How much CO2 does it take to grow a loaf of bread?∗
Jesse Brunner
Introduction: Agriculture and greenhouse gas emissions
Agriculture provides important ecosystem services in the forms of food and fiber, but can
also convey many disservices to agroecosystems themselves and to the ecosystems affected by
agricultural practices. In particular, agricultural activities contribute substantial amounts of
greenhouse gases, including more methane and nitrous oxide than any other human activity.
For example, Duxbury (1994) estimated that agriculture contributes 25%, 65% and 90%
of all anthropogenic emissions of carbon dioxide (CO2 ), methane (CH4 ) and nitrous oxide
(N2 O), respectively.
Several processes identified below are responsible for greenhouse gas emissions in production agriculture:
• Fossil fuels are oxidized to provide energy for machinery involved in tilling, planting
and harvesting.
• Initial cultivation of previously untilled soil results in substantial losses of carbon previously stored in soil organic matter (Robertson and Grace, 2004). This occurs because
tillage increases oxygen supply to soil organisms and exposes previously protected soil
organic matter to decomposers.
• Inputs such as nitrogen fertilizer, irrigation and manure can increase plant productivity
and soil carbon sequestration, but don’t necessarily result in a net decrease in carbon
dioxide emissions due to the fossil fuel energy requirements to provide these inputs
Schlesinger (1999)
• Nitrogen fertilization and tillage decrease the amount of CH4 sequestered by soils
because of a decrease in the abundance of methanotrophic bacteria in soil (Conrad,
1996).
∗
This lab has been inspired by (and the introduction largely taken from) Wilke, B. J. and J. Kunkle.
February 2009. What does agriculture have to do with climate change? Teaching Issues and Experiments
in Ecology, Vol. 6: http://tiee.ecoed.net/vol/v6/figure_sets/climate_change/abstract.html.
1
• Nitrogen fertilization and tillage increase the amount of N2 O given off to the atmosphere through the processes of nitrification and denitrification (Mosier et al. 1991).
• Nitrogen fertilizer is produced using energy from fossil fuels, and applications of nitrogen fertilizer can result in high nitrous oxide emissions.
Certain management activities have been shown to reduce agricultural greenhouse gas emissions after accounting for all inputs and emissions (i.e., the Net Global Warming Potential)
(Robertson et al., 2000). For example, growing winter cover crops increases net primary
productivity and inputs of organic carbon to the soil. Perennial plants have expansive root
systems and have long growth periods, thus increasing soil carbon storage (Cox et al., 2006).
Thus, depending how, where, and when a crop is grown the amount of CO2 and other
greenhouse gases released can vary quite a lot.
Since it was first recognized that agriculture could be a significant source of greenhouse
gas emissions, and that many aspects of food production could be changed to alter these
emissions, there has been a series of comprehensive, global studies of our food systems (e.g.,
West and Marland, 2003; Lal, 2004; Garnett, 2011). I, however, have a very simple question:
How much CO2 is released to grow the wheat needed to make a single loaf of whole wheat
bread? We are going to ignore the other greenhouse gases, energy required to take the wheat
and turn it into bread and drive it to a shop. Still, I think you’ll find that answering even
this little question becomes rather involved.
Class exercise: A wheat field as an ecosystem
As you will recall, an ecosystem is comprised of all of the interacting biotic and abiotic
components or features of a given place. This could be a watershed or a wheat field. In
our case, we can be even more specific—our ecosystem is the section of wheat field needed
to produce one loaf of bread. So how much is that? Well, this lab will involve a lot of
conversions, so let us begin by sorting this out. Here are the relevant facts1 :
• An average yield is 82 bushels of wheat per acre.
• A bushel produces 60 pounds of whole wheat flour.
• A standard pound-and-a-half loaf of whole wheat bread requires 16 ounces (1 pound)
of whole wheat flour.
