a 36.06% increase compared to the standard venturi [32]. A research team has further developed an advanced vortex micro-nano bubble generator that intensifies the shear-induced bubble rupture through a variable-pitch spiral cavitation reactor, enabling the production of bubbles down to 0.301 μm. This generator also exhibits superior oxygen enrichment performance relative to conventional Venturi-based systems [33]. Additionally, In a new type of swirling MNBs generation strategy, when the outlet diameter is 15 mm, the minimum static pressure at the outlet is the lowest, reaching −1.644 × 10⁵ Pa. Under this condition, the formation, development, and collapse of gas cavitation inside the liquid are intensified, resulting in the most significant cavitation effect [34].

2.2.2. Electrolysis Method

The electrolysis technique utilizes an electric field generated by a charged metal capillary to induce the formation of gas bubbles with varying sizes within the liquid phase. Electrolysis leads to supersaturation of oxygen and hydrogen in the anodic and cathodic regions of the solution, respectively, promoting bubble nucleation. Larger bubbles rapidly rise to the surface, where they rupture and leave behind nanoparticles approximately 100 nm in size [35]. The dispersion of bubbles into the liquid, either as discrete entities or in a spray-like manner, is governed by factors such as electric field intensity, capillary dimensions, electrode spacing, and capillary tip geometry. Notably, bubble size decreases with increasing applied voltage and decreasing gas flow rate [36]. Consequently, both bubble size and the mode of their dispersion can be tailored to meet specific application requirements.

2.2.3. Membrane Dispersion Method

The membrane dispersion method utilizes the shearing effect of a porous membrane on gas to break it into fine bubbles. When the gas pressure on one side of the membrane reaches a critical threshold, referred to as the “bubble point”, the gas passes through the membrane pores and emerges on the opposite side, generating relatively uniform microbubbles. The resulting bubble size is closely determined by the membrane’s pore size, surface characteristics, and the applied gas pressure. In systems comprising a surfactant-containing dispersed gas phase and a continuous aqueous phase, Shirasu-porous-glass (SPG) membranes with uniform pore structures can produce monodisperse MNBs, with bubble diameter being controllable through adjustment of the membrane pore size [37].

The cavitation method (especially hydrodynamic cavitation) uses Venturi tubes or modified swirling devices, and can rely on the high-pressure water flow of irrigation systems to achieve large-scale continuous bubble generation, which meets the demand for large-area aeration in farmland. Although the bubble size fluctuates slightly, it does not affect oxygen supply in the root zone. The electrolysis method requires additional power supply to maintain the electric field, resulting in high energy consumption costs, making it difficult to adapt to large-scale field irrigation. Although the membrane dispersion method produces uniform bubbles and allows for precise control over bubble dimensions, its membrane pores are easily clogged by sediment and impurities in irrigation water, leading to high maintenance costs, so it is not suitable for complex field irrigation environments. Therefore, in agricultural irrigation scenarios, the hydrodynamic cavitation method is the optimal choice considering comprehensive efficiency, cost, and applicability.

3. The Mechanism of Rhizosphere Hypoxia Affecting Plant Physiology, Plant Tolerance to Hypoxia, and the Alleviation of Hypoxia by MNBs

