Current status of application of seaweed fertilizer in modern agricultural production and the progress of research on its mechanism of action

Abstract: Seaweed is the largest reserve of renewable biomass resources in the ocean, and seaweed fertilizer is an important type of products related to seaweed. In order to fully recognize the role of seaweed fertilizer in modern agriculture and its development prospects, this paper reviews its main raw materials, functional components, production processes, common types of seaweed fertilizers in recent years and the current application status of seaweed fertilizers in agricultural production, and analyzes and discusses the mechanism of seaweed fertilizers, the current bottlenecks in application and the prospects of development, with a view to providing references for the further research and development and popularization of new types of high-efficiency seaweed fertilizers.

The continuous irrational application of chemical fertilizers in China's agricultural production and successive cropping have led to increasingly serious problems such as soil sclerosis, acidification, declining fertility, declining microbial diversity, aggravating soil diseases, and decreasing crop yields and quality. With the continuous development of social economy, green agriculture and organic agriculture have become the main development direction of modern agriculture. The National Strategic Plan for Quality Agriculture (2018-2022) puts forward key tasks such as accelerating the green development of agriculture, and the Ministry of Agriculture and Rural Development has also formulated the Technical Guidelines for Green Development of Agriculture (2019-2030), and the development of green agriculture has risen to the level of a national strategy.

Seaweed is a chloroplast-containing, embryo-free, leafy marine sporophyte, characterized by fast growth and strong adaptability, and is the largest and renewable biomass resource in the ocean. China is rich in seaweed resources, and the common macroalgae resources include kelp, Edelweiss, wakame, purslane, goat weed, unicorn weed, seaweed and rockweed. China's total output of seawater algae in 2018 was 2.3444 million tons, as recorded in the 2019 China Fisheries Statistical Yearbook. Seaweeds are rich in active ingredients such as polysaccharides, natural plant regulators, amino acids, alginic acid, mineral nutrients, vitamins and unsaturated fatty acids, and are widely used in food, medicine, cosmetics, agriculture, etc. In the 1950s, seaweeds and their extracts were used in agricultural production.

Seaweed fertilizer is an organic high-efficiency fertilizer made from the active ingredients of algae, which can improve the soil, promote plant growth, increase crop yield and quality as well as enhance crop resistance. Compared with other fertilizers, seaweed fertilizer has the advantages of being safe and non-toxic, comprehensive and efficient, and environmentally friendly. Therefore, the production of seaweed biofertilizer with seaweed or seaweed waste residue as substrate is one of the effective methods to develop green and sustainable agriculture, which is in line with the national strategic needs. In this paper, we review the raw materials, functional components, production process of seaweed fertilizers and the current status of their application in the field of agriculture in recent years, and discuss the bottlenecks in the process of research and development and promotion of seaweed fertilizers, with a view to providing references for the development of new and efficient seaweed fertilizers.

1. Raw material of seaweed fertilizer and its functional analysis

Seaweeds used in the production of seaweed fertilizer are mainly brown algae, red algae and green algae, and the most commonly used ones are brown algae, kelp, kelp, kelp, sargassum and so on. China is rich in wild seaweed resources, and the seaweed artificial culture industry is developed. Algae itself and algae processing by-products can be used as raw materials for the development of seaweed fertilizer. In addition, a large amount of seaweed biomass such as seaweed biomass produced by seaweed outbreaks due to the eutrophication of seawater can also be used as raw materials for the preparation of seaweed fertilizer. Compared with traditional chemical fertilizers and other land-based organic fertilizers, algal fertilizers are not only green and environmentally friendly, supplementing land-based fertilizers lack of biofunctional substances, helping to improve the quality and efficiency of agricultural products, but also able to promote the economic development of the algal industry, which is an organic combination of green agriculture and the algal industry.

Seaweed fertilizer is rich in natural growth regulators, alginic acid, brown algae polyphenols, mannitol and betaine, fatty acids and other functional components. With the deepening of seaweed research, new functional substances have been isolated and identified.

1.1 Seaweed growth regulators

Plant growth regulator-like components in seaweeds include cytokinins, polyamines, gibberellins, abscisic acid, indole 3-acetic acid, agonins and oleoresin-like steroids, etc., and their roles in plant growth and development have also been confirmed. Betaine is widely found in seaweeds and their extracts, which can enhance the ability of seaweeds to resist osmotic stress, and its structure and function vary according to the seaweed species.Valverde et al. determined the content of glycine betaine in 18 Irish seaweeds, and found that the content of betaine in different seaweed species ranged from 1.39 to 105.11 ng/mL. The seaweed sterols were mainly fucosterol and streptosterol. In addition, seaweed phenolic substances also have good growth-promoting effects.

1.2 Seaweed polysaccharide and oligosaccharide

Seaweeds are rich in a variety of polysaccharides that land plants lack, including alginic acid, fucoidan sulfate, fucoidan and fucoidan starch, etc., which are also important index components of seaweed fertilizer. Alginic acid has the characteristics of gel, chelating and hydrophilic, etc. It has a good effect on keeping soil and water, promoting the formation of granular structure, activating mineral elements and regulating soil pH value. The seaweed fertilizers produced at present mostly take alginic acid as the main functional component, which is also fully reflected in the standard of seaweed fertilizers formulated by the state. Seaweed polysaccharide can also be degraded into fucooligosaccharides of different small molecular weights, which have growth-promoting active functions.

