Science1 publisher2 min readPublished
Wetting sterilized soil released methanol and formaldehyde with no microbes present
Heidelberg chemists report that iron and reactive oxygen species cleave lignin's methoxy groups into methanol and then formaldehyde at room temperature in water. In living soil, microbes eat the methanol before it can reach the air.
The Scientist · Science desk
What happened
- Earth scientists at Heidelberg University report in Nature Communications that iron linked to reactive oxygen species converts lignin into methanol and then formaldehyde with no microorganisms involved.
- Isotope-marked molecules let the team follow the pathway and showed that the methoxy group leaves lignin as a whole unit, which is the step that yields methanol.
- In sterilized soil samples, wetting alone produced methanol and then formaldehyde, without added iron or reactive oxygen species.
- Experiments run at different temperatures showed that formation of both compounds increases as the temperature rises.
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Why it matters
- constraint The sterile-soil result cannot be converted into an atmospheric flux: the sterilization that rules out microbes also removes the microbes that would have eaten the methanol first.
- capability Anyone attributing soil methanol to biology now has a second candidate source to control for, one that needs only iron minerals and a wetting event at ordinary temperature and pressure.
- precedent Because the yield climbs with temperature, an abiotic term becomes a plausible thing to look for in warming-response work, and the pathway is now specified well enough for a field team to try to measure it.
The result rests on a control soil. The Heidelberg group first removed the methoxy groups from a soil sample, then wetted it, and no methanol formed [12]. Sterilizing excludes the microbes; stripping the methoxy groups excludes every other constituent of the soil as the carbon source [23]. Production continued through several wetting-drying cycles [11].
Lignin is a considerable part of terrestrial nonfossil organic carbon, and the methoxy groups (-OCH3) are what the iron chemistry attacks [2][3]. Decay pathways known before this work liberate only the methyl share of those groups [4]. Quantum chemical calculations by Peter Comba's group at Heidelberg's Institute of Inorganic Chemistry found the lowest energy barrier for cleaving the whole group [6]. "That exactly matches the experimental observations," Comba said [7].
"The reaction takes place at ambient temperature and under normal pressure in water, and will only intensify at higher temperatures," said Jonas Hadeler, who ran the laboratory experiments in Frank Keppler's biogeochemistry group [8][22]. The same demethoxylation has been known industrially at temperatures above 285 degrees Celsius, at higher pressure, in a hydrogen atmosphere [9]. The soil version needs iron minerals, which many soils contain, and reactive oxygen species, which form there under changing conditions [19]. This is the first indication that an iron-driven route could be an important source of either compound [26].
Untreated, biologically active soils hold considerably less methanol, because microorganisms consume it quickly [13]; both compounds are carbon and energy sources for soil microbes [20]. So the sterile experiment measures gross abiotic production, while what a living soil emits is production minus that uptake [24]. "Hence it primarily functions as an intermediate product linking abiotic chemistry and microbial metabolism," Hadeler said [14].
"It surprised us that the compounds methanol and formaldehyde can also be released entirely chemically from lignin-rich organic materials," Keppler said [15]. Both compounds influence photochemical reactions if they reach the atmosphere, and the researchers describe the pathway as a previously overlooked abiotic process with implications for trace gas fluxes and atmospheric chemistry [16][21]. The account of the study does not include a production rate or an estimate of what share of any measured soil flux is abiotic [25]. "What this means in the climate context is something further field studies will have to clarify," Keppler said [18].
The pathway is real, and water alone sets it off in unamended soil [10]. Nobody has yet measured how much of it survives the microbial community that sits on top of it, and that is the number that decides whether any flux changes [24][25].
What to watch
- A field campaign measuring net methanol and formaldehyde emission from living soil, where abiotic production and microbial uptake compete, would settle whether any measured flux needs reattributing.
- Whether the laboratory temperature dependence is reproduced in field soils, which decides if warming raises this source at all.
- Whether trace-gas inventories add an abiotic lignin term, and what magnitude they assign to it.