Forest soils contribute more to climate change than most people realize, quietly absorbing methane from the air each year.
New long-term measurements in southwestern Germany show that in some forests, this underground methane sink is steadily strengthening, rather than declining.
researchers University of Göttingen They tracked the movement of methane across 13 forest patches for almost 25 years and found that methane uptake by soils was increasing by about 3% per year.
This steady increase, observed across wet and dry seasons and gradually warming conditions, challenges the simple assumption that climate change will uniformly reduce the ability of soils to act as methane sinks. It also raises new questions about why some forests improve faster than others.
Forest microorganisms remove methane
Reduced rainfall left more air space in the soil, allowing methane to move downward faster instead of staying near the surface.
Dry ground has more air-filled pores, making it easier for both methane and oxygen to move through the soil. This is an important factor because microorganisms that consume methane require oxygen to function.
The researchers correlated enhanced methane uptake with lower soil moisture and gradual increases in soil temperature across the plots they monitored.
When methane reaches the topsoil, methanotrophs occur. microorganisms It uses methane as fuel and breaks it down through a reaction that converts the gas into carbon dioxide and water.
Metabolism tends to be faster when soils are slightly warmer, consistent with the steady increase in methane uptake seen in many locations.
However, the researchers noted that while extremely dry conditions suppress microbial activity, prolonged periods of wet soil can reduce oxygen levels and give methane-producing microorganisms an advantage.
Track methane in soil
Thin tubes delivered samples from several depths every two weeks to build soil gas profiles and measure gas levels in the soil.
Laboratory tests tracked methane and other gases, showing whether concentrations decreased with depth, indicating uptake.
To confirm these calculations, the researchers also set up a sealed room. forest floor Then he observed methane falling. This double check was important because small errors can add up and false trends can emerge as data accumulates over years.
methane affects the climate
According to NASA metrics, methane traps more heat than carbon dioxide, but only for about 7 to 12 years.
When soil absorbs methane, less reaches the atmosphere, reducing warming pressure in the coming years. Forest soils rarely benefit from this feature, but their performance can increase or decrease depending on the weather.
large methane absorption That in and of itself won’t solve the problem, but it could buy time for emissions reductions to catch up.
Rainfall controls methane absorption
Previous studies have not necessarily found that methane intake is increasing. A 2018 US study that tracked forests across multiple sites found that as precipitation increased, methane uptake decreased sharply, with losses reaching 89 percent at one site.
Such contrasts highlight how results can differ when regions follow different rainfall trajectories and why meta-analyses that combine many studies can sometimes overgeneralize local patterns.
“In the forested areas we studied, we observed significant increases in methane uptake over time,” Meyer said.
Rainfall plays a central role in controlling it. soil moistureThis determines the rate at which methane and oxygen pass through the pores of the soil.
In southwestern Germany, precipitation gradually decreased during the monitoring period, while parts of the northeastern United States experienced wetter conditions. This helps explain why methane trends have moved in the opposite direction.
Climate projections generally estimate temperature changes more reliably than local rainfall. This means that a single global map of future methane uptake could be misleading if regional moisture patterns change differently due to warming.
Forests change and soils react
Forest disturbances can quietly reshape the way soils process methane by changing the structures through which the gas passes.
For some monitored plots, Bark beetle Because the outbreak forced the felling of trees, researchers tracked those areas separately in later years to account for the changes.
When the forest canopy opens, sunlight can reach the forest floor more easily, often drying the soil more quickly. At the same time, heavy equipment used during logging can compact the ground and squeeze the small pores needed for methane to pass through.
These local changes can outweigh broader climate trends, so understanding the actual causes of methane uptake requires long-term monitoring to track forest disturbances alongside weather patterns.
Future forest methane absorption
Even within a single landscape, methane uptake varied widely between plots, indicating that soil quality and land use history greatly influenced the results.
The researchers found that beech and spruce In forests, we still found large differences in the amount of methane absorbed by the soil.
Although cross-checking has reduced uncertainty, stony soils and patchy moisture still limit the precision with which the team can accurately measure uptake, and expanding this work across the country could introduce errors as adjacent soils and standing trees can behave very differently.
This long-term record also shows a link between increased methane uptake and drier soil conditions and warmer years, calling into question the simple global story of universally declining methane consumption.
Decades of consistent monitoring will be needed in more locations, particularly in regions that become wetter, before projections can reliably guide climate policy.
This study Agriculture and forest meteorology.
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