1
These factors come from the National Association of Wheat Growers (http:
//www.wheatworld.org/wheat-info/fast-facts/)
and
Quara
(http://www.quora.com/
How-many-square-feet-of-wheat-are-required-to-produce-a-loaf-of-bread), which is a favorite
place to find interesting facts, ideas, and perspectives.
2
Now of course we are scientists and so we would never use these quaint Imperial units.
So, your first task is to establish the size of our ecosystem in hectares (ha) or
meters-squared (m2 ).
Size of the ecosystem:
Next, use the picture of our wheat field, below, to draw a diagram of the fluxes
of CO2 into and out of our ecosystem, with a focus on the fluxes in and out of the
atmosphere. Be sure to label the processes associated with each arrow. Make sure
you think about the ways that C accumulates in the ecosystem and all of the ways that C
is produced. You need to think broadly about this, too. For instance, the wheat seeds are
generally produced somewhere else and so while they weigh very little, they are associated
with a lot of CO2 emissions; that is, they are a net source of CO2 to the atmosphere.
Before you move on, you should discuss your diagram with your lab group, other groups,
and the class as a whole.
3
Class exercise: Adding numbers to the fluxes
So now we have the basic understanding of
the fluxes (arrows) of CO2 into and out of our
ecosystem, but we do not know how big these
arrows should be. How can we sort this out?
Well, one way would be to set up a wheat
field and measure biomass accumulation both
above and below ground; the amount of each
fertilizer added (and back-track the energy inputs to create those fertilizers); the amount
of petroleum burned to till the soil, plant the
wheat, harvest it, etc.; and so forth. In fact,
this has been done by a long list of curious
scientists over the years. We are lucky; we
get to benefit from their long hours and intricate research!2 And you are even luckier!
I’ve done the work of finding useful sources of
data on many, many fluxes. Your job is easy:
YOUNGET AL.: TILLAGE, WEEDMANAGEMENT,
AND W
6600
conservation
6400
conventional
74
70
6200
66
6000-
x
62
58
5800-
54
5600-
50
~oo-
46
5200-
42
5000-
74
70
4800
66
4800-
620
4400-
58
4200-
540
4000
500
VVVV-SP
WW-SW
WW-VVVV
1. Find the appropriate values for
Rotation Position
each flux (or fluxes) in your diaFig. 1. Five-year average of winter wheat yield in response to
gram, convert them to the approtillage systems
and rotation
position.
Within
tillage systems
Five-year
average
winter
wheat
(WW)
yield
(conservation, a-c; conventional, x-z), rotation positions with
priate size for our little ecosystem,
same letters County
are not significantly
different (P < 0.05).
intheWhitman
under conservation
(no
**,*** Significant differences at the 0.01 and 0.001 probability
and add them to your diagram.
till)
andrespectively,
conventional
whenwithin
WW
was
levels,
betweentillage
tillage systems
rotation
position.
WW-SP= winter wheat after spring pea; WWplanted
after
spring
pea
(SP),
spring
wheat
2. Determine whether our small
SW = winter wheat after spring wheat; WW-WW
= winter
wheat after
winter wheat.
or winter
wheat (WW) from Young
agroecosystem is a net Carbon (SW),
source or sink. That is, does grow- et al. (1994). For simplicity let’s assume our
Weeds, especially
downybrome,
reduced
yield
ecosystem
is WW-WW.
Also,
let’sgrain
assume
ing wheat fix more Carbon than is in
winter wheat after winter wheat under conservation
that the
yield
biomass
that
leaves
released in all of the processes that tillage
(Fig.
2). isAsthe
withonly
diseases,
downy
brome
populations
increased
over
time
and
became
a
severe
pest
the
ecosystem.
go into growing it?
problem after 6 yr (F.L. Young, unpublished data).
no-till wheat yields following spring wheat were
In the next few pages you will find several 1991,
20% higher with maximumweed management compared
useful tables and figures from which to glean with minimum weed management (data not shown).
