Nowhere do bacteria remove more methane than in forest soils. This makes these soils very important for stabilizing the climate, because one tonne of methane in the atmosphere warms the Earth 30 times more than one tonne of CO₂. So, if the bacteria in forest soils therefore remove less methane, the gas stays longer in the atmosphere and drives climate change more strongly than already.
Most studies suggest that this danger is real: The bacteria need oxygen to convert methane into the less climate-damaging CO₂ – the more air in the soil, the better. But climate change is making rainfall more intense. Then the pores in the soil become smaller, the air supply declines, and the bacteria can convert methane less effectively. Other bacteria species that produce methane, however, can thrive without oxygen and continue to release methane merrily.
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The Study
Now, while it is scientifically established that climate change makes it rain more because warmer air can hold more water, that does not necessarily mean that it will rain more intensely in forests — especially not in temperate forests, where soil bacteria are most effective at converting methane. This is what a study by German researchers investigates. They measured, at six sites in Baden-Württemberg, for up to 24 years how much methane forest soil absorbs.
For this, in addition to the long-running measurements with tubes in the soil for extended periods, they set chambers on the soil for gas measurement to check the long-term measurements. “Methodologically, that is very good,” says Friederike Lang, professor of soil ecology at the University of Freiburg. She was not involved in the study. “The investigation is truly unique; there are rarely continually collected measurements over such a long period.”
The result: The bacteria converted more methane in recent years, not less. On average, the studied forest soils absorbed 3 percent more methane each year than the year before. According to the study authors, this is probably mainly because rainfall in the forests has become less frequent and the soils have accordingly dried out — favorable conditions for methane-processing bacteria. “In addition, more methane is being emitted, so the bacteria are also processing more,” says co-study author Verena Lang, who is not related to the professor.
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What’s the point?
That forest soils worldwide take up more methane rather than less cannot be inferred from the study results. But for some regions, yes: “I would expect that this could be observed at more sites where it has become drier, for example in Brandenburg,” says Professor Friederike Lang.
The study author Verena Lang says: “This is crucial for greenhouse gas accounting. Normally, in each federal state a single value is used to calculate how much methane forest soil absorbs.” This value should be looked at closely, perhaps it is underestimated.
