Annual CO2 Exchange in
Irrigated and Rainfed
Maize-Based
Agroecosystems
S.B. Verma, A.E. Suyker, G.G. Burba, T.J. Arkebauer, D.T.
Walters, A. Dobermann, K.G. Cassman, B. Amos, H. Yang, K.G.
Hubbard, A.A. Gitelson, E.A. Walter-Shea, and D. Ginting.
University of Nebraska-Lincoln
X/SBV/Presentations/USDA Symposium_March 2005_Verma
Objective
Report results from the first three years of
measurements of net ecosystems CO2 exchange (NEE)
from the tower flux systems
Address the following questions:
– How does the seasonal and annual CO2 exchange of maize
compare with that of soybean?
– What is the impact of irrigation on the CO2 exchange of
these crops?
– How does the annual CO2 exchange of a continuous maize
system compare with a maize-soybean rotation?

University of Nebraska Agricultural Research
and Development Center near Mead, NE

Three cropping systems
Site 1: Irrigated continuous maize (Zea mays, L.)
Site 2: Irrigated maize-soybean (Glycine max. [L.]
Merr.) rotation
Site 3: Rainfed maize-soybean rotation

Sites 1 and 2: Center-pivot irrigation systems

Prior to initiation of the study
– Sites 1 and 2 had a 10-year history of
maize-soybean rotation under no-till
– Site 3 had a variable cropping history of
primarily wheat, soybean, oats, and
maize grown in small plots with tillage


All three sites were uniformly tilled by
disking prior to the initiation of the study to
humogenize the top 0.1 m of soil and
incorporate P and K fertilizers, as well as
previously accumulated soil residues
Since initiation of study in 2001
– All sites under no-till
– BMP
Landscape-level
(Eddy Covariance)
Measurement of CO2
and Other Fluxes
Close Up of
Eddy Covariance
Flux Sensors
Measuring Components
of Solar Radiation
Net Ecosystem CO2 Exchange (NEE): Three Years
Daily NEE (g C m-2 d-1 )
25
maize
20
maize
Site 1
maize
15
10
P
P
H
H
H
P
Irrigated
Continuous
Maize
5
0
-5
-10
5/1/01
7/30/01
10/28/01
1/26/02
4/26/02
7/25/02
10/23/02
1/21/03
4/21/03
7/20/03
10/18/03
1/16/04
4/15/04
Daily NEE (g C m-2 d-1 )
25
maize
20
maize
soybean
Site 2
15
10
H
P
H
P
P
H
Irrigated
Maize-Soybean
Rotation
5
0
-5
-10
5/1/01
7/30/01
10/28/01
1/26/02
4/26/02
7/25/02
10/23/02
1/21/03
4/21/03
7/20/03
10/18/03
1/16/04
4/15/04
Daily NEE (g C m-2 d-1 )
25
maize
20
maize
soybean
Site 3
15
P
P
H
P
H
H
10
Rainfed
Maize-Soybean
Rotation
5
0
-5
-10
5/1/01
7/30/01
10/28/01
1/26/02
4/26/02
7/25/02
10/23/02
1/21/03
4/21/03
7/20/03
10/18/03
1/16/04
4/15/04
NEE-Biomass Relationship
Estimating Microbial Respiration (Rg)

Soil surface CO2 flux measurements (Fs)
• Two different kinds of chambers:
 Model LI-6200, Li-Cor, Lincoln, NE
 Hutchinson & Mosier (1981) type chamber
• Used data from field measurements of maize soil
respiration in root excluded and non-root excluded
soil to estimate Rg