Downy
brome populations
plants here
m-2 inthan
the maxinformation on the magnitude of these fluxes. Be
careful!
There is awere
lot3more
you
imumweed managementlevel and 75 plants m-2 in the
will probably need.
minimumlevel.
Root diseases had no measurable effect on yields of
2
Don’t you love science! As long as you acknowledged
(i.e.,
cite) the
sources
you take
information
from
either
moldboard
plowed
continuous
wheat
or winter wheat
it isn’t considered stealing!
in the conservation tillage, 3-yr rotation (R.J. Cook,unpublished data). This agrees with previous research that
plowingor a 3-yr rotation reduces root diseases (7). For
4 wheat, the type of crops rotated is not as important as
the length of rotation, which permits time for the soil to
be sanitized by microorganisms(7).
Weed Management x Tillage
Effect
Whendiseases are suppressed or eliminated, as in the
3-yr cropping system, weeds become the limiting yield
factor, and wheat yields increase in response to weed
management
levels if sufficient moisture is present (Fig.
2a). In the 3-yr cropping system, yield of winter wheat
460
420
740
700
660
620
580
MO
5O
460
420
Fig. 2. Five-ye
tillage
syst
(a) spring p
tillage syste
managementl
different (P
probability
levels.
WW
= winter w
wheat after
following pe
managementfo
at the moder
With convent
erate and m
higher than w
of no-till w
creased with
and were 11
imumlevels o
minimumleve
T.O. West, G. Marland / Agriculture, Ecosystems and Environment 91 (2002) 217–232
223
Carbon dioxide emissions for currently used pesticides
pumping (US Department of Commerce, 1997) and
T.O. West, G. Marland / Agriculture, Ecosystems and Environment 91 (2002) 217–232
were estimated by assigning 64 herbicides, insectienergy price estimates (EIA, 2000b). The energy use
cides,
and
fungicides
used
on
US
corn
(Zea
mays
L.),
and C emissions from pumping water were applied to
Table 6
wheat
(Triticum
aestivum
L.),
and
soybean
(Glycine
both on-farm wells and off-farm surface reservoirs.
Fossil fuel energy requirements and carbon dioxide emissions from seed production
max L.) crops in 1996 (Fernandez-Cornejo and
Jans,
It was assumed
that the average energy and
CO2
Seed
Costa (US$ kg−1 )
Energyb (MJ kg−1 )
C emissionsc
1999) to their respective pesticide classes and calcucost of pumping water is the same (kg
perC ha-m
of
waper kg seed)
lating weighted averages of the C emissions based on
ter for the two sources (USDA, 1997a). The energy
Grain seed
the
relative amounts of pesticides used. It 0.26
was again
cost of collecting
and distributing 0.11
on-farm surface
Barley (Hordeum vulgare L.)
5.57
assumed,
unless
specified
otherwise,
that
steam
was
water,
powered
primarily
by
gravitational
Corn (Zea Mays L.)
2.49
53.36
1.05 forces, was
raised
burninghirsutum
natural L.)
gas. Energy balances
considered33.00
to be negligible.
Cotton by
(Gossypium
1.54for pro0.65
Oats (Avena
satira L.)
0.29
6.21
0.12
duction
of some
pesticides are rough approximations
Sorghum
bicolor L.)
2.03may be
43.50
0.86
only,
but(Sorghum
Green (1987)
suggested that values
2.5. Seed 12.86
production
Soybean (Glycine max L.)
0.60
0.25
within
±10% for some of the best known0.31
and most
Wheat, spring (Triticum aestivum L.)
6.64
0.13
widely
used (Triticum
pesticides.
Different5.57
methods for calculating
Wheat, winter
aestivum L.)
0.26
0.11energy use in
seed
production
have
been
reviewed
and compared
Forage seed
(Heichel,
1980).
Heichel
(1980)
concluded
that the
2.4.
Irrigation
Alfalfa
(Medicago sativa L.)
