Influenced by the international geopolitical landscape and adjustments to export policies, China has currently implemented export restrictions on fertilizer products with high nitrogen content—including certain seaweed fertilizers and complex amino acid fertilizers. Concurrently, against the backdrop of global climate change, the demand for agricultural inputs possessing stress-resistance capabilities continues to rise. Accordingly, the following section introduces several categories of fertilizers and biostimulants that serve as viable alternatives to seaweed fertilizers while offering stress-resistance functions.
Humic Acid Compounds
Key Representative
1.Humic Acid: Characterized by a high molecular weight; primarily acts upon the soil to improve its physicochemical properties.
2.Fulvic Acid: Characterized by a low molecular weight and excellent water solubility; can be directly absorbed by plants and is suitable for both foliar spraying and root application.
Mechanisms of Stress Resistance
1.Soil Structure Improvement: Promotes the formation of soil aggregates, increases soil porosity, and enhances aeration and water-holding capacity within the rhizosphere, thereby indirectly bolstering the root system’s tolerance to drought.
2.Nutrient Chelation: Forms chelate complexes with mineral elements present in the soil (such as iron, zinc, and calcium), thereby increasing nutrient availability and preventing a decline in stress resistance caused by nutrient deficiencies.
3.Stimulation of Root Development: Promotes root elongation and the proliferation of lateral roots, enhancing the root system’s capacity to absorb water and nutrients, and consequently boosting overall stress resistance.
4.Regulation of Plant Hormones: Humic acids can influence the levels of plant hormones—such as auxin (IAA)—within the plant body, thereby promoting growth and alleviating the effects of environmental stress.
Applicable Scenarios
1.Plots suffering from severe soil salinization or compaction.
2.Arid or semi-arid regions, utilized to enhance the soil’s water-holding capacity.
3.During the pre- and post-transplantation periods, to facilitate the rapid recovery of the root system.
Amino Acids
Key Representatives
1.Proline: An osmoregulatory substance that plants naturally accumulate in large quantities under stress conditions.
2.Glycine Betaine: Protects the activity of cell membranes and enzymes under conditions of drought, salinity/alkalinity, and low temperatures.
3.γ-Aminobutyric Acid (GABA): Regulates ion channels and participates in stress signal transduction pathways.
4.Compound Amino Acids: Obtained through the enzymatic or acid hydrolysis of animal and plant proteins; contain a variety of free amino acids.
Mechanisms of Stress Resistance
1.Osmoregulation: Substances such as proline and betaine accumulate in the cytoplasm, lowering the cellular osmotic potential. This enables plants to maintain water uptake under drought or saline/alkaline conditions, thereby preventing cellular dehydration.
2.Scavenging Reactive Oxygen Species (ROS): Amino acids serve as precursors for the synthesis of antioxidant enzymes (e.g., superoxide dismutase, catalase). They enhance the plant’s ability to scavenge free radicals and mitigate damage caused by membrane lipid peroxidation.
3.Direct Energy Source: Under stress conditions, plant photosynthesis is often inhibited; exogenous amino acids can be directly absorbed and utilized, thereby conserving energy and sustaining basal metabolic processes.
4.Chelation of Metal Ions: Certain amino acids can chelate excess heavy metal ions, thereby alleviating heavy metal stress.
Applicable Scenarios
1.Drought or Salinity/Alkalinity Stress: Proline- and betaine-based products demonstrate significant efficacy.
2.Low- or High-Temperature Stress: GABA and compound amino acids can help stabilize membrane structures.
3.Recovery from Herbicide or Fertilizer Injury: Rapidly replenishes organic nitrogen sources to promote recovery and regrowth.
Biopolymers (Chitin/Chitosan and Derivatives)
Key Representatives
1.Chitosan: Extracted from the exoskeletons of crustaceans and obtained through deacetylation.
2.Chitooligosaccharides: Degradation products of chitosan, characterized by lower molecular weights and higher biological activity.
3.Alginate Oligosaccharides: One of the active components found in brown algae extracts; while overlapping with the scope of seaweed fertilizers, they can also be utilized independently.
Mechanisms of Stress Resistance
1.Induction of Systemic Resistance: Acting as an elicitor, chitosan is recognized by receptors on plant cell membranes, thereby activating the jasmonic acid (JA) and ethylene (ET) signaling pathways. This process induces Systemic Acquired Resistance (SAR) in plants, enhancing their defensive capabilities against pathogenic bacteria and environmental stressors.