Night NEE data
• Adjusted for plant respiration based on leaf gas
exchange measurements
• Adjusted for night/day temperatures
• Applied measurements of root excluded vs. non-root
excluded soil respiration to estimate Rg, as mentioned
above
NEE – Biomass Relationship
(Cumulative values between planting and physiological maturity)
+15%
1500
1:1
-2
ENEE++ R
ERh (g
C m ) -2
NEE
g (g C m )
-15%
2001
2001
2001
2002
2002
2002
2003
2003
2003
1000
500
Site
Site
Site
Site
Site
Site
Site
Site
Site
1
2
3
1
2
3
1
2
3
0
0
500
1000
1500
Total Biomass (g C m-2)
+15%
1500
Rg: Chambers
1:1
-2
ENEE + ERh (g C m )-2
+ Rg (g C m )
NEE
-15%
1000
500
0
0
500
1000
Total Biomass (g C m-2)
1500
Rg: Night NEE
Ecosystem Respiration (Re) and Gross
Primary Production (GPP)
Renight = NEEnight
Reday = NEEnight * Q 10 (Ta,day - Ta,night)/10
GPP = NEE + Re
Integrated magnitudes of GPP and Re
(over a growing season)
A.
2000
2002
Carbon exchange (g C m-2)
1800
1600
Year 2
Irrigated Maize S1
Irrigated Soybean S2
1400
1200
Irrigated Maize Site 1
Irrigated Soybean Site 2
1000
800
400
Ecosystem Respiration
site
0
Grow Season Grow Season Grow Season
GPP
Re
NEE
site1
site1
site1
B.
2000
2001
1800
Carbon exchange (g C m-2)
Year 2
Irrigated Soybean S2
Rainfed Soybean S3
600
200
1600
site2
site2
site2
site3
site3
site3
1400
1200
Irrigated Maize Site 1
Rainfed Maize Site 3
1000
800
Avg Chambers
Site/Year
600
400
200
0
Grow Season Grow Season Grow Season
GPP
Re
NEE
C.
2000
2002
1800
Carbon exchange (g C m-2)
Year 1
Irrigated Maize S1
Rainfed Maize S3
1600
1400
1200
Irrigated Soybean Site 2
Rainfed Soybean Site 3
1000
800
600
400
200
0
Grow Season
GPP
Grow Season
Re
Grow Season
NEE
Non-Growing Season
(Autumn/winter/spring)CO2 Exchange
Daily Re: Correlated with soil temperature
(R2 = 0.59 to 0.71, P < 0.01)
Highest Re: Near harvest time
Spring
Renon-growing = 0.15 to 0.25 Regrowing
Carbon exchange (g C m-2)
Annually integrated magnitudes of GPP and Re
1900
Site 1: Irrigated Continuous Maize
1500
annual GPP
annual Re
annual NEE
1100
700
300
-100
2001
2002
Carbon exchange (g C m-2)
1900
2003
Site 2: Irrigated Maize-Soybean Rotation
annual GPP
annual Re
annual NEE
1500
1100
700
300
-100
2001
2002
Carbon exchange (g C m-2)
1900
2003
Site 3: Rainfed Maize-Soybean Rotation
annual GPP
annual Re
annual NEE
1500
1100
700
300
-100
2001
2002
2003
Annual Carbon Balance
NBP = Annual NEE – Cg + Ic
where: NBP = Net Biome Production
NEE = Net ecosystem CO2 exchange
Cg = Amount of C removed with harvested grain
Ic = CO2 released from irrigation water
(Estimated using in vitro direct measurements of
CO2 release from irrigation water applied to soil)
Annual Carbon Budget (g C m-2 yr-1)
Year 1
2001-02
Year 2
2002-03
Year 3
2003-04
Site 3: Rainfed maize-soybean rotation
Maize
Soybean
Maize
Annual NEE
510
-18
397
Grain C removal during
harvest (Cg)
335
153
297
NBP
175
-171
100
Annual Carbon Budget (g C m-2 yr-1)
Year 1
2001-02
Year 2
2002-03
Year 3
2003-04
Site 1: Irrigated continuous maize
Maize
Maize
Maize
Annual NEE
517
424
381
Grain C removal during
harvest (Cg)
521
503
470
Estimated CO2 release
from irrigation water (Ic)
43
39
49
7 to 28
-69 to -50
-77 to -52
NBP
Annual Carbon Budget (g C m-2 yr-1)
Year 1
2001-02
Year 2
2002-03
Year 3
2003-04
Site 2: Irrigated maize-soybean rotation
Maize
Soybean
Maize
Annual NEE
529
-48
572
Grain C removal during
harvest (Cg)
518
183
538
Estimated CO2 release
from irrigation water (Ic)
41
26
45
21 to 42
-225 to -212
45 to 68
NBP
Annual Carbon Budget (g C m-2 yr-1)
Year 1
2001-02
Year 2
2002-03
Year 3
2003-04
Site 3: Rainfed maize-soybean rotation
Maize
Soybean
Maize
Annual NEE
510
-18
397
Grain C removal during
harvest (Cg)
335
153
297
NBP
175
-171
100
Annual Carbon Budget (g C m-2 yr-1)
Cropping system
NBP
Rainfed maize-soybean
rotation
(Site 3)
+2 (years 1 and 2 average)
-36 (years 2 and 3 average)
Irrigated continuous maize
(Site 1)
-46 to -25 (3 year average)
Irrigated maize-soybean
rotation (Site 2)
-102 to -85 (years 1 and 2 average)
-90 to -72 (years 2 and 3 average)
Conclusions
Accounting for the grain C removed during harvest and the CO2
released from irrigation water, our tower eddy covariance flux
data over the first three years suggest that, at this time:
• The rainfed maize-soybean rotation system is C neutral
–
Consistent with results from ongoing studies in Illinois
and Minnesota
• The irrigated continuous maize is nearly C neutral or a
slight source of C
• The irrigated maize-soybean rotation is a moderate source
of C