6.21
133.08
2.63
Orchardgrass (Dactylis glomerata L.)
2.62
56.15 method would be to1.11
most accurate
calculate a deRed
clover (Trifolium
87.01budget for each crop, 1.72
tailed energy
including energy
Irrigation
water pratense
in the L.)
US is obtained4.06
primarily
Ryegrass (Lolium perenne L.)
1.28
27.43
0.54
for seed cleaning
and packaging of the
seed. Heichel
from
on-farm wells, on-farm surface reservoirs,
and
Timothy (Phleum pratense L.)
1.61
34.50
0.68
further concluded that, lacking these detailed energy
off-farm surface reservoirs (Table 5). Fossil fuels used
a
prices
from USDA
(1997c).
budgets, the next best method was to estimate energy
tob Seed
power
pumps,
which
distribute irrigation water,
Using dollar to energy conversion of 21.43 MJ US$−1 for agricultural products (US Office of Technology Assessment, 1990).
costs using the retail cost of seeds in conjunction
were
calculated
using
energy
expenses
for
on-farm
c
224
Fuel mix contributing to C emissions is assumed to consist of fuel oil (50%), natural gas (20%), and electricity (30%) (Börjesson, 1996).
The
Table
5
energy and C-emissions involved in producing seeds (West and Marland, 2002).
with
thefossil
current
dollar-to-energy
transformacovercollection,
after planting.
Reduced
tillage water
(RT) represents
Annual
fuel average
energy requirements
and carbon
dioxide emissions from
storage, and
use of irrigation
tion coefficient for agriculture. Energy used in seed
practices
that
leave
15–30%
residue
cover. ConFuel use and irrigation
Area irrigated by fuel
requiredb
CO2 emissions
irrigated by
production,
packaging, and
distribution
(Table Energy
6) was
servation
tillage is any practice thatArea
leaves
greater
a
−1
type
type (million ha)
(GJ ha )
irrigation typea (%)
−1
−1c
In kg Cresidue
ha
In kg Cplanting;
ha-m
estimated from current seed prices (USDA, 1997c) and
than 30%
after
this latter catea On-farm
dollar-to-energy
conversion factor for general agrigory includes no-till (NT) (Conservation Technolpump
cultural
products (US Office
ogy Information
Center,
tillage
Electricity
8.00 of Technology Assess5.32
266.00
– 1998). Conventional
–
Natural
gas Carbon emissions
2.46
19.61
– of plowing, while
–
ment,
1990).
were calculated
with
usually285.13
involves the use
reduced
0.65used in seed production
6.70
the LPG
assumption that energy
tillage125.22
involves using –disks or chisels,– without the
Distillate fuel
3.33
7.53
165.28
–
–
consisted of a 50, 20, and 30% mix of fuel oil, natural
use of plows. No-till leaves
the soil undisturbed.
In
Gasoline
0.07
5.92
125.92
–
–
gas, and electricity, respectively (Börjesson, 1996).
this analysis, CT is any practice that uses a moldTotal on-farm pump
14.48
8.31
597.93practices that –do not use a
board 239.17
plow, RT includes
d
Total on-farm wells
–
8.31
239.17 plow, and
597.93
moldboard
NT leaves the 62.08
soil relatively
on-farmemissions
surface
–
0.00
0.00
0.00
12.77
undisturbed.
3.Total
Carbon
relative
to tillage practice
Total off-farm surface
–
15.17
436.49
597.93
29.99
and crop type
Total US averagee
–
9.26
266.48machinery525.10
104.84
3.1. Farm
a
Emissions
generated
Data fromofUSCO
Department
of Commerce are
(1997).
Totals for columns may not equal sums of individual values due to independent
2 from agriculture
rounding.
