Hi I’m Ali Sealander with UTD GeoNews and today we are looking down, where the plants, soil, and water
interact with each other in what scientists call “The Critical Zone”. It is critical because
understanding the interactions and processes that happen here are immensely helpful in understanding
Earth’s climate, specifically the levels of carbon dioxide, or CO2, within the atmosphere.
Carbon dioxide is naturally occurring within our atmosphere, but the levels have risen dramatically
since the industrial revolution and the burning of fossil fuels. But there are natural ways to
pull down CO2 from the atmosphere, the most important are plant growth and soil formation.
You might remember that plants use photosynthesis to grow. This is the
process that takes CO2 and water and uses the energy from sunlight to facilitate growth.
The CO2 gets pulled in from the surroundings and the water gets sucked up by the roots through
the plant and evaporates off its leaves, stems, and flowers in a process called transpiration.
More plants and plant growth means more intake of CO2 from the atmosphere
But what about soil formation? How does that lower CO2? Well soil is mostly made up of minerals. When
rainwater drains into the soil it helps to break down these rocks, and it is these little pieces of the rock,
the minerals, that make up a large part of the soil. But it's also made up of organic material,
like microorganisms or plants. This organic matter holds carbon and releases CO2 when they decompose.
The soil actually sequesters this carbon, using it to weather more rock,
and exporting the carbonate products for burial in ocean sediments.
One thing you might notice about both plant growth and soil formation
is that they are both dependent on water. Water is needed for plant transpiration
and water is needed to break down rocks to create soil. So, we can see that a vital
control for the two most important methods to draw down CO2 is water distribution.
This implied competition going on here is further
complicated by the fact that these competitors depend on each other. Plants need soil to grow
and soil needs plants to provide the organic matter that keeps it healthy.
So the best outcome of this competition is one where some of the water goes to plant
transpiration and some goes to soil formation in such a way as to maximize the drawdown of
atmospheric CO2. So what is this ratio? Well, Allen Hunt, a physics professor
at Wright State University, and his colleagues answered this question.
Publishing their results in the September 2020 issue of GSA Today.
What they found is that the best situation for drawdown of atmospheric CO2 is when
2/3 of the precipitated water that makes it below ground goes to the plants and 1/3 to soil formation.
And as it turns out this ideal situation happens naturally.
After it rains the water that makes it into the ground can either get sucked up by the roots of
plants or flow through the ground creating soil. In controlled environments scientists
can actually measure how much water was transpired by plants and how much went to subsurface run-off
It has been found that the global mean value of the amount of water transpired by plants
is about 2/3 of the total precipitated water that makes it into the ground. It is the same value
that Allen Hunt and his colleagues found to be the ideal ratio to remove CO2 from the atmosphere.
These results are even reflected in the growth patterns of plant roots.
In a 2-dimensional plane we can see that the roots spread out laterally. The vertical growth provides
the 3rd dimension to the root’s growth pattern. The lateral growth is dependent on transpiration
and the downward growth depends on the soil depth.
So, 2 /3 of the dimensions of plant growth are related to transpiration and 1/3
is related to the soil, just like Hunt’s results and the observed global mean values.
These results and observations are fascinating in their own right but we can also use them to
predict plant growth and soil development as well as their respective effect on atmospheric carbon
in terms of the water within the soil. But this special zone beneath our feet
and the optimal process between plants, soil, and water can be disrupted. Factors like changes
in land use or changes in climate can throw this ecosystem out of balance and reduce its
ability to take CO2 out of the atmosphere. So, yeah, this zone is pretty critical.
Thanks to Allen Hunt for helping us put this video together.
Check out the links to his research in the description box below.
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Thanks for watching!