Microorganisms: We are the engine of soil material circulation!

Produced by: Science Popularization China

Author: Wang Qingkui and Zhao Xuechao (Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang)

Producer: China Science Expo

Microorganisms are everywhere in nature, and soil is their home base and the natural "culture medium" for their growth and reproduction. Soil microorganisms are a general term for all tiny organisms in the soil that are difficult to see with our naked eyes, mainly including bacteria, fungi, actinomycetes and algae.

They are important components of soil, but they are tiny, usually measured in micrometers or nanometers. The number and variety of microorganisms in soil are amazing. There can be hundreds of millions to tens of billions of them in 1 gram of soil, and there are thousands or even tens of thousands of species. Bacteria are the most numerous, followed by actinomycetes and fungi, and algae are less numerous. Their number and variety in soil will constantly change according to the soil environment.

Figure 1 Microorganisms under a microscope

(Photo source: Veer Gallery)

Soil microbes’ recipes

Soil microorganisms are very important for soil formation and development, material circulation and plant growth. It can be said that they are the basis for soil to produce products, purify water and become the most effective carbon dioxide storage site.

Like us humans, microorganisms also need to constantly obtain food from the external environment in order to survive and maintain a healthy body. Our human food is called by the name of the food, such as rice, tomatoes, beef, etc., while the food of microorganisms is mainly classified according to the physiological role in their bodies, such as carbon source, nitrogen source, growth factor, water, etc.

The carbon sources that microorganisms often consume mainly include sugars, organic acids, carbon dioxide, proteins, etc., while the nitrogen sources that can be used by them mainly include proteins and their degradation products, ammonium salts, nitrates, molecular nitrogen, urea, amines, amides, etc.

In addition, microorganisms also need to consume vitamins, amino acids, purines and pyrimidines to meet their own growth needs. Inorganic salts are also substances required for microbial growth, generally phosphates, sulfates, and compounds containing sodium, potassium, calcium, magnesium, iron and other elements. Water is also a key link in the growth of microorganisms, mainly acting as a solvent and transport medium. Not only that, water can also participate in a series of chemical reactions in its cells.

Soil microbial residues: an important component of forest soil carbon pool

The forest soil carbon pool refers to the carbon-containing organic matter present in the forest, and is the most important carbon pool in the terrestrial ecosystem. Trees absorb carbon dioxide from the atmosphere through photosynthesis to maintain their own growth, and produce some dead branches and leaves and dead roots. The dead branches and leaves, dead wood, stumps and dead roots we see in daily life are all sources of forest soil carbon pools. Soil microorganisms use these organic matter as food to meet their own growth and reproduction.

The life of soil microorganisms is very short, generally from tens of minutes to hours or days. They have strong reproductive capacity and can multiply several times or even dozens of times in a short period of time. After the death of microorganisms, some of the substances in their bodies will be absorbed and used by other microorganisms, while the remaining difficult-to-decompose substances will remain in the soil. We call these substances retained in the soil soil microbial residues, which can usually remain in the soil for a long time and are an important component of the forest soil organic carbon pool.

Figure 2 Common microorganisms in forest ecosystems

(Image source: Reference 1)

What is the difference between bacterial residues and fungal residues?

Soil microbial residues mainly include bacterial residues and fungal residues, which are both important components of forest soil carbon pools, but they are different. First, the "food" that bacteria and fungi eat is not exactly the same, and there are also differences in their ability to absorb food. Because bacteria have weak enzyme systems, they consume more energy to "digest" these foods, and more carbon is emitted in the form of respiration, while relatively less carbon is used to synthesize biomass. On the contrary, fungi can produce more biomass because of their "strong digestion ability, good absorption, and high utilization rate of food."

In addition, after the death of soil microorganisms, the cytoplasm will be reused by other microorganisms, and the cell wall will gradually form microbial residues after turnover. However, the differences in the composition of the cell wall between bacteria and fungi will also affect the contribution of their microbial residues to the soil carbon pool. The bacterial cell wall is mainly composed of relatively easy-to-decompose substances such as peptidoglycan, while the fungal cell wall is mainly composed of difficult-to-decompose substances such as chitin and melanin.

Therefore, compared with bacterial residues, fungal residues are less edible and most microorganisms do not like to eat them. Over time, the contribution of fungal residues to the organic carbon pool in forest soils will be higher than that of bacterial residues.

Microorganism: It’s so cold, I don’t want to work!

my country has a vast territory, with great climate differences between the north and the south, and rich forest types. Forests in cold regions are dominated by conifers, with a relatively short growing season. Dead branches and fallen leaves contain many difficult-to-decompose substances that microorganisms do not like to eat, which cannot satisfy the "appetite" of soil microorganisms.

At the same time, like us humans, microorganisms are also very "delicate". Too hot or too cold is not conducive to their activities. The cold climate causes soil microorganisms to enter a "hibernation" state during the long winter, and organic matter that is not fully utilized by microorganisms remains in the soil, ultimately making the contribution of soil microbial residues to the soil organic carbon pool in cold forests lower than that in warm forests.

Figure 3 Schematic diagram of the dynamic changes of forest soil microbial residues

(Image source: Reference 2)

Forests are the largest carbon pool in terrestrial ecosystems and play a vital role in maintaining soil fertility, ensuring food security and regulating climate change. Soil microbial residues contribute almost 50% to forest organic carbon pools.

Therefore, promoting the formation of soil microbial residues is beneficial to improving the carbon sequestration capacity of forests and enhancing the carbon sink function of forest ecosystems. Therefore, the accumulation year after year makes the contribution of soil microbial residues to the forest organic carbon pool not to be ignored. Don't underestimate these little guys!

Editor: Sun Chenyu

References:

1.Peay, KG, Kennedy, PG, Talbot, JM, 2016. Dimensions of biodiversity in the Earth mycobiome. Nature Reviews Microbiology 14, 434-447.

2.Zhao,