Energy and CO2 emissions associated with diffrom
three sources: machinery used for cultivating
b Based on 1994 irrigation data (US Department of Commerce, 1997) and 1994 energy prices (EIA, 2000b).
ferent
tillage practices (Table 7) are a consequence
the land,
production
and
application
of
fertilizers
and
c Average depth of water applied in 1994 was 0.40, 0.43, and 0.73 m for on-farm pump, on-farm surface, and off-farm surface, respectively
pesticides,
and
the
SOC
that
is
oxidized
following
of
the
fuel used by farm machines and the en(US Department of Commerce, 1997). Data were not available for the amount of water applied with respect to the primary fuel used.
d Irrigation water
soil disturbance.
The
amount
of
soil
that
is
disturbed,
ergy
consumed
manufacture,
transportation,
and is
from on-farm wells is primarily derived from a pump system; energyinused
for off-farm surface
water collection
inassumed
turn causing
decomposition
andwater
oxidation
of on-farm
SOC, pumprepair
of the
machines
(Bowers,
1992).
COand
to be the
same per ha-m of
as that for
water, and
energy
used for on-farm
surface
waterWhile
collection
2
is assumed on
to be
(USDA, 1997a).
isdistribution
largely dependent
thenegligible
tillage practices
used. The
emissions associated with the application of fertilize The total area is greater than 100% because some areas are irrigated using more than one irrigation practice and are counted twice in
amount
of fertilizers and pesticides applied varies
ers and pesticides were calculated along with other
the US agriculture survey data. The total weighted energy and carbon emission values shown here have been normalized to 100% coverage.
among crop types, crop rotations, and tillage practices.
farm operations (Table 7), they do not occur on all
The term conventional tillage (CT) represents
fields and in all years, as do other farm operations.
The
energythat
and
C-emissions
involved
and
Marland,
2002).of
tillage
practices
leave
less than 15%
residuein irrigating
Therefore,crops
CO2 (West
emissions
from
the application
5
T.O. West, G. Marland / Agriculture, Ecosystems and Environment 91 (2002) 217–232
225
Table 7
Annual fossil fuel energy requirements and carbon dioxide emissions from agriculture machinery for different tillage practices in the United
States, circa 1990
Farm operation
Moldboard plow
Disk
Planting
Single cultivationi
Fertilizer application
Pesticide application
Harvest w/combine
Diesel fuel used in
machine operation
In l ha−1
In MJ ha−1
21.78c
6.70d
4.93e
3.26f
9.82g
1.22g
11.14g
1122
345
254
168
506
63
574
Energy in MTRa
(MJ ha−1 )
Carbon emissions
(kg C ha−1 )
CTb
(kg C ha−1 )
RTb
(kg C ha−1 )
NTb
(kg C ha−1 )
102
55
58
42
60
56
186
26.75
8.72
6.79
4.57
12.35
2.54
16.47
26.75
17.44h
6.79
4.57
–j
–j
16.47
–
17.44h
6.79
4.57
–
–
16.47
–
–
6.79
–
–
–
16.47
72.02
67.45
45.27
40.70
23.26
23.26
Total C emissions
Corn
Soybean and wheati
a Energy embodied in manufacturing, transportation, and repair of machinery is from residual fuel (25%), distillate fuel (10%), coal
(45%), electricity (8%), and human labor (12%) (Bowers, 1992; Boustead and Hancock, 1979; and Graedel and Allenby, 1995). Energy
from human labor is not included in calculations for carbon emissions, because it is assumed that humans will respire carbon dioxide
regardless of whether they are working.
b CT, RT, and NT are conventional till, reduced till, and no-till, respectively.
c Sources of data for calculations of average fuel use are Collins et al. (1980), Gumbs and Summers (1985), Plouffe et al. (1995),
Shelton (1980), Sijtsma et al. (1998), Tompkins and Carpenter (1980).
d Sources of data for calculations of average fuel use are Collins et al. (1980), Shelton (1980), Sijtsma et al. (1998), Smith (1993),
Tompkins and Carpenter (1980).
e Sources of data for calculations of average fuel use are Collins et al. (1980), Tompkins and Carpenter (1980).
f Sources of data for calculations of average fuel use are Shelton (1980), Smith (1993).
g Source of data for calculation of average fuel use is Bowers (1992).
h Disking was counted twice to represent two passes over the field.
i Single cultivation is not included in analyses for wheat, soybean, or other non-row crops.
j Since fertilizer and pesticide application does not necessarily occur on an annual basis, the associated C emissions need to be weighted
with respect to the percentage of crops using fertilizers and pesticides (see Table 8).
