Do you know about the "Man Jiang Hong Incident" which is more exciting than the drama?

Recently, a movie with the same name based on the Song Dynasty poem "Man Jiang Hong" is being released. Speaking of "Man Jiang Hong", I believe everyone must be familiar with it. The one that impressed me the most is probably "Man Jiang Hong: Nu Fa Chong Guan" which I had memorized by heart in elementary school. After many years, most of the content may have been forgotten, but I can definitely remember the passion in my heart when I recited this poem and the indignation when I learned the history behind it.

As an important ci tune in the Chinese poetry system, there are different opinions about the origin of the ci tune "Man Jiang Hong". One is that it comes from the line "The sunrise over the river is redder than fire" in Bai Juyi's "Remembering Jiangnan"; another is that it comes from the song title "Shang Jiang Hong" in Zhu Qingyu's Tang Dynasty strange story novel "Ming Yin Lu", which is obtained by transposing the two characters "Shang" and "Hong"; but there is another theory that the name of the ci tune comes from a kind of water grass, also named Man Jiang Hong.

This aquatic plant is mostly distributed in rice fields, ponds, lakes and other waters in the southern region. It is a very common aquatic plant. Such an inconspicuous aquatic plant has such a domineering name. What is the secret behind it? Let us learn about it together.

Source: Wikipedia

1

What is Azolla?

Azolla (Azolla), also known as red duckweed or green duckweed, belongs to the Salviniales, Azollaceae, and is a floating water fern.

Floating plants refer to plants whose leaves float on the water surface and whose roots are suspended in the water or grow underwater. Their leaves floating on the water surface can absorb carbon dioxide in the air very well for photosynthesis, while their underwater roots can directly absorb nutrients from the water . This way of nutrient intake allows them to grow rapidly.

Source: China Plant Image Database, photo by Bai Chongyan

Normally, the water lily is green, and most people cannot distinguish it from duckweed at first glance. However, if you look closely, you will find that it has small round leaves distributed in feathery shapes. In autumn and winter, the leaves of the water lily will synthesize a large amount of anthocyanins, making the leaves covering the water surface appear bright red, hence the name of the water lily.

Source: China Plant Image Library, photo by Zhou Jianjun

Azolla is one of the fastest growing plants, and its biomass can double in just 2 to 3 days. This is due to the symbiotic relationship formed between Azolla and Anabaena in its leaves. Anabaena has efficient nitrogen fixation ability, which can concentrate and store nitrogen in the atmosphere. Azolla thrives with the help of this nitrogen, and then absorbs large amounts of carbon dioxide with its advantage in numbers. In this way, Azolla can achieve efficient carbon and nitrogen accumulation.

Azolla grows fast, but its vitality is not too strong. It has certain requirements for the growth environment. It is difficult to grow in low temperature or high temperature environment. It can only grow at the fastest rate when the water level is suitable (above 3 cm) and the temperature is suitable (20~25 degrees Celsius). Otherwise, it will soon wither and die.

In comparison, duckweed, which is also a floating plant, has much stronger vitality. Not only can it convert the nutrients in its body into starch and store it when encountering low temperature environments, but it can also temporarily insert its roots into the soil to hibernate during droughts, and will resume growth when the environment is suitable.

Duckweed Source丨Wikipedia

2

The role of Azolla in agriculture and ecology

Azolla plays an important role in agricultural development.

First of all, it is a high-quality natural green manure, especially suitable for rice cultivation. Growing duckweed in rice fields to increase soil fertility has a long history. There are records of growing duckweed in rice fields to feed pigs in "Wang Zhen's Book of Agriculture" in the Yuan Dynasty in my country. Farmers will sow Azolla in irrigated rice fields and let it grow with the seedlings. Spreading Azolla on the water surface can inhibit the growth of weeds. After the rice is mature, farmers will drain the water in the fields, and the Azolla will soon die. The accumulated nitrogen will be released into the soil, increasing soil fertility.

Amaranthus in the rice fields

Source: geolsoc.org.uk

Beijing Agricultural University once conducted a rice planting experiment using Azolla as base fertilizer. Applying 3,000 kg of fertilizer per mu increased the yield by 35% in the current season and 5-10% in the next season. The soil structure was significantly improved and the nitrogen content was significantly increased.

Secondly, Azolla is also a cheap and high-quality feed. It contains a large amount of nutrients and amino acids needed by livestock. In addition, it grows rapidly. As long as the indigestible lignin contained in it can be removed, Azolla can be used as a clean, safe and low-cost feed.

Source: pexels

3

The Man Jiang Hong incident

In addition to nitrogen fixation, the extremely fast growth rate also gives Azolla a strong carbon fixation ability .

We are used to the fact that the Arctic is always covered with ice and snow, so it is probably hard for us to believe that a long time ago the Arctic was a large lush jungle. Moreover, the most critical process of the Arctic changing from forest to snowfield only occurred in a short period of time, almost overnight on a geological scale. This critical process is related to the ability of Azolla to fix carbon.

During the Eocene Epoch about 50 million years ago, the Earth's carbon dioxide concentration was astonishingly high, reaching 2500~3500 ppm, much higher than the current 400 ppm. The resulting greenhouse effect made the entire Earth very hot. This event was called the Paleocene-Eocene Thermal Maximum (PETM).

At that time, the Arctic Ocean was surrounded by mountains. The fresh water injected by the rivers that intersected it formed a water layer on the surface of the Arctic Ocean, with fresh water in the upper layer. The Azolla multiplied in the desalinated Arctic Ocean. The vast waters, suitable temperatures and high concentrations of carbon dioxide provided excellent conditions for the growth of the Azolla, allowing it to grow almost exponentially and soon cover a large area of ​​water. When winter came, they would wither and sink to the bottom of the sea. Due to the lack of microorganisms that could decompose them, the large amount of carbon source they gathered also sank to the bottom of the sea. When the temperature was right, the Azolla that survived on the sea surface began to multiply in large numbers again.

The plant morphology in the fossil is the same as that of modern Azolla

Source: Dr Ian Miller of the Denver Museum of Science

This cycle lasted for nearly 800,000 years until the Fram Strait opened and the seawater from the nearby ocean was injected, and the Azolla gradually disappeared from the surface of the Arctic Ocean. During this period , the carbon dioxide absorbed by the Azolla reduced the carbon dioxide concentration of the entire earth by nearly half, and the Arctic also accelerated its pace of becoming a world of ice and snow. This magical geological event is called the Azolla event , which is an important discovery in geology. The event shows that the Azolla played an important role in the formation of the current ecological environment of the earth.

Impact of the Azolla event on climate

Source: theazollafoundation.org

To sum up, we can see that Azolla, which has high efficiency in carbon and nitrogen fixation, can not only be used as high-quality green fertilizer, but can also be developed into cheap and safe livestock feed. In addition, from the perspective of its role in shaping the earth's current ecological environment, as long as it is used reasonably, Azolla can also be used as a carbon sink to cool down the earth which is now starting to "heat up" again.

As the concept of green development gradually gains popularity, organisms with excellent ecological value such as Azolla will surely play an important role in building a green world in the future, making such a world a reality as soon as possible.