Agricultural Soil N2O Emissions in the US
Greenhouse Gas Inventory:
A Comparison of Methodologies
Margaret Walsh1, Steve Del Grosso2, and Tom Wirth3
1
ICF Consulting
2
USDA Agricultural Research Service
3
Non-CO2 Gases and Sequestration Branch, US EPA (Formerly)
Overview
■
The US GHG Inventory
■
Ag N2O
■
Evolution of Methodologies
 IPCC’s Default Method
 DAYCENT Simulation
■
Differences
 Inputs
 Emission & Partitioning Factors
■
Conclusions
Inventory of U.S. Greenhouse Gas
Emissions and Sinks
■
Identifies and quantifies the US’s primary anthropogenic GHG
sources and sinks
■
Commitment under the UN Framework Convention on Climate
Change
■
Published annually
■
Undergoes extensive review process (internal, multi-agency,
expert, public, international, UN)
■
2005 Review Draft Available Online at:
http://yosemite.epa.gov/oar/globalwarming.nsf/content/Resource
CenterPublicationsGHGEmissionsUSEmissionsInventory2005.html
■
Final available at the same URL on 15 April
2003 US GHG Inventory Sources
Industrial Waste
Processes (2.7%)
(4.5%)
Agriculture
(6.3%)
LUCF
(0.1%)
Solvents
(0.1%)
Energy
(86.4%)
Total Gross Emissions: 6,900.2 Tg CO2 Eq.
Note: Total does not sum to 100% due to independent rounding.
2003 Agriculture Sector Emissions
Manure
Management
(N2O and CH4)
13.1%
Rice
Cultivation
(CH4) 1.6%
Agricultural
Residue
Burning (N2O
and CH4)
0.3%
Enteric
Fermentation
(CH4) 26.5%
Total Emissions: 433.3 Tg CO2 Eq.
Note: Total does not sum to 100% due to independent rounding.
Agricultural
Soil
Management
(N2O) 58.5%
In 2005, we’ve moved from a Tier 1 to a Tier 3
Methodology
Tier 1: IPCC Default Methods using Nationally Available
Data
Tier 3: Simulation Modeling, using a Spatially &
Temporally Explicit Representations of Biogeochemical
Processes
IPCC Method
Synthetic N
Manure N
Mineral
Soils
1.25%
Direct Emissions
Indirect Emissions
NH3 & NOx Volatilization (10% Synthetic N, 20% Manure N)
NO3- Leaching/Runoff (30%)
Organic
Soils
Temperate:
8 Tg N2O/ha
Subtopics:
12 Tg N2O/ha
Atmospheric N2O
2.5%
1%
Tier 3 Simulation: DAYCENT
■
Process
 County-Level N Inputs, Soil Info, Climate Data
 Daily Timestep
■
Results
 Calculates Volatilization and Leaching Based on Immediate
Conditions
 Does not Distinguish Organic vs. Mineral Soils
 Does not Distinguish N Sources
How do the differences in calculation
methodologies lead to different
results?
U.S. Cropland Soil N2O Emissions
300
IPCC
200
(Tg CO2 Eq)
N2O Emissions
250
DAYCENT
150
100
Average Difference = 14.2%
50
0
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003
Year
N Inputs
■
IPCC: 1.25% EF is applied to all consumed N fertilizer
and reported within the Ag Sector, regardless of
application.
■
DAYCENT: Fertilizer application rates are considered
by crop and geographic region. 90% of consumed
fertilizer is accounted for in this way.
■
The residual is applied (primarily) to settlement soils,
and (secondarily) to forest soils.
■
Isolated, the net effect would be to reduce emissions.
U.S. Managed Soil N2O Emissions
300
200
(Tg CO2 Eq)
N2O Emissions
250
IPCC
DAYCENT
150
100
Average Difference = 11.7%
50
0
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003
Year
Emission Factors
1.3%
1.2%
IPCC EF: 1.25%
(g N 2O/g Fertilizer N)
Implied EF
1.1%
1.0%
0.9%
0.8%
DAYCENT EF wrt Total Added N
Average EF = 0.95%
0.7%
0.6%
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003
Year
Nitrate Leaching & Runoff
30%
(g N leached/g added N)
Leaching & Runoff Proportion
35%
IPCC L&R Factor: 30%
25%
20%
15%
DAYCENT L&R Factor wrt Total N
Avg = 20.9%
10%
1990
1991
1992
1993
1994
1995
1996
1997
Year
1998
1999
2000
2001
2002
2003
NOx & NH3 Volatilization
25%
(g NOx- & NH 3-N/g added N)
Volatilization
20%
IPCC Volatilization Factor for Manure N (20%)
15%
10%
5%
0%
IPCC Volatilization Factor for Synthetic N (10%)
DAYCENT Vol Factor for Synth N (Avg = 2.6%)
DAYCENT Vol Factor for Manure N (Avg = 2.0%)
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003
Year
What do these Differences Mean?
Variable
N Inputs
Leaching & Runoff
Volatilization
Emission Factor
DAYCENT vs. IPCC
Net Effect
In Conclusion
■
Nationally, Ag Soil N2O is a Key Category.
■
IPCC Guidelines are being revised now.
Revised drivers (indirect fractionation & EFs) should be
considered.
■
For more info: [email protected]