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More bad news – new model shows global warming accelerates CO2 emissions from soil microbes.

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The increase in atmospheric carbon dioxide (CO2) concentration is a major factor in global warming, and about one fifth of atmospheric CO2 comes from soil sources. This is due in part to the activity of microorganisms, including bacteria, fungi, and other microorganisms that decompose organic matter in the soil using oxygen, such as dead plant materials. This process releases CO2 into the atmosphere, which scientists refer to as heterotrophic soil respiration.

According to a recent study published in the scientific journal Nature Communications, a team of researchers from ETH Zurich, the Swiss Federal Institute for Forest, Snow and Landscape Research WSL, the Swiss Federal Institute of Aquatic Science and Technology Eawag, and the University of Lausanne has reached a significant conclusion. Their study indicates that emissions of CO2 by soil microbes into the Earth’s atmosphere are not only expected to increase, but also accelerate on a global scale by the end of this century.

Using a projection, they found that by 2100, CO2 emissions from soil microbes will escalate, potentially reaching an increase of up to about forty percent globally, compared to current levels, under the worst-case climate scenario. According to Alon Nissan, the main author of the study, this is more bad news.

“Thus, the projected rise in microbial CO2 emissions will further contribute to the aggravation of global warming, emphasizing the urgent need to obtain more accurate estimates of the heterotrophic respiration rates.”

Soil moisture and temperature as key factors

These findings not only confirm earlier studies but also provide more precise insights into the mechanisms and magnitude of heterotrophic soil respiration across different climatic zones. Unlike other models that rely on numerous parameters, the novel mathematical model developed by Alon Nissan simplifies the estimation process by utilizing only two crucial environmental factors: soil moisture and soil temperature.

The model represents a significant advancement as it encompasses all biophysically relevant levels, ranging from the micro-scales of soil structure and soil water distribution to plant communities like forests, entire ecosystems, climatic zones, and even the global scale. Peter Molnar, a professor at the ETH Institute of Environmental Engineering, highlights the significance of this theoretical model which complements large Earth System models, stating,

“The model allows for a more straightforward estimation of microbial respiration rates based on soil moisture and soil temperature. Moreover, it enhances our understanding of how heterotrophic respiration in diverse climate regions contributes to global warming.”

Polar CO2 emissions likely to more than double

Peter Molnar and Alon Nissan’s research collaboration has identified a key finding: the increase in microbial CO2 emissions varies across climate zones. In cold polar regions, the main contributor to the increase is the decline in soil moisture, rather than a significant rise in temperature, unlike in hot and temperate zones. Alon Nissan emphasizes the sensitivity of cold zones, stating,

“Even a slight change in water content can lead to a substantial alteration in the respiration rate in the polar regions.”

According to their calculations, under the worst-case climate scenario, microbial CO2 emissions in polar regions are projected to increase by ten percent per decade by 2100, which is twice the rate anticipated for the rest of the world. This disparity is due to the optimal conditions for heterotrophic respiration, which occur when soils are in a semi-saturated state, i.e. neither too dry nor too wet. These conditions are prevalent during soil thawing in polar regions.

On the other hand, soils in other climate zones, which are already relatively drier and more prone to desiccation, exhibit a comparatively smaller increase in microbial CO2 emissions. However, regardless of the climate zone, the influence of temperature remains consistent: as soil temperature rises, so does the emission of microbial CO2.

How much CO2 emissions will increase by each climate zone

As of 2021, most CO2 emissions from soil microbes originate primarily in warm regions of the Earth. Specifically, 67 percent of these emissions come from the tropics, 23 percent from the subtropics, 10 percent from the temperate zones, and a mere 0.1 percent from the arctic or polar regions.

The researchers anticipate substantial growth in microbial CO2 emissions across all regions by the year 2100 compared to levels observed in 2021. Their projections indicate an increase of 119 percent in the polar regions, 38 percent in the tropics, 40 percent in the subtropics, and 48 percent in the temperate zones.

Will soils be a CO2 sink or a CO2 source for the atmosphere?

The carbon balance of soils, determining whether they act as a source or sink of carbon, depends on the interplay between two crucial processes: photosynthesis, whereby plants assimilate CO2, and respiration, which releases CO2. Therefore, studying microbial CO2 emissions is essential for understanding whether soils will store or release CO2 in the future.

Alon Nissan, explained:

“Due to climate change, the magnitude of these carbon fluxes—both the inflow through photosynthesis and the outflow through respiration—remains uncertain. However, this magnitude will impact the current role of soils as carbon sinks.”

The researchers have primarily focused on heterotrophic respiration in their ongoing study. However, they have yet to investigate the CO2 emissions that plants release through autotrophic respiration. Further exploration of these factors will provide a more comprehensive understanding of the carbon dynamics within soil ecosystems.

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