Soil is alive, and this life is soil microorganism. As we all know, the soil is the base of microorganisms and the natural medium for their growth and reproduction. Humic acid runs through soil life and is the “life core” of soil. Without humic acid, the soil will lose its activity. Therefore, clarifying the relationship between “soil humic acid microorganisms” has important practical significance for protecting land resources and promoting sustainable soil production.

1、 Soil humic acid

• The root of soil life humic acid is the most profound “dark substance” in soil, and every cycle of its life is associated with life. ① Living according to the living body: mainly animal and plant residues (mainly plant residues) as the material source; ② Transformation by living body: continuous decomposition, transformation and synthesis mainly through the participation and action of microorganisms; ③ Returning to life: it mainly feeds life through soil humic acid, promotes the growth of green plants (producers), enhances the immunity of livestock, poultry, aquatic products, people, etc. (consumers), and provides nutrients and energy sources for microorganisms (decomposers). Accordingly, humic acid, along with the rhythm of life, realizes the cycle of “survival → transformation → extinction → rebirth”, and becomes the original idea of glowing soil life vitality according to its existence. (2) The core of soil structure the structural characteristics of soil include three meanings: ① in the process of soil formation, organic matter and minerals form organic-inorganic complexes, small aggregates and large aggregates; ② Micro aggregate is the core of soil structure, which is the smallest and independent structural unit (equivalent to the cells in the soil) of soil; ③ In the process of soil utilization, the structure of soil has a great impact on the function of soil. Micro aggregates are the key to the structural stability of “soil houses”, and humic acid is the key component to build “soil houses”. In the process of soil formation, humic acid combines with organic macromolecules such as polysaccharide aldosides secreted by various microorganisms, clay minerals and hydroxides of iron and aluminum to form various types of organic-inorganic complexes through different forms and various forces. At the same time, under various surface charges and forces, micro aggregates with different particle sizes are formed through agglomeration. Therefore, the formation process of micro aggregates of humic acid endows the soil organic-inorganic complex with fresh vitality, which is the core and key to stabilize the soil structure. (3) Soil ecosystem functions mainly include: the ability and level of material transformation and energy flow in soil, the activity of soil organisms, the balance of nutrients and water in soil and its impact on the environment. Humic acid is the most active part of soil organic matter, which plays an important role in material circulation and energy conversion of soil ecosystem. ① Active participants and promoters of soil formation and soil fertility; ② Promoting and restricting the migration, fixation and leaching of soil metal ions and trace elements; ③ It is the stabilizer of soil structure; ④ Affecting the base exchange capacity of soil; ⑤ Affecting the water holding capacity of soil; ⑥ It is a warehouse for plant materials. Therefore, humic acid can regulate the material circulation, energy conversion and information transmission among biological system, soil system and environmental system, and is an important material to maintain the stability of soil function.

2. Soil microorganism

• Soil microbial composition soil is the base of microorganisms and the natural “culture medium” for microbial growth and reproduction. Soil microorganism is the general name of bacteria, fungi, actinomycetes, algae, etc. living in soil, and it is an important component of soil. Its individual size is small, which is generally calculated in microns or nanometers. Its type and quantity change with the soil forming environment and soil depth. Among them, bacteria are the most, followed by actinomycetes and fungi, and algae are less. It is estimated that there are thousands or even tens of thousands of species in each gram of soil, about hundreds of millions to billions of microbial individuals, and their species and quantities change with the soil forming environment and the depth of the soil layer. Generally speaking, the number of bacteria, actinomycetes, fungi and algae in each gram of cultivated soil layer is about 10^8 ~ 10^9, 10^7 ~ 10^8, 10^5 ~ 10^6 and 10^4 ~ 10^5, all of which live in the soil. (2) Soil microorganisms have a significant impact on the formation and development of soil, material circulation, fertility evolution and plant growth. ① Bacteria: suitable for neutral and slightly alkaline living conditions. Among them, autotrophic bacteria have the ability to assimilate carbon dioxide, directly affect the physical and chemical properties of soil, and balance the pH of soil; Heterotrophic bacteria exist in the state of symbiosis with crops, which can directly promote the growth of crops, such as Rhizobium of leguminous plants. They have a strong nitrogen fixation effect and produce obvious yield increase effect under the condition of less nitrogen fertilizer. ② Actinomycetes: suitable for neutral and slightly alkaline living conditions. Actinomycetes in the soil live in an aerobic heterotrophic state. Their main activities are to decompose cellulose, lignin and pectin in the soil. Through these functions, the soil nutrient status can be improved, which is convenient for crops to directly absorb and utilize soil nutrients. ③ Fungi: suitable for slightly acidic living conditions. Participating in the decomposition of animal and plant residues has become an indispensable driving force for the nitrogen and carbon cycle in soil. Especially in the early stage of decomposition of plant organisms, fungi are more active than bacteria and actinomycetes. ④ Algae: a kind of unicellular organisms, mainly existing in the aquatic environment. Cyanobacteria in algae can also fix nitrogen in the air. It is more suitable to function in alkaline environment. (3) Soil microorganism activity soil microorganism is not only an important part of soil, but also the vitality of soil ecosystem. ① Gases and organic acids produced by the metabolism of various microorganisms contribute to the formation of soil aggregate structure; ② Decompose organic matter, synthesize soil humus humic acid, and fertilize soil; ③ Decompose minerals and promote the dissolution of insoluble substances in soil (such as phosphorus bacteria, potassium bacteria, etc.); ④ Fixing atmospheric nitrogen and increasing soil nitrogen nutrient; ⑤ Regulating plant growth, such as rhizobia, mycorrhiza, fungi, etc. symbiotic with plants; ⑥ Secrete antibiotics (such as actinomycetes) to prevent soil borne diseases; ⑦ Secrete a large number of enzymes to promote the transformation of soil nutrients; ⑧ Degrade residual organic pesticides, urban pollutants and factory wastes in soil. The above 8 items reflect the important relationship between soil microorganisms and soil fertility, soil health and plant growth, and are the main force of material circulation and energy flow in soil ecosystem.