3.1. The Mechanism by Which Rhizosphere Hypoxia Influences Plant Physiological Processes

Plants experience hypoxic stress during waterlogging or flooding, which significantly reduces their survival rate. The root system is the first organ to perceive this stress [38]. At the same time, within a certain temperature range, the driving force for O₂ diffusion toward the root system along plant tissues depends on the O₂ concentration gradient rather than thermos-osmotic processes [39]. Consequently, hypoxia reduces tissue porosity, formed by gas-filled intercellular spaces and aerenchyma, which increases resistance to longitudinal O₂ diffusion from shoot tissues to root tips, thus further aggravating hypoxic stress [40]. Hypoxia triggers rapid proliferation of quiescent center (QC) cells followed by programmed cell death (PCD), leading to the differentiation of distal initial cells [38]. This disrupts homeostasis in the root apical meristem stem cell niche (SCN), ultimately impairing or halting root tip growth. In maize, hypoxic conditions have been shown to deplete starch and soluble sugar reserves in QC stem cells. Moreover, the activities of key metabolic enzymes, including pyruvate dehydrogenase (PDH), are suppressed, thereby blocking the tricarboxylic acid (TCA) cycle and forcing reliance on glycolytic fermentation for energy production [41]. The shift from aerobic respiration to fermentation results in NADH accumulation and reduced ATP synthesis, causing an energy deficit. Furthermore, fermentation leads to the buildup of metabolically toxic byproducts, such as ethanol, in plant tissues [42,43]. Hypoxia also compromises QC function by inhibiting both auxin biosynthesis and signaling pathways [44]. During anaerobic and hypoxic conditions, as well as the subsequent recovery phase, the absence of O₂, the final electron acceptor in mitochondrial respiration, disrupts the electron transport chain. This disruption leads to electron leakage from the inner mitochondrial membrane, resulting in excessive reactive oxygen species (ROS) production in plant roots [45]. ROS accumulation under hypoxia is particularly pronounced in the root tip region [46]. Concurrently, ethylene and CO₂ accumulate; elevated CO₂ levels acidify the cytoplasm of root cells, ultimately leading to root apex death, a damage that often prevents recovery of root growth even after re-oxygenation [47] (Figure 2).

Figure 2. Mechanism diagram of rhizosphere hypoxia in plants and rhizosphere hypoxia alleviation by MNBs.

3.2. The Mechanism of Plant Tolerating Hypoxia

A key characteristic of hypoxia tolerance is the ability to form aerenchyma [47]. Hypoxia resistance may also involve aquaporins and ion channel proteins [5,48]. For instance, overexpression of NtPIP1;3 under hypoxic conditions enables rapeseed to maintain higher root O₂ concentrations, root respiration rates, and ATP levels, thereby improving its growth and physiological performance [49]. Additionally, cytoplasmic acidosis induced by anaerobic metabolism can activate SlAH3, enhancing the plant’s response to flooding stress [50]. In intact plants exposed to hypoxia, increased O₂ delivery to root cells can be achieved through two mechanisms: first, by enhancing O₂ uptake directly at the roots; and second, by strengthening O₂ transport from shoots to roots, which may occur simultaneously [49].

3.3. The Mechanism of MNBs Alleviating Hypoxia

O₂ in the water–air mixture gradually dissolves into soil pore water, providing a continuous supply of oxygen to plant roots. This sustained O₂ availability enhances root respiration and development, improves root uptake of water and nutrients, and consequently supports overall plant growth and yield formation [51]. Under conditions of physical drought or physiological waterlogging, tomato plants accumulate high levels of abscisic acid (ABA), which is transported via the xylem to the shoots, triggering stomatal closure and limiting photosynthetic activity [52]. Improved soil aeration mitigates ABA accumulation, reduces its inhibitory effect on stomatal conductance, and ensures adequate CO₂ influx, resulting in an increased net photosynthetic rate [51]. Moreover, adverse environmental stresses often lead to excessive accumulation of reactive oxygen species (ROS). Antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), play critical roles in scavenging ROS by converting them into water and O₂. Rhizosphere O₂ enrichment has been shown to enhance the activity of these enzymes, particularly CAT and POD [51]. Furthermore, integrated analyses of RNA sequencing, qRT-PCR, and enzyme activity assays reveal that genes encoding peroxidases are central to the physiological benefits conferred by nanobubble water [53]. Gene Ontology enrichment and clustering analyses also demonstrate that nanobubble water promotes the expression of genes associated with cell division and cell wall loosening [53] (Figure 2).

3.4. Other Advantages of MNBs in Irrigation

MNBs can not only directly supply O₂ but also enhance soil moisture distribution under drought conditions, optimize soil aggregate structure, reconstruct the rhizosphere microbial community.

