“We’re just mayflies amid the universe or mere specks of millet in the boundless sea,” as Su Shi, an ancient Chinese poet and statesman, has observed. Compared with the Earth with a diameter of 12,742 kilometers, how small is the scale and means of human observation? If you only rely on human observation, even if you spend your whole life, it would be difficult to thoroughly investigate one corner of the Earth. For this reason, in the field of Earth science, it is particularly important to deeply combine Earth science with computer science with the help of advanced research methods such as information technology and artificial intelligence technology.

A study published not long ago vividly demonstrates the practice of this combination.

An international research team organized by Professor Huang Xiaomeng from the Department of Earth System Science (DESS), Tsinghua University, Class 2018 doctoral student Tao Feng and Professor Luo Yiqi from Cornell University in the United States has revealed the decisive role of microbial carbon use efficiency (CUE) on global soil organic carbon (SOC) storage through in-depth interdisciplinary research. On May 24, the paper was published online in Nature titled “Microbial carbon use efficiency promotes global soil carbon storage”.

(Subhead) “Tiny guys” that cannot be ignored

The Earth system refers to an organic whole composed of atmosphere, hydrosphere, terrestrial sphere and biosphere. The Earth system science, which originated from traditional Earth science, is a discipline studying this organic whole. The research related to carbon, the main factor of global climate change, is one of the “main jobs” of Earth system science.

Carbon pool, that is, the carbon storage pool on the Earth, can be divided into atmospheric carbon pool, marine carbon pool, lithospheric carbon pool, terrestrial ecosystem carbon pool and other parts. “Among them, SOC is one of the most important components of the terrestrial ecosystem carbon pool, and its carbon content is about four times that of terrestrial vegetation organic carbon,” said Huang Xiaomeng, co-author of the paper, “SOC plays a key role in the global carbon cycle. Its carbon storage capacity is so powerful that we can offset the total carbon emissions of mankind in one year by increasing the carbon storage capacity just by four thousandths.”

The role of SOC is so important, but how is it formed and stored stably in the soil? It is obviously impossible for inanimate things such as rocks and minerals to become the source of soil organic matter. The weight of animals in the biosphere is so extremely low that they are by no means the “first contributors” of organic carbon storage. Therefore, the traditional scientific research paradigm takes plants such as trees, shrubs and grasses as the biggest “heroes” in the process of SOC storage.

According to the introduction of Tao Feng, the first author of the paper, the traditional research paradigm takes plant transport-organic matter degradation as the core, and divides the storage of SOC into several processes such as photosynthesis input carbon distribution and soil respiration. “However, in recent years, more and more evidence shows that the process model established according to the traditional paradigm cannot completely and accurately simulate the total amount of SOC,” said Tao Feng, “Obviously, people have overlooked something, ignoring some ‘little guys’ invisible to the naked eye.”

The “little guys” in Tao Feng's words are soil microorganisms. Although microorganisms seem to be negligible on the macro scale, they are a “new force” that cannot be ignored in the carbon cycle process. “In recent years, with the evolution of the scientific research paradigm, it has become a mainstream view to take soil micro-process into account,” said Huang Xiaomeng.

(Subhead) Major factors that promote change

Like animals and plants, tiny soil microorganisms are not aloof from the world, but they also have metabolism, and will experience birth, death and illness. Therefore, soil microorganisms are both consumers of and contributors to SOC. A question arises as to whether soil microorganisms promote carbon sequestration, that is, contribute more SOC, or lead to carbon emission, that is, consume more SOC. “This is a big problem that we have to figure out,” said Tao Feng.

According to Huang Xiaomeng, microbial carbon use efficiency (CUE), which refers to the ratio of microbial biosynthesis carbon to total microbial metabolism carbon, is an important parameter to describe soil microbial physiology. Despite the fact that the higher the efficiency of microbial carbon use is, the better the biosynthesis of soil microorganisms is, it is still unclear whether more biosynthesis can eventually be transformed into higher SOC reserves. “There are two control paths with completely opposite results. High microbial synthesis may not only mean that more organic products and microbial residues are finally preserved in the soil, but also promote the production of microbial extracellular enzymes, which will catalyze the degradation of soil organic matter and lead to the loss of soil carbon,” said Huang Xiaomeng.

In this study, the research team first selected microbial CUE as a variable, combined the mechanism model describing complex soil carbon cycle with more than 50,000 soil carbon observation data, and determined the most possible control path of microbial process to SOC storage under the Bayesian framework. “This step is mainly to find out whether microorganisms are sequestrating or discharging carbon,” explained Tao Feng, “The results show that microbial CUE is positively correlated to SOC storage. In other words, microorganisms do help to promote carbon sequestration rather than carbon emission. The higher carbon distribution ratio of microorganisms to biosynthesis in micro-metabolism is finally transformed into SOC reserves with higher macro-scale.”

“It’s not enough to just arrive at this step. We want to develop our research to a deeper level,” said Huang Xiaomeng. Based on the self-developed PROcess-Guided Deep Learning and DAta-Driven Modelling (PRODA), the research team extended the site-scale data model fusion results to the global scale through the advanced computing power of artificial intelligence technology and the ability of matrix model construction, obtained the spatial distribution pattern of seven mechanisms including microbial CUE, plant carbon transport efficiency and non-microbial carbon transfer efficiency, and quantitatively evaluated their relative contributions to the global SOC storage and spatial distribution.

“Comparative research and comprehensive evaluation show that compared with other processes, microbial CUE has the most critical impact on SOC storage, which is at least four times higher than other processes such as plant carbon input,” said Tao Feng, “This study shows that small microorganisms are a big factor in promoting SOC storage!”

In the view of the research team, the conclusion of this study does not deny the importance of traditional carbon sequestration methods such as afforestation for SOC storage, but it points out a more efficient new path for future scientific research and environmental protection. Even so, the research team also admitted that it still needs more theoretical and practical research to regulate microbial CUE on a large scale, and with high efficiency and low cost under the current technical level.

“Imagine that if we can take measures to effectively improve the global microbial CUE, then the soil can store more organic carbon, so that a series of vicious climate changes caused by the rising atmospheric carbon dioxide concentration, such as melting ice, mountain wildfires and abnormal low temperatures, can be curbed,” said Huang Xiaomeng, “This study shows that in the future research on soil carbon cycle more attention should be paid to microorganisms.”

Science and Technology Daily, Li Zhaoyu