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Underground Fingerprints

25 Aug | By Biophotonics.World
Underground Fingerprints
Image source: Leibniz IPHT
By: Sven Döring

How do climate change and land use affect the soil and our groundwater? Torsten Frosch and Andreas Knebl are developing powerful gas sensors to investigate this issue 

Into the Depth

Gas sensors with innovative hollow-core optical fibers provide a key to understanding metabolic and exchange processes underground 

Between the treetops and the bottom of the groundwater – at the interface of atmosphere and geosphere – lies the critical zone: a living, breathing, constantly evolving environment in which rock, soil, water, air, and living organisms interact with each other. Their complex network of relationships regulates our natural habitat. The critical zone determines our supply of natural resources and the quality of our drinking water. And it is increasingly influenced and strained by our way of life. 

How do intensive land use, environmental pollution, and the climate change affect groundwater and the habitat under the earth? How safe are the vital underground water reservoirs? What do metabolic processes and gas exchange processes reveal about the state of the ecosystem? 

In order to better understand the interrelationships between subsurface and surface, between environmental factors and the processes in vegetation, soils, and groundwater, the University of Jena, Leibniz IPHT, and two other partners, have set up an open-air laboratory in Thuringia's Hainich National Park that is unique in the world. In one of the last original beech forests in Germany, scientists from the Collaborative Research Center (Sonderforschungsbereich, SFB) "AquaDiva" are investigating how the above and below ground habitats of plants and microorganisms interact with each other under different land use conditions. 

Investigating drought stress of trees 

The Hainich Critical Zone Exploratory (CZE) research platform is equipped with new types of measuring equipment and sampling facilities to obtain and analyze samples of gases, water, and substances from the subsoil, soil, and groundwater. From this, the researchers want to deduce how we can preserve these vital ecosystems for future generations in such a way that they too can use them as a basis for life. Torsten Frosch and Andreas Knebl from the Leibniz IPHT work group "Fiber Spectroscopic Sensors" are also involved. In their team, the scientists are researching innovative optical gas sensors to analyze biogenic gases in underground habitats. These provide clues to decipher the interactions between living organisms and environmental factors – such as climate or nutrient concentration – in this ecosystem. 

"Plants, animals, and microorganisms produce and consume various gases, especially oxygen and carbon dioxide," explains Torsten Frosch, who heads the work group. "With innovative Raman multi-gas sensors, we can continuously track depth profiles of these gases on site and find out how microbial life underground is connected to the environment. "We measure gases and isotopically labelled compounds," reports Andreas Knebl. In order to use stable isotopes to trace and decompose the gas exchange of plants, the fiber-spectroscopic sensor technology research team developed a novel Raman spectroscopic gas sensor whose core component is a hollow-core optical fiber. This sensor fiber makes it possible to investigate the oxygen and carbon dioxide exchange of plants with high sensitivity and selectivity in one experiment. "Thus, we can also use this method to determine the respiration coefficient and investigate the effects of drought stress on trees," explains Torsten Frosch. 

The rapid optical analysis technique allows gases to be clearly identified on the basis of their molecular fingerprint. "We can monitor all relevant gases simultaneously with one device, including the normally not easily measurable gases oxygen, nitrogen, and hydrogen," says Torsten Frosch. "This makes it particularly suitable for measuring the gas exchange of bacteria and plants in field trials," adds Andreas Knebl. 

With new concepts for highly sensitive Raman gas spectroscopy, the team wants to further break down the exchange of gases between atmosphere, soil, and groundwater. To this end, the researchers are working with the fiber technologists of Leibniz IPHT on the development of specific filigree hol- low-core fibers produced in-house. The gas sensors are intended to provide a better insight into natural processes. "By measuring gases and isotopes at different points in time, we want to further contribute to the mission of 'AquaDiva'", Torsten Frosch states. "We want to gain a better understanding of how the subsurface and the surface interact, and what influence humans have on this critical ecosystem." 

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Publication: Andreas Knebl et al., Fiber-Enhanced Raman Gas Spectroscopy for 18O−13C-Labeling Experiments, Analytical Chemistry 2019, 91, 7562-7569, https://doi.org/10.1021/acs. analchem.8b05684 


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