The scanning electron microscope image shows a colony of bacteria (coloured bluish) around a few micrometres small root (brown) in the soil. The image represents how bacteria and microbial communities influence CO2 release from the soil through heterotrophic respiration: The soil contains soil grains to which microbial communities adhere and process dissolved oxygen O2 and organic carbon DOC from organic matter (OM).
Newswise — Heterotrophic soil respiration is a significant contributor to the increase in atmospheric carbon dioxide (CO2) concentration, which is a major driver of global warming. Roughly one fifth of the CO2 present in the atmosphere originates from soil sources. This can be attributed, in part, to the activity of various microorganisms like bacteria, fungi, and other decomposers that thrive in the soil. These microorganisms break down organic matter, including deceased plant materials, using oxygen. As a result of this process, CO2 is released into the atmosphere. Scientists commonly refer to this phenomenon as heterotrophic soil respiration.
According to a recent publication in the scientific journal Nature Communications, a collaborative 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 made a noteworthy discovery. The findings of their study suggest that CO2 emissions from soil microbes are projected to not only rise but also intensify globally by the conclusion of this century.
By employing a projection, the researchers have determined that by the year 2100, CO2 emissions from soil microbes are expected to rise significantly, potentially increasing by approximately forty percent on a global scale compared to current levels. This forecast is based on the worst-case climate scenario. Alon Nissan, the lead author of the study and an ETH Postdoctoral Fellow at the ETH Zurich Institute of Environmental Engineering, emphasizes that this projected surge in microbial CO2 emissions will further contribute to the exacerbation of global warming. Consequently, it highlights the pressing need for more precise estimations of heterotrophic respiration rates.
Soil moisture and temperature as key factors
The recent findings not only validate previous research but also offer more precise understanding of the mechanisms and scale of heterotrophic soil respiration in various climatic zones. Unlike other models that depend on multiple parameters, the new mathematical model created by Alon Nissan simplifies the estimation process by focusing on two key environmental factors: soil moisture and soil temperature. This innovative approach enhances our ability to accurately assess the impacts of heterotrophic soil respiration.
The developed model marks a significant advancement as it encompasses all essential biophysical levels, ranging from the micro-scales of soil structure and soil water distribution to larger entities such as plant communities, ecosystems, climatic zones, and even the global scale. Peter Molnar, a professor at the ETH Institute of Environmental Engineering, emphasizes the importance of this theoretical model in conjunction with large Earth System models, stating that it facilitates a simpler estimation of microbial respiration rates based on soil moisture and soil temperature. Furthermore, it deepens our comprehension of how heterotrophic respiration in diverse climate regions contributes to the phenomenon of global warming.
Polar CO2 emissions likely to more than double
One noteworthy discovery from the collaborative research led by Peter Molnar and Alon Nissan is the variation in the increase of microbial CO2 emissions across different climate zones. In cold polar regions, the primary driver of the increase is the reduction in soil moisture, rather than a significant rise in temperature as observed in hot and temperate zones. Alon Nissan emphasizes the sensitivity of cold regions, explaining that even a slight change in water content can result in a substantial alteration in the respiration rate in polar regions.
According to their calculations, in the worst-case climate scenario, microbial CO2 emissions in polar regions are estimated to increase by ten percent per decade by the year 2100. This rate is twice as high as the anticipated increase for the rest of the world. The disparity can be attributed to the favorable conditions for heterotrophic respiration, which occur when soils are in a semi-saturated state—neither too dry nor too wet. These optimal conditions prevail during soil thawing in polar regions.
In contrast, soil in other climate zones, which are already relatively drier and susceptible to additional drying, experience a comparatively smaller rise in microbial CO2 emissions. Nonetheless, regardless of the climate zone, the impact of temperature remains consistent: as soil temperature increases, so does the emission of microbial CO2.
How much CO2 emissions will increase by each climate zone
As of 2021, the majority of CO2 emissions from soil microbes originate from the warmer 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 negligible 0.1 percent from the Arctic or polar regions.
The researchers highlight a significant observation that anticipates substantial growth in microbial CO2 emissions across all regions, surpassing the levels observed in 2021. Their projections indicate that by the year 2100, there will be an estimated 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. This projected rise underscores the concerning trend of escalating microbial CO2 emissions globally.
Will soils be a CO2 sink or a CO2 source for the atmosphere?
The carbon balance in soils, which determines whether they function as a carbon source or sink, relies on the interaction between two fundamental processes: photosynthesis, where plants absorb CO2, and respiration, which releases CO2. Consequently, studying microbial CO2 emissions is crucial for understanding whether soils will store or release CO2 in the future. By examining these emissions, we can gain valuable insights into the potential impact on carbon storage in soils.
Alon Nissan explains that the magnitude of carbon fluxes, encompassing both the influx through photosynthesis and the outflow through respiration, remains uncertain due to climate change. Nevertheless, it is crucial to recognize that this magnitude will have a direct impact on the current role of soils as carbon sinks. As climate change continues to unfold, understanding and quantifying these carbon fluxes become increasingly important for assessing the future carbon storage capacity of soils.
In their ongoing study, the researchers have primarily directed their focus towards heterotrophic respiration, without delving into the CO2 emissions resulting from autotrophic respiration by plants. Further investigation into these aspects will contribute to a more comprehensive understanding of carbon dynamics within soil ecosystems. By exploring both heterotrophic and autotrophic respiration, researchers can gain deeper insights into the overall carbon balance and its intricate processes within soil ecosystems.
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