SOIL CONSERVATION COUNCIL OF CANADA
CONSEIL DE CONSERVATION DES SOLS CANADA
Global Warming and Agriculture
The Carbon Cycle
I n this, the second in a series of
information sheets aimed at dis-
cussing global warming and its rela-
This give-and-take has been going on
for centuries and things have stayed
pretty much in balance.
photosynthesis into C-containing
compounds like carbohydrates, sug-
ars, cellulose and lignin. A portion
tionship with agriculture in Canada, of this is used by the plant for its own
The problem started a relatively short
we'll look at carbon dioxide (CO2). energy and released back into the
time ago with the advent of the indus-
Specifically, we'll explore the role atmosphere as CO2. Some of the C
trial revolution. For centuries, Earth
CO2 plays in the global warming sce- remaining in the plant is removed
had been packing excess C away
nario and the affect agriculture can during harvest with the rest returning
deep below its surface, and in its
have on global CO2 levels. to the soil as plant residue. This
forests and soils. The industrial rev-
residue, along with the roots, becomes
olution required energy. Coal and oil
part of the soil organic matter. Soil
Carbon dioxide - resources were extracted and burned,
microorganisms decompose the
releasing their stored C into the
what is it? organic matter returning some of the
atmosphere in the form of CO2. This
C to the atmosphere in the form of
situation was aggravated by the clear-
Carbon dioxide is considered the CO2. While rates of transformation
ing of forests for industrial and agri-
largest contributor to global warm- vary due to such things as climate,
cultural purposes. Agriculture wors-
ing. What is it? Where does it come soil and crop type, this cycle is essen-
ened the situation by breaking the
from? tially the same in all cropping sys-
land thereby releasing centuries old
There are vast amounts of carbon (C) soil-stored C into the atmosphere in
in global circulation. The oceans the form of CO2. Our Earth's system Livestock adds another variable into
hold most of the C while the atmos- of balance, that kept CO2 levels rela- this situation. The carbon in crops
phere and the world's soils and tively constant for 10,000 years, has that might otherwise be harvested
forests hold much of the rest. been thrown out of whack with CO2 and exported can be fed to livestock
levels rising 30% in a mere 140 years and used as bedding. Some of the
Of these C pools, the atmosphere
and becoming a major contributor to carbon is released into the atmos-
holds the smallest amount, but is the
global warming. phere through animal respiration and
most active. Atmospheric C com-
some carbon is removed as meat
bines with oxygen to form CO2. This
products. Most of the C can be
CO2 is continually on the move.
The carbon cycle returned to the soil as manure.
Some of it is absorbed and released
by oceans. Some is used by plants
and other photosynthetic organisms What farmers can do -
As discussed in the previous informa-
on land and water. They absorb CO2
from the atmosphere, and use energy
tion sheet all segments of society will increasing soil carbon
have to do something if the CO2
from the sun to combine it with
imbalance is to be effectively turned The farming community has a lot to
water into organic matter (composed
around. Agriculture will be no offer in terms of reducing C emis-
of carbon, hydrogen and oxygen). As
exception. sions. While no one knows for certain
well, CO2 is continually being
the amount of CO2 that could be
released into the atmosphere through Within the agricultural C cycle, CO2
stored (sequestered) in agricultural
respiration, organic matter decay and is absorbed from the atmosphere by
soils, some predict that by using best
burning. plant leaves and is transformed via
Volume 1 Number 2, December 2000
management practices, approximate- decomposition is another way to l Remove land permanently from
ly 763 million tonnes of CO2 could increase soil-stored C. Carbon diox- cultivation. This could mean
be sequestered in Canadian agricul- ide is released from organic matter removing marginal land from cul-
tural soil over a 20 year period. through microbial activity. A man- tivation by establishing woodlots,
There are basically two ways farmers agerial goal is to make conditions less planting shelterbelts, creating pas-
can reduce C02 emissions, one is to favorable for these microbes. For tures, grassing waterways, etc.
l Eliminate summerfallow. During
increase the amount of C stored, the example, crop residue left on the soil
a fallow year, virtually no residue
other is to use less fossil fuel energy. surface slows microbial activity. As is added and the soil remains
well, keeping growing plants on the warm and moist hastening organic
Farmers can influence the amount of
surface as long as possible slows matter decay.
C taken from the atmosphere to be
decay. Decay can also be reduced by Grow cover crops. When the sea-
stored in the soil. Management prac- l
shielding soil organic matter from son allows, a winter cover crop
tices that accomplish this must con-
microbes. Soils are usually granulat- can be sown after the main crop.
centrate on adding as much C to soil
ed and organic matter is often pro- This practice can add residue and
as possible and on slowing the rate of prevent erosion.
tected inside the granule (or aggre-
CO2 release by decomposition. Avoid burning of crop residues.
gates). Intensive tillage can break l
Carbon dioxide in the air enters the these aggregates open and expose the When residues are burned almost
soil by way of photosynthesis. It is organic matter to soil microbes. all their C is returned to the
trapped in organic forms and enters atmosphere.
l Use crops and varieties that yield
the soil as residue (including roots).
Farming practices that higher volumes of residue.
Practices that encourage higher levels l Improve water management.
of photosynthesis enhance C accu- increase stored carbon Irrigation, snow trapping, etc.
mulation, in other words, practices increase yields and thus residue
that increase plant yields. This can be Specifically, the following manageri- returned to the soil.
done by using higher yielding crops al practices will encourage C seques- l Restore wetlands. By putting pre-
and varieties, and by reducing water tration. viously drained wetlands back
stress, where possible, through prac- under water, oxygen is cut off,
l Reduce tillage. Tillage hastens the preventing organic matter decom-
tices such as irrigation, water conser- release of soil carbon into the
vation or drainage. As well, any position.
atmosphere and practices such as Integrate livestock into farm man-
action that improves soil quality and summerfallow reduce inputs into
thus promotes higher yields helps in agement plans.
the soil during the year when no l Improve grazing management.
this area. Another key goal is to keep crop is growing. Reducing tillage Overgrazing can result in large
plants actively growing, and thus also protects soil resources from losses of C via erosion.
photosynthesizing, on the land as erosion.
much and as long as possible. l Apply more nutrients. Fertilizers, Agriculture also adds CO2 into the
animal manures or green manure atmosphere through direct or indirect
Increased photosynthesis helps build increase yields leading to higher use of fossil fuel energy. The next
soil C only if some of the trapped C is inputs of C. Manure can also information sheet will investigate
returned to the soil. The more of the improve soil tilth adding further agricultural fossil fuel use and what
plant that’s removed the more C is yield increases and residue addi- can be done to reduce it.
removed. Using practices that leave tions.
a maximum amount of residue in the l Grow more perennial forage Sources
field and including in crop rotations, crops. Perennial crops can trap Agriculture and Agri-Food Canada, Research
forages that store much of their C in more CO2 than annual crops Branch. 1998. The health of our air.
their roots, can promote soil C accu- because they continue growing Ottawa, Ontario.
more months of the year. As well, The Soil Conservation Council of Canada.
many have extensive root systems 1998. Carbon sequestration and trading
Slowing the rate of organic matter that place C below ground. implications for Canadian agriculture, dis-
cussion paper. Saskatoon, Saskatchewan.
Agriculture and Agriculture et
Agri-Food Canada Agroalimentaire Canada
Agriculture and Agri-Food Canada (AAFC) is pleased to participate in the production of this publication. AAFC is committed to
working with our industry partners to increase public awareness of the importance of the agri-food industry to Canada. Opinions
expressed in this document are those of the Soil Conservation Council of Canada and not necessarily the Department's.