US agricultural GHG emissions can be reduced through improved fertilizer management and conservation practices, though achieving absolute reductions while increasing yields is challenging. Process-based models are better at large scales but have high uncertainty at the whole-farm level. Stabilizing nitrogen sources may reduce emissions while maintaining production, but observations are needed to verify impacts at larger scales given uncertainties. Continued efficiency gains may not decrease absolute emissions due to potential rebound effects like Jevon's Paradox. Carefully defined goals and incentives are needed to balance production and emission reductions.

1. US Agricultural GHG Emissions:
challenges and opportunities for
mitigating plot scale vs. whole farm
(and larger) emissions
Steve Del Grosso, et al.

2. OUTLINE
• US Agricultural GHG Sources and Sinks
• GHG intensity and Nitrogen Use Efficiency
• Scaling paradox
• Farm system emissions: conventional vs. organic
• To what extent does Jevon’s paradox apply?

3. US Agricultural GHG Emissions
300
200
Tg CO2 eq.
100
0
-100
Soil CO2 energy use Soil N2O enteric CH4 manure
CO2 CH4+N2O

4. x
F lu
et
US Cropped Land Δ Soil C
N
ils
So
ic
an
rg
O
d
te
ga
ri
RP Ir
C &
o ps
Cr
er
th
O
s1
in
ra
ay lG
H al
Sm
p s&
ro n
C tio
ow ta
R Ro
i n
re
stu
/ Pa
ay
H
40
30
20
10
0
-10
-20
-30
-40
Tg CO2 eq. yr-1

5. IPCC METHODOLOGY FOR N2O
Direct Emissions Indirect Emissions
Synthetic N
Organic N
Residue N NH3 & NOx Volatilization (10% Synthetic N, 20% Manure N)
NO3- Leaching/Runoff (30%)
Mineral
Soils
Organic 0.75%
Soils
1%
Temperate:
8 kg N2O-N/ha
1.0% (applied N)
Subtopics:
2.0% (PRP N) 12 kg N2O-N/ha
Atmospheric N2O

7. Uncertainty Framework
Input
Uncertainty PDF
Input PDF
Simulation Scaling
Results
Uncertainty Model Uncertainty
PDF 95%
Confidence
Input Interval
PDF
Uncertainty
Structural Uncertainty

8. CO N2O gN ha-1 d-1
dr
CO yla
nd
0
5
10
15
20
25
30
35
40
dr w
yla he
nd at
NE cr
op
dr pi
yl ng
an
d
wh
ea
M NE t
Ic
or gr
IPCC
n/ as
so s
y/
measured
al
DAYCENT
fa
lfa
CO TN
CO co
irr
irr ig rn
ig at
ed
at
ed co
co rn
rn
/b
ar
le
CO y
gr
O as
nt
ar s
io
co
Site Level Mean N2O Emissions
rn
PA
cr
op
perform better than simple models
PA
gr
as
s
At plot level, process based models usually
av
er
ag
e

9. As scale increases, simple models become more reliable
US major Crops N2O
0.6
0.4
Tg N
0.2
0.0
DAYCENT Lower bound DAYCENT Upper bound IPCC
Global Anthropogenic N2O
8
6
Tg N
4
2
0
Top Down lower bound Top down upper bound IPCC
Del Grosso et al. 2008

10. as scale decreases, CIs get wider
1200%
1000%
800%
CI Width
600%
400%
200%
0%
20 25 0 5 10 15 20
ion Regional N2O Emisisons (gG N)
Most of this uncertainty is due to model structure
Del Grosso et al. 2010

11. US Corn, Wheat, Cotton, Sorghum Yields
and N Fertlizer Applied
N fert Tg
45
grain yield Tg * 10
40
35
30
25
20
15
10
5
0
90
91
92
93
94
95
96
97
98
99
00
01
02
03
04
05
06
07
08
09
19
19
19
19
19
19
19
19
19
19
20
20
20
20
20
20
20
20
20
20
NUE is increasing

12. GHG Intensity for N2O from US Corn and Soy
0.22 g CO2 eq./
g grain
0.29 gCO2/
g grain
GHG intensity is decreasing

13. 0.12 Theoretical GHG Intensity
N2O CO2-C eq./grain-C
0.1
0.08
0.06
0.04
0.02
0
0 5 10 15 20 25 30 35
N inputs g N m-2

14. 0.12 DayCent GHG Intensity
N2O CO2-C eq./grain-C
0.1
0.08
0.06
0.04
0.02
0
0 5 10 15 20 25 30 35
N inputs g N m-2
Irrigated Corn CO - N2O GHG Intensity
0.06
0.05
CO2-C eq/grain-C
0.04
0.03
0.02
0.01
0
0 N NT no-
corn 0-N, corn NT
0 N 0-N, corn Hi-N, no- High Hi-N,
High N NT corn N CT
till conventional- till conventional-
till till

15. Global Warming Potential
CO2 eqvt
(kg CO2eqvt ha-1 y-1)
System Soil C N2O Energy GHGnet
No-Till 0b 303 b 807 1110 b
Chisel Till 1080 a 406 ab 862 2348 a
Organic -1953 c 737 a 344 -872 c
Cavigelli et al. 2009

16. Greenhouse Gas Intensity
GHGnet Crop Yield GHGI
System (kg CO2eqvt ha-1 y-1) (Mg ha-1 y-1) (kg CO2eqvt Mg grain-1)
No-Till 1110 b 7.25 a 153 a
Chisel Till 2348 a 7.14 a 330 a
Organic -872 c 5.12 b -169 b
Cavigelli et al. 2009

17. California Annual Grassland
Compost Addition Study
Change in System Carbon due to Enhanced Production
1800
1600
Compost carbon
1400 Change in system carbon
1200
g C m-2
1000
800
600
400
200
0
2008 2012 2016 2020 2024 2028
The change in system carbon is the total system carbon from the compost simulation
minus the compost carbon. This represents the carbon sequestration due to
enhanced plant production.
Updated 13 July 2011

18. California Annual Grassland
Compost Addition Study

19. Current US Farming Practices and Mitigation Outlook
50% employ conservation tillage or no till
Small minority use improved fertilizers
Most farmers do not greatly exceed recommended rates
Slow release fertilizers and nitrification inhibitors are
more effective in the drier western US
Practices that reduce direct N2O are also likely to reduce
indirect N2O
Urease inhibitors may increase direct N2O
Model results suggest that rates could be reduced with
stabilized N sources, but need observations to verify

20. Jevon’s Paradox
• Many assume that increases in efficiency will solve our problems
• Is increasing yield but keeping emissions constant (or decreasing) the same
as deceasing emissions but keeping yield constant (or increasing)?
• Probably not – as efficiency increases people tend to consume more
• So absolute emissions will continue to increase

21. Summary and Issues
• Currently available technologies can reduce emissions (in at least some
regions) but incentives are needed
• at small scales, fluxes can be measured precisely and accurately, but models
usually do better at large scales.
• Benefits of stabilized N likely scale up to the farm system level and include
improved water and air quality
• However, entity level observations are not feasible and model uncertainty at
the farm scale is huge
• If GHG intensity is minimized then yields would plummet
• If increasing NUE is the goal then BAU is sufficient
• If the goal is to increase yields and decrease absolute emissions then things
get challenging
• Need to define life cycle analysis boundaries carefully