Under arid and water-deficient climatic conditions, irrigation with MNBs enhances soil water retention capacity, optimizes moisture distribution, and improves the availability of soil water [54]. MNBs exhibit high interfacial potential and strong stability, enabling them to aggregate saline–alkali-dispersed soil particles into stable micro-aggregates. The reduction in the absolute value of zeta potential in MNB-treated soil indicates increased coagulation tendency of colloidal particles, thereby promoting the formation of soil aggregate structures [55]. Irrigation with nanobubble water (NBW) alters the composition of soil bacterial communities [56]. As NB concentration increases, O₂-rich bubbles are more effectively delivered to the plant rhizosphere, creating an environment favorable for obligate and facultative aerobic bacteria while significantly suppressing anaerobic populations. Consequently, rising O₂ levels lead to progressive differentiation in bacterial community structure, accompanied by a linear decline in overall microbial diversity. However, this shift concurrently increases the abundance of key bacterial taxa (i.e., Thermomonas, Aeromonas, Hyphomonadaceae_SWB02) involved in critical soil nutrient cycling processes, such as organic matter mineralization and nitrogen fixation [57,58].

As a result, soil fertility is substantially enhanced, as micro-nano bubble water delivers increased nutrient availability to plants, thereby promoting crop growth [59]. Furthermore, MNBs significantly reduce the diversity and abundance of microbial communities in biofouling deposits on drip irrigation emitters while simultaneously increasing the population of beneficial water-purifying bacteria. This shift in microbial composition effectively mitigates clogging risks in subsurface drip irrigation systems and extends their operational lifespan [60].

3.5. The Potential Effects of ROS Generated After Their Collapse

The collapse of MNBs generates shock waves and produces ROS [61]. ROS include superoxide anion (O₂⁻·), hydroperoxyl radical (HO₂·), hydrogen peroxide (H₂O₂), hydroxyl radical (·OH), and singlet oxygen (¹O₂) [62]. Numerous studies have demonstrated that MNBs containing oxygen or air can produce ·OH radicals [63]. Although ROS are traditionally recognized as harmful agents, they are naturally regulated within plant cells through a dynamic equilibrium between their production and scavenging by antioxidant systems [64]. When ROS production exceeds the scavenging capacity, this balance is disrupted. Upon surpassing a toxic threshold, ROS induce oxidative stress, leading to the oxidation of proteins, peroxidation of lipids, inactivation of enzymes, degradation of chlorophyll (CHL), and damage to nucleic acids under adverse conditions [65]. This ultimately results in extensive oxidative damage to critical biomolecules, including lipids, proteins, DNA, carbohydrates, and polynucleotides [66,67]. Due to their strong oxidative potential, ROS are widely utilized in applications such as disinfection and advanced oxidation processes.

However, some studies have indicated that ROS play a dual role in seed physiological activities: on the one hand, they act as toxic products accumulated under stress conditions; on the other hand, they are participants in cellular signaling pathways [68]. In addition, from seed development and germination to the development of roots, shoots, and flowers, the production of ROS and ROS-related signal transduction are associated with almost all aspects of plant growth and development in various organs and tissues [69]. Excessively high concentrations of ROS induce oxidation, while moderate concentrations of ROS can exert physiological promoting effects [70]. It is a key regulatory component in metabolic processes [71]. For example, exogenous ·OH generated by nanobubble water significantly upregulate the expression of genes related to peroxidases and nicotinamide adenine dinucleotide phosphate (NADPH), thereby increasing the content of endogenous superoxide anions O₂⁻· in barley seeds [53]. Water containing NBs can continuously generate an extremely small amount of ROS in water [66]. It is more beneficial to the metabolism of organisms [70]. Therefore, micro-nanobubbles do generate additional ROS after collapse. However, concerns about their adverse effects on plants can be largely dispelled, as ROS within a certain concentration range is also beneficial to plants.

4. The Application of MNBs in the Field of Agricultural Sciences

In agricultural production, rhizosphere hypoxia represents a major constraint on crop growth. As a central factor in agricultural systems, the synergistic optimization of irrigation and oxygen supply to the root zone is of critical importance. Whether through improving the rhizosphere environment via integrated approaches such as drip irrigation in conventional systems, enhancing dissolved oxygen levels in nutrient solutions for soilless cultivation, or extending to other agriculture scientific applications, MNBs have shown substantial potential.