The
energy
C-emissions
involved
in various
stages
growing
row
crops
(West and
fertilizers
and and
pesticides
are weighted
by their extent
to RT
to NTof
(USDA,
1997b)
(Table
8). Although
the
Marland,
2002).
of application
and are included in Table 8.
decreased use of insecticides with no-till appears to
3.2. Crop inputs
Carbon dioxide emissions from specific crop inputs are given for corn, soybean, and wheat (Table 8).
Agronomic inputs were calculated from the US national average use of fertilizers, pesticides, irrigation,
and other production inputs. National average data
were available as a function of crop type and tillage
intensity for all inputs except lime and irrigation,
and for these, data were available for crop type only.
This analysis assumes that the need for lime does not
change with the intensity of tillage.
United States data for 1995 show that herbicide use
was greater, and insecticide use less, going from CT
6
be contrary to traditional agronomic findings, a recent
study that reviewed past estimates of national insecticide use confirmed this trend and concluded that
insecticide use with NT is no more than that with CT,
and is often less (Day et al., 1999). Fungicides were
not included in the accounting, because the contribution is negligible and data were not available. Carbon
emissions from the application of fertilizers, pesticides, and lime were combined with their respective C
emissions from production on a per crop basis, and the
emissions total weighted by the percentage of planted
area using the respective treatments. Emissions from
application were included with emissions from production separately for herbicides and insecticides since
they are typically applied separately. Emissions from
from crude petroleum or natural gas products. The
total energy input is thus both the material used as
feedstock and the direct energy inputs. Carbon dioxide emissions from production of pesticides (Table 4)
consist of both of these contributions to manufacture the active ingredient. Post-production emissions
include those from formulation of the active ingredients into emulsifiable oils, wettable powders, or
granules; and those from packaging, transportation,
and application of the pesticide formulation.
Carbon dioxide emissions from pesticide use were
estimated for specific pesticide classes by calculating average values of energy input for the production
and application of individual pesticides (Green, 1987).
emissions.
Similarly, production of P fertilizers typically
results in generation and export of excess steam. Sulfuric acid plants, that are generally run in conjunction
with phosphoric acid production, generate and export
excess steam that can equal 40% of the gross energy
requirement for P2 O5 production. Because much of
this steam export appears to be generated from the
burning of sulfur, and hence without CO2 emissions,
it is included in the energy balance summarized here
but ignored in the CO2 balance. Recently, revised
calculations by Anthony Turhollow (personal communication, 2000) indicate that natural gas used in
the production of P2 O5 may be more than previously
Table 4
Fossil fuel energy requirements and carbon dioxide emissions from production of pesticides
Herbicide
In
Productiona
Naphtha
Natural gas
Coke
Distillate fuel
Electricityb
Steamc
Production total
GJ Mg−1
Insecticide
In
kg C Mg−1
In
GJ Mg−1
Fungicide
In kg C
Mg−1
In GJ Mg−1
In kg C Mg−1
71.99
43.04
0.32
12.31
71.68
44.22
1572.98
625.80
9.80
270.20
1228.80
642.96
63.31
49.78
0.77
7.86
92.13
47.97
1383.32
723.80
23.59
172.53
1579.37
697.48
92.20
31.10
0.00
11.10
78.15
53.34
2014.57
452.19
0.00
243.65
1339.71
775.56
243.56
4350.54
261.82
4580.09
265.88
4825.68
2.00
1.00
20.00
43.90
17.14
290.80
2.00
1.00
20.00
43.90
17.14
290.80
2.00
1.00
20.00
43.90
17.14
290.80
Post-productiond
Distillate fuel
Electricityb
Natural gas
Post-production total
Pesticide total
23.00
351.84
23.00
351.84
23.00
351.84
266.56
4702.38
284.82
4931.93
288.88
5177.52
a Based on weighted amount of pesticides used on corn, wheat, and soybean crops in the United States in 1996, using pesticide energy
values from Green (1987).
