Forests: A free "nitrogen meal"? Too expensive to afford

Produced by: Science Popularization China

Author: Zheng Mianhai (South China Botanical Garden, Chinese Academy of Sciences)

Producer: China Science Expo

The human body needs to constantly replenish nutrients, and in nature, animals and plants also need to constantly replenish nutrients.

Similar to humans, in the big family of forests, plants and animals need many nutrients, such as nitrogen, phosphorus, potassium, calcium, sodium, magnesium, iron, etc. Among them, nitrogen is the most basic and very important element. This is because without nitrogen, there is no protein, and without protein, there is no life.

The protein content in plants can reach up to 80-85%, while the nitrogen content in proteins accounts for an average of 16-18%. In the forest, plants are an important source of nutrition for many animals. So what is the source of nutrition for plants? To go further, where does the nitrogen in plants come from?

How do forest plants “eat” nitrogen?

The plants in the forest seem to have no hands or feet, and cannot seem to "move", so how do they obtain nutrients such as nitrogen?

Let's first look at where nitrogen is distributed on Earth. In nature, most nitrogen exists in the air in the form of gas (nitrogen), which accounts for 78% of the volume of air. With such a high nitrogen content in the air, does it mean that plants can easily absorb nitrogen by just opening the stomata on their leaves?

In fact, nitrogen cannot be directly used by organisms. Even if it is inhaled, it will be excreted along the same route because nitrogen cannot be directly "digested" by plants.

Nitrogen in the air

(Photo source: Veer Gallery)

So what exactly do plants need to do?

In fact, plants have developed a variety of ways to obtain nitrogen during the long process of evolution.

The first type is specialized plants doing specialized things . There is a type of plant that specializes in obtaining nitrogen, called nodulation nitrogen-fixing plants . Unlike other types of plants, this type of plant can cooperate symbiotically with nitrogen-fixing bacteria to form "nodules" with high nitrogen-fixing capabilities from the roots. In this way, nitrogen in the air can be fixed in the form of nitrogen fixation by the nodules, and then further converted into "ammonia" that can be digested by the plant.

Because of this basis for cooperation between plants and nitrogen-fixing bacteria, plants only need to "please" the nitrogen-fixing bacteria in the nodules (for example, provide them with some carbohydrate food) to exchange the nitrogen they want from the nitrogen-fixing bacteria.

Nodules of nitrogen-fixing plants

(Photo source: Veer Gallery)

The second type is specialized microorganisms that do more. In nature, if only nodulated plants can fix nitrogen, wouldn’t that mean that the nitrogen supply market is “monopolized” by them? In fact, there is another type of nitrogen-fixing bacteria in the forest, called autotrophic nitrogen-fixing bacteria . As the name suggests, they do not choose to cooperate with plants, but come out to fight on their own (survival of the fittest). The nitrogen retained in the remains formed after their death can be decomposed by microorganisms and further used by plants.

Self-generating nitrogen-fixing bacteria are widely distributed in every corner of the forest, including soil, fallen leaves, dead branches, rotten wood, epiphytes, and plant leaves, etc. The forest provides them with a variety of habitats to meet their needs, and their appearance also benefits the nitrogen needs of many plants in the forest.

Microbial nitrogen fixation

(Image source: self-made by the author)

The third is that humans provide a free "free meal". Factories and enterprises built by humans, as well as the exhaust gas discharged into the air by transportation, all contain a large number of nitrogen-containing active compounds. These nitrogen-containing compounds can float in the air with the wind, and finally enter the forest by directly falling (dry deposition) or dissolving in rainwater and then falling (wet deposition), and are absorbed and used by plants. Therefore, humans provide a free "meal" for forest plants.

Atmospheric nitrogen pollution caused by human activities

(Photo source: Veer Gallery)

On the surface: free "nitrogen meal", in reality: huge cost

However, the free "feast" provided by humans to forests seems to have disrupted the "market order" of forests.

Both nitrogen-fixing plants and nitrogen-fixing bacteria consume a lot of energy to fix nitrogen. In contrast, with this "pie" falling from the sky - nitrogen-containing compounds falling from the air, why wouldn't the forest be happy? Gradually, nodulated nitrogen-fixing plants stopped nodulating, and nitrogen-fixing bacteria seemed to no longer want to fix nitrogen. They all tended to take this unearned nitrogen from the air.

Nitrogen supply method

(Image source: self-made by the author)

In fact, is there a real "free lunch" in nature? The answer is no. When the forest lacks nitrogen (malnutrition), it is the instinctive reaction of organisms to obtain nitrogen. However, when the nitrogen in the forest is already abundant (overnutrition), it will be counterproductive.

The "pie" of nitrogen from air deposition simply cannot obey orders. Over time, more and more nitrogen will fall into the forest, and eventually plants may "die" because of too much nitrogen, or become "thin" because other nutrients (phosphorus, potassium, calcium, etc.) cannot keep up (imbalance in the proportion of various nutrients). Therefore, too much nitrogen supply is not a good thing, and free things also come at a price.

In fact, not all nitrogen-fixing plants and bacteria in the forest have the same idea. After all, different choices require different costs. Therefore, there will still be a considerable number of nitrogen-fixing plants and bacteria who choose to be "self-reliant" and "earn" as much as they need.

In the forest, some "smart" (or "lazy") plants and microorganisms directly use the nitrogen that settles from the air to supplement their nutrients, but there are also some not-so-"smart" (or "hardworking") nitrogen-fixing bacteria that consume energy to fix nitrogen.

In fact, we found that these seemingly not very "smart" nitrogen-fixing microorganisms are not "stupid". They cleverly use the low-cost energy in the forest (for example, carbon dissolved in fallen leaves and soil organic matter) to fix nitrogen. This "using carbon to promote nitrogen" method saves their own energy costs. This is also a true portrayal of the "battle of wits and courage" in the forest society.

Even so, it is undeniable that the waste gas emitted by human production and life has already affected the normal order of the forest.

We further discovered that nitrogen that settles into the forest from the air first accumulates in plants and soil. Nitrification of nitrogen in the soil releases more hydrogen ions (H+), leading to soil acidification. Acidification is not conducive to the growth of plants and microorganisms. The respiration of organisms is weakened, leading to the death of plant fine roots and microorganisms.

At the same time, the soil will also release more nitrogen-containing greenhouse gases (such as N2O), exacerbating climate warming. Therefore, if the waste gas produced by human beings is not controlled, many forests will have an excess of nitrogen in the future, which may cause an environmental crisis.

Forest degradation

(Source: Veer Gallery)

Protecting the forest home shared by mankind

Forests are not only the home of animals and plants, but also the environment for us humans to survive. Although we do not want forest plants to be "malnourished", we should also respect the laws of nature. In the short term, nitrogen-containing waste gas emitted by human activities may be beneficial to plant growth, but in the long term, the harm caused will be immeasurable. For example, it will lead to a decrease in plant diversity, deterioration of the forest environment, and an intensification of the greenhouse effect. These will eventually affect human life and health.

At present, human society is facing the threat of climate change. Achieving the "dual carbon" goal (carbon neutrality and carbon peak) is both a national strategy and a social need. Forests are important ecosystems for increasing carbon sinks (absorbing greenhouse gases from the atmosphere). In the future, scientists will further study the mechanism of enhancing forest carbon sink capacity, and will also explore more carbon sequestration technologies based on biological pathways.

For us, it is our responsibility to further enhance our awareness of environmental protection, respect the laws of nature, plant and protect the flowers, plants and trees around us in a scientific and rational manner, and protect our shared forest home!