3. Soil humic acid and microorganism

(1) Microbiological mechanism of soil humic acid formation there are mainly seven hypotheses about the microbiological mechanism of humic acid formation: ① Waxman theory holds that in the process of plant to humus formation, nitrogen-containing organic matter and nitrogen-free organic matter are decomposed by aerobic microorganisms to produce microbial bodies (soil protein) and lignin, which then combine with alkali to form humus soil. ② According to Williams theory, humic acid is the secretion of soil microorganisms; ③ According to the microbial synthesis hypothesis, microorganisms can use plants as carbon and energy sources to synthesize macromolecular humus substances in cells, which are released into soil after death and then degraded into humic acid and fulvic acid outside cells; ④ According to the hypothesis of cell autolysis, humus is the product of cell autolysis after the death of plants and microorganisms; ⑤ According to the coalification theory, there are two main conditions that affect the accumulation of plant residues in the swamp and the formation of peat. One is the physical condition that separates the plant residues from the air, and the other is the chemical condition that maintains the good activity of microorganisms; ⑥ Konova’s theory holds that the formation of humus includes two stages of “decomposition condensation”, with the participation of soil microorganisms; ⑦ According to the theory of anaerobic fermentation, the formation of humic acid includes three stages of “hydrolysis acid production synthesis”, and the role of microorganisms is mentioned in varying degrees. No matter which hypothesis, it fully affirmed the important role of microorganisms in the formation of humic acid. (2) The effect of humic acid on the reproduction of soil microorganisms the formation of humic acid is inseparable from soil microorganisms, and the reproduction of soil microorganisms is also inseparable from humic acid. ① Humic acid provides material and energy sources for soil microorganisms. Soil microbial nutrition mainly includes carbon, hydrogen, oxygen, nitrogen and some mineral elements. Humic acid contains carbon, oxygen, hydrogen, nitrogen and other elements, which can provide carbon and nitrogen sources for soil microorganisms to survive and multiply. ② Humic acid creates a suitable living environment for soil microorganisms. The survival of soil microorganisms requires appropriate temperature, humidity, pH, etc. Humic acid can directly or indirectly improve soil temperature, moisture and air permeability, adjust soil pH, promote the growth and reproduction of soil microorganisms, and increase their number and types. ③ Humic acid provides a living home for soil microorganisms. The total number of microorganisms in the soil with deep soil layer, loose soil, good physical properties and rich available nutrients is also high. Humic acid is the core of soil fertility; If the soil flourishes, the microbes thrive. (3) Humic acid and microorganism are the soil life community. Humic acid is the carrier of soil life. It provides carbon and nitrogen sources for microorganisms for life, and promotes the number, variety and diversity of microorganisms; Microorganism is the embodiment of soil life, playing the role of decomposer, and plays an important role in the formation and decomposition of humic acid. The formation of humic acid is inseparable from soil microorganisms, and the survival and reproduction of soil microorganisms are inseparable from humic acid. They are interdependent and jointly promote the formation and development of soil, fertility evolution, material circulation and energy transformation. It can be seen that “soil humic acid microorganism” are inseparable and belong to the community of life.