The dead branches and leaves under your feet actually hide the big secret of global change!

Produced by: Science Popularization China

Author: Zhang Yunyu and Sun Tao (Institute of Applied Ecology, Chinese Academy of Sciences)

Producer: China Science Expo

Editor's note: In order to unveil the mystery of scientific work, the China Science Popularization Frontier Science Project launched a series of articles called "Me and My Research", inviting scientists to write articles themselves, share their scientific research experiences, and create a scientific world. Let us follow the explorers at the forefront of science and technology and embark on a journey full of passion, challenges, and surprises.

The sky and earth were dark and the universe was vast. About 3.8 billion years ago, the first life was born on Earth. After billions of years, about 2.4 million years ago, Homo erectus began to try to use stone tools, thus humans appeared.

Looking back at the vast history of life is also a bird's-eye view of the history of life and the environment. After knowing the birth, evolution and destruction of millions of creatures, we can't help but think: What is the environment in which we live? What is nature? What is our relationship with nature? How should we get along with nature?

Winter snow scene in Daxing'anling, Heilongjiang Province (cold temperate forest)

(Photo source: veer photo gallery)

The study of the relationship between man and nature has existed since ancient times, and people's long observation of wind, frost, rain, snow, sunrise and moonset gave birth to the well-known "Song of the Twenty-Four Solar Terms". However, the climate is not static, especially today when global changes are becoming increasingly significant.

Since the Industrial Revolution in the 18th century, many man-made problems, such as the massive use of nitrogen fertilizers, waste pollution, increased greenhouse gases and soil pollution, have greatly affected the environment we live in. "How to protect the nature on which we depend in the context of global change" is the problem that our generation needs to face.

Decomposition of litter

Light energy from the sun and carbon dioxide (CO₂) in the air are converted into starch by photosynthesis in plant leaves, and energy and matter enter the ecosystem from then on; when autumn comes, trees shake off dead branches and leaves (litter), which become food for soil organisms and are decomposed by animals, fungi and bacteria in the soil. Energy is gradually lost, part of the matter continues to circulate, and part of the matter is sealed underground.

In a sense, decomposition is the "reverse process" of photosynthesis. Without decomposition, our planet would be filled with various biological remains, and energy flow and material circulation would stagnate. If the rate of decomposition increases, the carbon stored in the soil will be released in the form of CO2, exacerbating the greenhouse effect and global warming.

Oceans, rivers, grasslands, deserts, swamps... Among the complex ecosystems on Earth, forests are the most complex, complete and "resilient" ecosystems. The temperate forest soils in the northern hemisphere store a large amount of carbon, which is a huge carbon pool that cannot be ignored.

Colorful dead leaves and branches

(Photo source: veer photo gallery)

After many long-term litter decomposition experiments, the academic community now widely believes that climate, decomposers and litter quality are the main regulators of litter decomposition rate, and on this basis proposed the "litter decomposition triangle" to explain the decomposition mechanism of litter.

Among climate factors, temperature and precipitation are the main drivers of decomposition at the global level. Decomposers include bacteria, fungi and soil animals. The more active the decomposer, the faster the decomposition rate it leads. Litter quality includes more content, which can be roughly divided into the chemical properties of litter (the type and content of chemical substances) and structural properties (such as leaf shape).

Among them, nitrogen content is a very important indicator. Because decomposers in the forest mainly obtain the energy and nitrogen necessary for survival by decomposing litter, people have long believed that "the higher the nitrogen content of litter, the faster the decomposition rate." This conclusion is mostly based on short-term experiments, but litter decomposition is a slow process that lasts for several years to decades.

At the same time, in the current global change, due to the burning of fossil fuels and excessive use of fertilizers, a lot of nitrogen elements enter the ecosystem, which accelerates the earth's nitrogen deposition process and may also affect the nitrogen content of plants and the quality of litter, thereby affecting the rate of decomposition.

A 10-year litter decomposition experiment in a temperate forest

Different tree species have different nitrogen contents in litter, so researchers from our Shenyang Institute of Applied Ecology collected litter from 62 woody plants in China's temperate forests and conducted a 10-year field decomposition experiment. In the first five years of the experiment, litter with higher nitrogen concentrations decomposed faster than litter with lower nitrogen concentrations.