2.Cell Wall Reinforcement: Promotes the deposition of lignin and callose, resulting in thickened cell walls. This creates a physical barrier that impedes the invasion of pathogens while simultaneously reducing water transpiration.
3.Activation of Antioxidant Systems: Enhances the activity of defensive enzymes—such as peroxidase (POD) and polyphenol oxidase (PPO)—thereby mitigating oxidative damage.
4.Regulation of Stomatal Opening and Closing: Certain chitooligosaccharides can induce stomatal closure, thereby minimizing water loss under drought conditions.
Applicable Scenarios
1.Drought Stress: Induces stomatal closure to reduce transpiration.
2.Low-Temperature Stress: Enhances cell membrane stability to alleviate cold injury.
3.Periods of High Disease Incidence: Offers a dual function, acting as both a resistance inducer and a microbial inhibitor.
Microbial Inoculants
Key Representatives
1.Plant Growth-Promoting Rhizobacteria (PGPR): Such as *Bacillus subtilis*, *Bacillus amyloliquefaciens*, *Pseudomonas fluorescens*, etc.
2.Arbuscular Mycorrhizal Fungi (AMF): Form symbiotic relationships with over 80% of terrestrial plants, primarily facilitating phosphorus uptake.
3.Trichoderma spp.: Possess dual functions of plant growth promotion and biological control.
Mechanisms of Stress Tolerance
1.Secretion of Phytohormones: Microorganisms can produce compounds such as Indole-3-acetic acid (IAA) and Gibberellins (GA), thereby promoting root system development—specifically the proliferation of lateral roots and root hairs—and significantly increasing the nutrient absorption surface area.
2.Nutrient Solubilization: Facilitate phosphorus solubilization, potassium mobilization, and nitrogen fixation, thereby improving nutrient availability under stressful conditions and preventing the exacerbation of stress-induced damage caused by nutrient deficiencies.
3.Induction of Systemic Resistance: Similar to the action of chitosan, beneficial microorganisms—upon colonizing the rhizosphere—induce plants to develop ISR (Induced Systemic Resistance), thereby enhancing their tolerance to both biotic (biological) and abiotic (environmental) stresses.
4.The “Bridging” Role of Mycorrhizal Networks: AMF hyphae can extend into soil pores inaccessible to plant roots, absorbing water and nutrients and transporting them to the host plant; this mechanism demonstrates particularly significant stress-mitigating effects in drought-prone or nutrient-poor soils.
5.Improvement of the Rhizosphere Microenvironment: Beneficial microbes occupy ecological niches, thereby inhibiting the proliferation of pathogenic microorganisms, while simultaneously secreting extracellular polysaccharides that improve the aggregate structure of the rhizosphere soil.
Applicable Scenarios
1.Continuous Cropping Fields: Alleviating issues associated with continuous cropping, such as soil-borne diseases and the accumulation of autotoxic substances.
2.Drought-Prone or Nutrient-Poor Soils: Scenarios where the synergistic action of AMF and PGPR yields particularly significant benefits.
3.Organic Farming or Reduced Chemical Fertilizer Use: Serve as partial substitutes for chemical inputs within agricultural production systems.
Synergistic Effects of Combined Application
A primary reason for the excellent stress-resistance effects of seaweed fertilizers lies in their complex composition (containing a diverse array of active substances such as plant hormones, polysaccharides, amino acids, and minerals). Alternative products, when applied in combination, can achieve effects similar to—or even superior to—those of seaweed fertilizers.
Common synergistic combinations include:
| Combination | Synergistic Effect |
| Humic Acid + Microbial Inoculant | Humic acid provides carbon sources and a habitat for microorganisms, thereby promoting the colonization of beneficial bacteria; in turn, the microorganisms decompose the humic acid to release active small molecules, establishing a virtuous cycle. |
| Amino Acids + Silicon Fertilizer | Amino acids supply organic nitrogen and osmoregulatory substances, while silicon reinforces physical barriers; these two components complement each other, resulting in a significant enhancement of overall resistance against drought, salinity-alkalinity stress, and high temperatures. |
| Chitosan + Complex Amino Acids | Chitosan induces plant resistance, while amino acids provide energy and metabolic support; their synergistic action mitigates the energy expenditure associated with the resistance-induction process, thereby ensuring “induced resistance without yield loss.” |
| Fulvic Acid + Trace Elements (e.g., Zinc, Selenium) | Fulvic acid chelates trace elements, thereby improving the efficiency of their uptake and utilization by plants under stressful conditions. |