b Energy input from electricity is given as the primary energy input required for power generation and is based on 10.5 MJ kWh(e)−1
(0.0105 GJ kWh(e)−1 ).
c Demands for steam are assumed to be met by combustion of natural gas.
d Includes formulation, packaging, and transportation (Green, 1987). The energy for formulation is assumed to be from natural gas,
the energy for packaging an equal mix from electricity and distillate fuel, and the energy for transportation from diesel fuel. Energy used
in post-production processing is assumed to be the same for the different pesticides and for their respective formulations. Energy used in
pesticide application is included in later calculations (see Table 7).
The energy and C-emissions involved in producing herbicides, insecticides, and fungicides
(West and Marland, 2002).
7
T.O. West, G. Marland / Agriculture, Ecosystems and Environment 91 (2002) 217–232
221
8
The energy and C-emissions from producing three common fertilizers (and agricultural lime,
which we can probably ignore) (West and Marland, 2002). It may help to note that “Mg”
does not mean “milligram,” but instead “megagram” or 106 grams.
Group exercise: adding complexites
Putting together your energy diagram this wheat field ecosystem, you probably started to ask
yourself about a lot of the details of the system. How much fertilizer is added? What kind?
Does that change productivity a little or a lot? Some of these complexities can change how
Carbon flows through the system a great deal while others do not. So now is your chance to
satisfy your curiosity. In this last part of the lab you will use published or governmental
data sources and the same approach to CO2 budgets we used above to address
one of the following questions:
• Do increased yields (i.e., CO2 fixation) offset the C involved in applying fertilizer?
That is, from the perspective of CO2 emissions, is it better to fertilize wheat fields or
not?
• Does this change with the form of fertilizer used (Ammonia, nitrates, organic)?
• How do farming practices such as tilling or burning influence CO2 fluxes and accumulation in the ecosystem?
• How would CO2 accumulation and fluxes change if wheat were a perennial crop?
• Or, if you are adventurous or curious, a question that your TA approves.
You will be writing your results in a 12-sentence abstract again, so look ahead to the
guidelines.
Here are several potentially useful references (click to go the pdfs):
• West, T. O., and G. Marland. 2002. Net carbon flux from agricultural ecosystems:
methodology for full carbon cycle analyses. Environmental Pollution 116:439-444.
• West, T. O., and G. Marland. 2003. Net carbon flux from agriculture: Carbon emissions, carbon sequestration, crop yield, and land-use change. Biogeochemistry 63:73-83.
• Cox, T. S., J. D. Clover, D. L. Van Tassel, C. M. Cox, and L. R. DeHaan. 2006.
Prospects for developing perennial grain crops. Bioscience 56:649.
• Halvorson, A. D., and C. A. Reule. 1994. Nitrogen fertilizer requirements in an annual
dryland cropping system. Agronomy Journal 86:315-318.
9
Format of abstract
You will need to turn in an abstract detailing your hypothesis and related logic, your tests of
this hypothesis, your prediction, and your findings. In science—indeed, in powerful writing
in general—brevity and clarity are essential. Thus, this assignment will be very similar to
the 12-sentence paper you turned in for your first lab. Make every word count!