Fallen leaves

(Photo source: veer photo gallery)

However, starting from the sixth year, this pattern was reversed, and the decomposition rate of high-nitrogen litter gradually decreased. At the end of decomposition, the average litter residue of high-nitrogen tree species was 1.78 times that of low-nitrogen tree species. This shows that the decomposition degree of litter with lower nitrogen content is greater.

The residues of decomposing microorganisms can be expressed as amino sugars. In order to explore the possible reasons for the above results and the activities of decomposers in this process, we used isotopes to label the amino sugars in the litter in this experiment. Starting from the sixth year, the residual mass value of the litter was positively correlated with the amino sugar concentration.

The above two results can describe what happened during the experiment: litter with high nitrogen content attracted many microbial decomposers, so it decomposed quickly in the early stage; as the decomposition progressed, many microbial products and residues accumulated on the high nitrogen litter, where they were tightly bound and inhibited further decomposition; therefore, the experimental process was reversed, and the decomposition of litter with high nitrogen content was more incomplete compared to litter with low nitrogen content.

Does the above rule also apply to the same temperate forest tree species?

We selected two tree species with very different leaf masses (Acer truncatum and Quercus mongolica) and conducted a 6-year foliar nitrogen application experiment. During the 6-year decomposition period of both tree species, leaves treated with nitrogen enrichment decomposed faster; however, after 3 years, the decomposition rate of these leaves began to slow down. This suggests that the relationship between nitrogen concentration and decomposition can also reverse over time within the same temperate tree species.

Acer pictum subsp. Mono

(Image source: China Plant Image Library)

Mongolian Oak (Quercus mongolica)

(Image source: China Plant Image Library)

However, all the above experiments were conducted in my country. So, is this rule also effective in temperate forests around the world?

Temperate forest in Corcovado National Park, Chile

(Photo source: veer photo gallery)

We collected 32 published long-term decomposition experiments and established a database of 437 measurements from 120 species, testing for the first time whether there is a general relationship between litter nitrogen content and litter residual mass in both boreal and temperate forests.

The range of nitrogen concentrations in litter in the database varies by about 11 times, but the results show that the nitrogen content of litter in different tree species is still positively correlated with the residual mass of litter. This means that in boreal and temperate forests around the world, the higher the nitrogen content of litter, the greater the residual mass of litter produced by decomposition.

Relationship between litter nitrogen content and decomposition rate may be reversed

In the modern era of information revolution, scientists use various models to make predictions, such as daily weather forecasts, the possible impact of urban development on land, agricultural and animal husbandry management, climate simulation, and even simulation of the earth itself... But models are not completely accurate. For a model to be put into use, a framework that conforms to scientific facts, perfect parameter settings, and sufficient data are essential.

In many existing ecosystem models and Earth system models, the relationship between litter nitrogen content and decomposition rate is often determined as a positive proportional relationship. However, our research results show that this positive proportional relationship will be reversed over time.

Eco-friendly world, green world concept

(Photo source: veer photo gallery)

We may need to modify parameters to include the staged effects of litter nitrogen content on decomposition rates, as predicting long-term decomposition dynamics from factors affecting early decomposition may be misleading. The contrasting effects of litter matrix nitrogen content on different decomposition stages will be critical to understanding and improving predictions of future terrestrial ecosystem carbon cycling in the face of increasing atmospheric nitrogen deposition, changing climate conditions, and other environmental changes.

Some people may raise questions. Similar to carbon, we know that nitrogen exists in organic and inorganic forms in nature. So what kind of experiment can simulate the "real" nitrogen deposition? Does the difference between organic and inorganic nitrogen affect the results? How many pathways does nitrogen have in the decomposition process? Does it interact with other elements?

As far as we can see, we can only see a short distance ahead. But we can still see that the lonely truth is waiting for those who are determined behind the fog.

References:

[1] MA Bradford, B. Berg, DS Maynard, WR Wieder, SA Wood, Understanding the dominant controls on litter decomposition [J]. The Journal of Ecology, 2016, 104, 229-238.

[2] AM Turing. Computing Machinery and Intelligence [J]. Mind, 1950, 49: 433-460.