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2. Soil Fundamentals
Fueled by sunlight and drawing CO
2
(carbon
dioxide) from the atmosphere, plant leaves
make sugars (carbohydrates) that store this
converted solar energy. A byproduct of
photosynthesis is oxygen, which is released
back into the atmosphere. The manufacture,
storage, and use of carbohydrates is
essential to plant growth, maintenance,
and reproduction. Through the process of
photosynthesis, carbohydrates are formed in
the cells of leaves and stems during daylight
hours of the growing season.
It is important to remember that plants also
respire. In addition to drawing CO
2
, from the
atmosphere in the process of photosynthesis,
plants release CO
2
as they burn stored
carbohydrates to fuel maintenance, growth,
and reproduction. And because burning
always requires oxygen, plants must consume
oxygen, just as animals do. Aboveground parts
of plants can access oxygen from the air, but
Plants and Carbon
Glossary
Photosynthesis
the reaction in green plants that converts
solar energy (light) to chemical energy.
Glossary
Respiration
produces energy that cells can use, by
taking in oxygen, breaking apart (burning)
carbohydrates, and releasing carbon dioxide.
roots belowground also need oxygen for their
cells. For this reason, soils need to be moist
but not saturated to allow for oxygen to flow
through pore space to the roots.
Plant biomass
The aboveground biomass of a plant, literally
the weight of all its leaves and stems, is largely
determined by how much carbon it takes in
through photosynthesis and the amount of
carbon it respires. At any given moment in
a plant’s life, total biomass may be less than
the total produced because insects and other
animals, parasites, and disease consume plant
tissues.
Some organisms (e.g. cows, grasshoppers)
use the carbohydrates and other compounds
from living plants to drive their own activities.
Some microbes use root exudates, which are
carbohydrates released from living plants,
either actively or due to roots breaking.
(These exudates are sometimes called “liquid
carbon.”)
Other organisms use carbohydrates and
other compounds from dead plants to drive
their activities. This is a part of the process of
decomposition, which also includes breaking
down the bodies of other dead organisms or
their waste (manure).
In turn, plants rely on other organisms to help
them gain nutrients or defend them from
parasites or grazers. And, of course, some
plants rely on animals for pollination, which
rarely occurs in soil and so is not a feature of
soil biology that needs consideration here.