increasing amount of carbon dioxide absorbed by plants is triggering the soil
to release nitrous oxide and methane, two potent greenhouse gases. Artist’s
rendering by Victor Leshyk
are not as efficient in counteracting the effects of global warming as once
believed, according to a study released in this week’s edition
study, “Increased Soil Emissions Of Potent Greenhouse Gases Under Increased
Atmospheric Carbon Dioxide,” details how the research team found the opposite
is occurring in several regions in the world—higher levels of carbon dioxide in
the atmosphere triggers soil to release methane and nitrous oxide, two potent
feedback to our changing atmosphere means that nature is not as
efficient in slowing global warming as we previously thought,” said Kees
Jan van Groenigen, lead author of the study and visiting research fellow at
Northern Arizona University.
Groenigen, along with co-authors Bruce Hungate, professor of biological
sciences at Northern Arizona University, and Craig Osenberg, professor of
ecology at the University of Florida, analyzed all published research to date
from 49 different experiments conducted in forests, grasslands, wetlands, and
agricultural fields, including rice paddies, in North America, Europe and Asia.
researchers measured how extra carbon dioxide in the atmosphere affects how
soils take up or release methane and nitrous oxide.
research team used a statistical technique called meta-analysis, or
quantitative data synthesis, a powerful tool for finding general patterns in a
sea of conflicting results.
now, there was no consensus on this topic because results varied from one study
to the next,” Osenberg said. “Two strong patterns emerged when we analyzed all
the data—more carbon dioxide boosted soil emissions of nitrous oxide in all the
ecosystems, and in rice paddies and wetlands, extra carbon dioxide caused soils
to release more methane.”
analysis pointed to wetlands and rice fields as two major sources of methane
emissions in the atmosphere.
culprits responsible for the greenhouse gas emissions are specialized
microscopic organisms in soil that respire the chemicals nitrate and carbon
dioxide like humans respire oxygen. The microbes also produce methane, a
greenhouse gas 25 times more powerful than carbon dioxide, and nitrous oxide,
300 times more potent than carbon dioxide. Their oxygen-free habit is one of
the reasons these microorganisms flourish when atmospheric carbon dioxide
higher concentrations of carbon dioxide reduce plant water use, making soils
wetter and reducing the availability of oxygen in soil, which provides
favorable conditions for these microorganisms,” van Groenigen said.
other reason these microorganisms become more active is that increasing carbon
dioxide makes plants grow faster, and the extra plant growth supplies soil
microorganisms with extra energy, pumping up their metabolism. This extra plant
growth is one of the main ways ecosystems could slow climate change: with more
carbon dioxide, plants grow more, soaking up carbon dioxide through
photosynthesis, and, the hope is, locking away carbon in wood and soil.
this new work shows at least some extra carbon also provides fuel to
microorganisms whose byproducts end up in the atmosphere and counteract the
cooling effects of more plant growth.
an ecological point and counterpoint—the more the plants soak up carbon
dioxide, the more microbes release these more potent greenhouse gases,” Hungate
said. “The microbial counterpoint is only partial, reducing the cooling effect
of plants by about 20 percent.”
an ecological surprise too, and one that climate models will need to reckon
with as they further refine pictures of the climate of the future.
overlooking the key role of these two greenhouse gases, previous studies may
have overestimated the potential of ecosystems to mitigate the greenhouse
effect,” van Groenigen said.