4.1. The Application of MNBs in the Field of Traditional Cultivation

In traditional irrigation practices, the integration of MNBs with drip irrigation technology has been shown not only to enhance crop yield and plant stress resistance (the scenario diagram is shown in Figure 3), but also to sustainably promote growth across all stages of crop development. This approach significantly improves maize’s phosphorus uptake efficiency, partial factor productivity of phosphorus, and agronomic use efficiency of phosphorus [72]. When delivered via subsurface drip irrigation, micro-nano bubble hydrogen (MNBH) water markedly increases yield, plant height, and root morphological characteristics in water spinach (Ipomoea aquatica Forssk). It also boosts chlorophyll content, photosynthetic rate, and levels of metallothionein and stress-responsive plant hormones such as abscisic acid (ABA) and salicylic acid (SA), thereby enhancing the plant’s tolerance to cadmium (Cd) stress. Furthermore, MNBH application improves the rhizosphere soil environment by modulating pH and enzyme activity, and reshapes the soil microbial community structure [73]. The integration of MNBs aeration with subsurface drip irrigation enhances soil aeration by 5%, promoting plant dry matter accumulation, increasing crop yield, and improving the concentration of soluble solids and sugars in fruits through enhanced tomato leaf photosynthesis. This approach effectively mitigates the negative impacts associated with a 25% reduction in nitrogen topdressing [74]. Furthermore, the combined use of MNBs and humic acid (HA) alleviates drought stress-induced limitations on tomato yield [75]. The synergistic application of biochar and micro-nano bubble water (MNBW) improves soil aeration and aeration porosity, thereby stimulating cucumber root development and photosynthetic activity, which contributes to greater dry matter accumulation [76]. In rice cultivation, applying three different concentrations of MNBW, compared to conventional tap water, significantly increases dissolved oxygen levels in the soil solution. This improvement supports the growth of various plant organs, enhances nitrogen uptake and utilization efficiency, and ultimately boosts grain yield in double-cropping rice systems [77].

Figure 3. Simulation diagram of the application scenario for MNBs integrated with water and fertilizer irrigation.

In saline–alkali soils, a large number of cations such as Na⁺ and Ca²⁺ will neutralize the negative charge on the bubble surface, reducing the absolute value of the zeta potential of MNBs. This impairs the dispersibility of the bubbles, leading to their premature collapse and failure to continuously supply oxygen to the deep root zone. However, while the absolute value of the MNBs zeta potential decreases, the cations in the saline–alkali soil are also adsorbed, which reduces their contact with crop roots. Furthermore, when used in combination with other agents (such as microbial agents, MA), MNBs—acting as a carrier that enhances dissolution and dispersion—can improve the properties of saline–alkali soils more significantly. The specific improvements include reducing soil electrical conductivity and pH value, increasing soil fertility, and promoting the formation of soil aggregates [55].

4.2. The Application of MNBs in the Field of Soilless Cultivation

In the field of soilless cultivation, MNBs demonstrate a significant growth-promoting effect. Studies have employed micro-nano bubble generators in combination with CO₂ or air to treat hydroponically grown amaranth. Results indicate that MNBs positively influence both the growth performance and nutritional quality of amaranth green (Amaranthus viridis), suggesting that MNB-treated water exerts a favorable and statistically significant effect on plant development [22]. In substrate-based lettuce cultivation, the use of nutrient solutions enriched with micro-nano bubbles (O₂ and O₃) increased dissolved oxygen levels, which in turn enhanced lettuce yield and improved key physiological parameters, including net photosynthetic rate, water conductivity, and intercellular CO₂ concentration [78]. Seed germination experiments were conducted on lettuce, carrot, broad bean, and tomato using nanobubbles generated from air, O₂, N₂, and CO₂. The presence of nanobubbles in irrigation water led to a 6–25% increase in seed germination rates [79]. These outcomes imply that integrating micro-nano bubble technology into soilless substrate systems can substantially promote plant growth and increase crop yield.