1.3 Proteins, peptides and amino acids of seaweeds

The protein content of seaweeds varies according to the species of algae and different growth environments, the protein content of red algae and green algae is higher, at 10% ~ 47%, and the protein content of brown algae is relatively lower, at 3% ~ 15%. In addition to aspartic acid and glutamic acid and other conventional amino acids, seaweed protein also contains fucoxanthin, sea manchuic acid, chondroitin, taurine and lectin and other seaweed-specific amino acids, which have good functional activity. Seaweed proteins have inhibitory effect on banana anthracnose bacteria, and different sources have different mechanisms of action. Peptides, amino acids and other fertilizers also have a good effect, compared with other kinds of foliar fertilizers, "peptide - amino acid" foliar fertilizers can obviously improve the absorption of nutrients, prolong the time of nutrient absorption, improve the quality of the crop, increase crop yield, and effectively inhibit the occurrence of soil-borne diseases in the soil, while microorganisms have a good effect on the development of soil-borne diseases. The microorganisms have good degradation effect on marine macromolecule substances. Therefore, the preparation of seaweed polypeptide-amino acid fertilizers by bio-enzymatic digestion or microbial fermentation with seaweed as raw material has a broad application prospect.

1.4 Lipid compounds

Seaweeds are rich in fatty acids such as palmitic acid, octadecenoic acid, linoleic acid, eicosapentaenoic acid and their derivatives, and lipophilic compounds such as pigments are also closely related to the potential biological activities of seaweeds.Lipophilic antioxidants extracted by Rajauria from brown algae showed strong antioxidant capacity. Analysis using LC-PDAESI-MS/MS showed that the seaweed extract contained cyanidin-3-o-glycoside, fucoxanthin, purple plastocyanin, carotenoids, chlorophyll a derivatives and chlorophyll b derivatives. In addition, polar lipids such as glycolipids, sphingolipids, galactose ceramides, inositol phosphate ceramides, glycerophospholipids, phosphatidylcholine, phosphatidic acid, phosphatidylglycerol, and betaine lipids are found in red algae. The lipids in seaweeds were mainly monogalactosyl diacylglycerol, di-lactosyl diacylglycerol, sulphoquinolyl diacylglycerol, and phosphatidylcholine, which accounted for 43.0% of the total lipid distribution. Although these substances are rarely reported in seaweed fertilizer studies, it is assumed that they have a positive effect on plant growth and development and stress tolerance in view of their biological effects. In addition, because the organic nitrogen and phosphorus contained in fatty acids can only be absorbed and utilized by crops after being decomposed by microorganisms, the fertilizing effect of seaweed fertilizers containing fatty acids is more lasting, and the effect may be better when combined with microbial fungicides.

1.5 Minerals

In addition to the above components, seaweed also contains minerals such as potassium, calcium, magnesium, strontium, iodine and other minerals incomparable to terrestrial plants, as well as manganese, molybdenum, zinc, iron, boron, copper and other trace elements necessary for plant growth.

2. Seaweed fertilizer production process and product types

2.1 Production process

The production methods of seaweed fertilizer mainly include chemical method, physical method and biological method. The chemical method mainly utilizes acid and alkali or organic solvents to treat seaweed, so that the components inside the seaweed cells are dissolved. At present, most seaweed fertilizer producers at home and abroad still adopt this method because of its simple operation and low cost. However, due to the serious damage of chemical raw materials to intracellular active substances and the environmental pollution caused by the waste liquid, this method is not suitable for long-term use. Physical methods mainly use high pressure, ultrasound, low temperature and other ways to achieve the effect of seaweed wall-breaking, relatively high requirements on the instrument, and the processing scale is relatively small, taking into account the cost of production is not suitable for small and medium-sized enterprises. Bio-extraction process utilizes the enzyme system produced during microbial fermentation to degrade the cell wall of seaweed, so that the active ingredients inside the cell can be dissolved and the macromolecules can be decomposed into small molecules that can be easily absorbed by plants. This method is the most promising seaweed fertilizer production method because of its mild reaction, safe products and no pollution in the production process.

2.2 Common types of seaweed fertilizer

According to different classification standards, seaweed fertilizer can be divided into different types. The common types of products on the market at present can be divided into flush fertilizers and matrix fertilizers. There are liquid and water-soluble powder types of flush fertilizers, which are concentrated from seaweed extracts and are suitable for most crops. Fertilizers usually need to be diluted when used, and the dilution factor depends on the foliar spraying, root flushing, drip irrigation, etc. or different growth periods of plants. Seaweed matrix fertilizer refers to seaweed as the main source of organic matter, compounded with inorganic nutrients, other types of organic fertilizers or microbial fertilizers of fertilizers, can be seaweed organic + inorganic blend, seaweed organic + other organic blend, seaweed organic + other organic + inorganic blend, seaweed organic + microbial blend and other combinations.

2.3 Seaweed fertilizer scientific application program

Seaweed fertilizer will show differences in effect depending on product type, crop type, growth period and spraying method. In the application of seaweed fertilizer research, we need to comprehensively consider various factors, optimize the combination of different product forms, and obtain diversified seaweed fertilizer application programs to make it better play its role.

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