Focusing on one of the seven tests we conducted in lab, your abstract will consist of:
1. Introduction (1–2 sentences). Here you need lay out the context and logic that leads
to your hypothesis and research. If we were writing about a test of the hypothesis that
adding worms to wheat fields led to increased yields high enough to offset the loss of
organic C from decomposition, we might start out by discussing the energetic costs
of hibernation: “Although worms aerate the soil, which increases aerobic microbial
decomposition, and directly consume soil organic matter, their action also increases
crop yields. . . ”
2. Hypothesis (1 sentence). A brief, clear statement of the hypothesis you set out to
test. This is not the same thing as an expectation. If you are unclear on the different
consult your TA. It might start out with, “We hypothesized that . . . ”
3. Methods & Predictions (2–3 sentences). Describe the data and analyses you used
to test your hypothesis. This will include the sources (i.e., where the data came from),
the spatial and temporal scale (e.g., county, state, USA; time period), and a brief
description of the data (e.g., crop yields with and without worms), as well as how you
went about using these data (e.g., including them in your Carbon budget). You also
need to state your prediction, which should be closely related to both your hypothesis
and your data and analyses.
4. Results (1–2 sentences). Summarize in text format your results (i.e., “When worms
were included in our Carbon budget wheat fields became net Carbon sinks because
of large changes in. . . ”). The focus should be on changes to your system that are
meaningful to your hypothesis.
5. Conclusion (1 sentence). Here you just need to make a conclusion about whether
your hypothesis was strongly/weakly supported, or not. Keep it brief!
10
Grading rubric for Wheat field ecosystem lab assignment
Criteria
Introduction provides context and logic clear
Points Possible
1
Hypothesis is clear and coherent
1
Data and assumptions are accurately described (sources, scale, etc.)
Method of testing hypothesis is clearly explained
Prediction is laid out in relation to hypothesis and test
2
1
1
Results and meaningful relationship(s) clearly describe
2
Conclusion is logical and flows from results
2
Total
10
11
References
Conrad, R. 1996. Soil microorganisms as controllers of atmospheric trace gases (H2 , CO,
CH4 , OCS, N2 O, and NO). Microbiological Reviews 60:609–640.
Cox, T. S., J. D. Clover, D. L. Van Tassel, C. M. Cox, and L. R. DeHaan. 2006. Prospects
for developing perennial grain crops. Bioscience 56:649.
Duxbury, J. M. 1994. The significance of agricultural sources of greenhouse gases. Fertilizer
Research 38:151–163.
Garnett, T. 2011. Where are the best opportunities for reducing greenhouse gas emissions
in the food system (including the food chain)? Food Policy 36, Supplement 1:S23–S32.
Lal, R. 2004. Carbon emission from farm operations. Environment International 30:981–990.
Robertson, G. P. and P. R. Grace. 2004. Greenhouse gas fluxes in tropical and temperate
agriculture: the need for a full-cost accounting of global warming potentials. In Tropical
Agriculture in Transition—Opportunities for Mitigating Greenhouse Gas Emissions?, 51–
63, Springer.
Robertson, G. P., E. A. Paul, and R. R. Harwood. 2000. Greenhouse gases in intensive
agriculture: contributions of individual gases to the radiative forcing of the atmosphere.
Science 289:1922–1925.
Schlesinger, W. H. 1999. Carbon Sequestration in Soils. Science 284:2095–2095.
West, T. O. and G. Marland. 2002. A synthesis of carbon sequestration, carbon emissions,
and net carbon flux in agriculture: comparing tillage practices in the United States. Agriculture, Ecosystems & Environment 91:217–232.
West, T. O. and G. Marland. 2003. Net carbon flux from agriculture: Carbon emissions,
carbon sequestration, crop yield, and land-use change. Biogeochemistry 63:73–83.
Young, F., A. Ogg, R. I. Papendick, D. Thrill, and J. Alldredge. 1994. Tillage and Weed
Management Affects Winter Wheat Yield in an Integrated Pest Management System.
Agronomy Journal 86:147–154.
12