In soilless cultivation, plant roots are in direct contact with nutrient solutions or substrates, and dissolved oxygen concentration is a key limiting factor for plant growth. For field crops, especially deep-rooted crops, traditional irrigation methods struggle to meet the oxygen demand of their deep-root zones. However, MNBs can reach the deep-root zones by releasing oxygen slowly and improving soil porosity. In protected cultivation, intensive soil tillage often leads to soil compaction, which easily causes rhizosphere hypoxia. Protected cultivation fruits and vegetables have high requirements for yield and quality (such as sugar content and vitamins). MNBs can simultaneously address the issues of hypoxic stress and inefficient nutrient absorption. Therefore, certain crops with the above-mentioned needs—including lettuce, maize, and tomatoes and other plants—can particularly benefit from MNB irrigation (Table 1).

Table 1. Summary Table of the Effects of MNBs on Different Systems (Soilless Cultivation, Conventional Irrigation, Saline–Alkali Soil).

System Type	Core Mechanism of Action	Practical Impacts
Conventional Irrigation System	Enhance photosynthesis [74]. Alter microbial community composition [80].	Maize: Phosphorus absorption efficiency and partial factor productivity of phosphorus significantly improved [72]. Double-cropping rice: Grain yield of early rice increased by 4.84–10.95%, and yield of late rice increased by 6.10–14.31% [77]. Tomato: Dry matter accumulation increased by 3–59% and plant nitrogen accumulation increased by 2–70%, tomato yield increased by 12–44% [74].
Soilless Cultivation System	Enhance dissolved oxygen in nutrient solution [78]. Reshape microbial community [58].	Amaranth: Growth quality and nutritional indicators optimized, ant height increased by 62%, leaf area increased by 414%, and net photosynthetic rate improved [22]. Seed germination rate: Increased by 6–25% for lettuce, tomato, etc. [79].
Saline–Alkali Soil System	Synergize with microbial agents (MA) to reduce soil electrical conductivity and pH [55]. Promote soil particle aggregation (reduce salt crust) [55].	Soil CEC increased 1.3–1.7 times, pH reduced by 5–5.8% [55].

4.3. The Application of MNBs in Other Agricultural Science Fields

The MNBs exhibit broad application prospects in water treatment, production system optimization, and post-harvest processing. For instance, in a circulating disinfection system integrating a micro-nano bubble generator with a selenium catalyst and solar disinfection (SODIS) technology, the incorporation of MNBs has been shown to enhance dissolved O₂ levels, improve O₂ mass transfer efficiency, and optimize interfacial photoelectric effects, thereby enabling effective low-temperature disinfection of raw groundwater [81]. Antibiotic pollution has emerged as a critical global environmental challenge; however, immobilized Chlorella augmented by MNBs offers a promising solution for pollutant degradation. MNBs increased Chlorella biomass by 2.48 times while significantly enhancing the removal efficiency of ofloxacin (OFLX), sulfadiazine (SD), and chloramphenicol (CAP) [82,83]. In treating wastewater containing high concentrations of ibuprofen (IBU), MNB-assisted ozonation achieved 99.0% IBU removal within 70 min [84]. Furthermore, injecting MNBs into rice–crayfish co-cultivation systems not only promotes the growth of both rice and crayfish but also increases soil total nitrogen and phosphorus content, thereby improving soil fertility. The integration of this rice-fish symbiotic system with MNBs enhances rice yield by 26.8% and is projected to increase economic efficiency by approximately 35% [85].

In the field of post-harvest treatment, the research groups led by Deng, Malahleha, and Shi have developed an ozone micro-nano bubble water or ice (O₃-MNBW/O₃-MNBI) system for preserving lychees, guavas, and parsley [86,87,88]. Additionally, a piezoelectric catalytic sterilization system based on synergistic spontaneous polarization ceramics (SPC) and ozone micro-nano bubbles (OMNB) has been introduced, enabling efficient fruit disinfection and prolonged preservation [89]. In cut flower preservation, nanobubble (NB) treatment has been shown to delay flower bud opening in inflorescences and retard petal senescence [90]. Similarly, treatment with hydrogen nanobubble water (HNW) at optimal concentrations can suppress reactive oxygen species (ROS) accumulation and inhibit the initial activity of senescence-related enzymes, thereby delaying senescence in cut carnations (Dianthus caryophyllus L.) [91]. In aqueous environments, pesticide degradation primarily occurs through hydrolysis, photolysis, and redox reactions. Following degradation, residual organic compounds containing unsaturated carbon chains react with O₃ to form acids, alcohols, amines, carbonyl compounds, carbohydrates, and carboxylates, products that are generally water-soluble and thus removable via rinsing. Although aqueous O₃ exhibits high reactivity, it rapidly self-degrades; however, micro-nanobubble ozonated water (MNBO), due to its high mass transfer efficiency and sustained release from nano-sized bubbles [92], maintains elevated levels of dissolved O₃ over extended periods, effectively addressing the instability of conventional ozonation [93]. Consequently, the integration of micro-nanobubbles (MNBs) and O₃ demonstrates significant potential for the removal of pesticide residues from fruits and vegetables [94].

5. Conclusions and Prospects

In traditional irrigation systems, MNBs boast unique characteristics: a large specific surface area, high oxygen dissolution efficiency, and long retention time. They rise slowly in water while releasing oxygen continuously. Introducing such oxygen-enriched MNBs through irrigation offers multiple benefits. It overcomes the drawback of rapid dissolved oxygen loss in conventional methods, continuously raises rhizosphere oxygen levels, and balances the oxygen supply and demand for plant roots’ aerobic respiration. This effectively addresses the “restricted oxygen diffusion” issue caused by soil compaction or waterlogging. Additionally, MNBs carry a strong negative surface charge. This charge enhances their interaction with soil particles, reduces migration resistance in clayey soils, and allows them to penetrate deeper into the rhizosphere. In turn, this alleviates hypoxic stress induced by water-filled soil pores.

High-efficiency protected agriculture—centered on soilless cultivation, integrated water and fertilizer management—is a key direction for agricultural development. Integrating MNB generators with water–fertilizer delivery systems enables precise control over bubble generation, release location, and dosage. In scenarios like substrate cultivation or hydroponics, direct injection of MNBs into nutrient solutions not only increases dissolved oxygen concentration but also improves the rhizosphere microbial environment. This ultimately boosts crop yield and provides an effective solution to rhizosphere hypoxia in protected agriculture.

Existing MNBs generation technology requires optimization for agricultural scenarios. It is necessary to develop low-cost, anti-clogging generators suitable for the high-flow demands of field irrigation, while improving bubble production efficiency and reducing energy consumption to support large-scale applications. In addition, in-depth research should be conducted on the optimal matching relationship between bubble parameters, crop types, and soil conditions: for deep-rooted crops, the rising speed and stability of bubbles need to be enhanced to ensure effective oxygen delivery to the deep root zone; for sensitive crops, the concentration of reactive oxygen species generated by bubble rupture must be strictly controlled. In the future, a real-time monitoring system for rhizosphere oxygen levels, soil moisture, and mineral ion concentrations can be established. By installing sensors at the end of irrigation pipelines to continuously transmit data, intelligent regulation of water, fertilizer, and gas supply can be realized. This system will dynamically monitor soil or substrate conditions, adapt to the oxygen and nutrient requirements of plants at different growth stages, and ultimately improve yield and resource utilization efficiency.

In the long term, it is essential to evaluate the continuous impact of repeated MNBs irrigation on soil physical and chemical properties. Efforts should be made to upgrade the technology from generalization to precision, customizing bubble parameters based on crop root characteristics and soil texture. For specialized systems such as ozone or hydrogen MNBs systems, ecotoxicological studies are required to clarify their potential impacts on soil microbial diversity, groundwater environments, and crop secondary metabolites, thereby avoiding ecological risks arising from technical application. Meanwhile, multi-regional field-scale experiments should be carried out, with statistical replication to ensure data reliability, to verify the applicability of the yield-promoting conclusions of MNBs obtained under laboratory conditions in actual farmland scenarios. This will provide empirical support for formulating standardized application schemes of MNBs technology, ultimately helping it become one of the core technologies to address rhizosphere hypoxia and enhance agricultural productivity.