Agricultural Soil N2O Emissions in the US Greenhouse Gas Inventory:

A Comparison of Methodologies

Margaret Walsh1, Steve Del Grosso2, and Tom Wirth3

1 ICF Consulting2 USDA Agricultural Research Service3 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/ResourceCenterPublicationsGHGEmissionsUSEmissionsInventory2005.html

■ Final available at the same URL on 15 April

2003 US GHG Inventory Sources

Total Gross Emissions: 6,900.2 Tg CO2 Eq.

Note: Total does not sum to 100% due to independent rounding.

Waste (2.7% )

Industrial Processes (4.5% )

LUCF (0.1% )

Solvents (0.1% )Agriculture

(6.3% )

Energy (86.4% )

2003 Agriculture Sector Emissions

Agricultural Soil

Management (N2O) 58.5%

Enteric Fermentation (CH4) 26.5%

Manure Management (N2O and CH4)

13.1%

Rice Cultivation (CH4) 1.6%

Agricultural Residue

Burning (N2O

and CH4) 0.3%

Total Emissions: 433.3 Tg CO2 Eq.Note: Total does not sum to 100% due to independent rounding.

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

Manure N

Synthetic N

Mineral Soils Organic

Soils

Atmospheric N2O

Direct Emissions

Indirect Emissions

Temperate:8 Tg N2O/haSubtopics:

12 Tg N2O/ha

1.25%

NH3 & NOx Volatilization (10% Synthetic N, 20% Manure N)

NO3- Leaching/Runoff (30%)

1%

2.5%

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

0

50

100

150

200

250

300

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Year

N2O

Em

iss

ion

s(T

g C

O2

Eq

)

IPCC

DAYCENT

Average Difference = 14.2%

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

0

50

100

150

200

250

300

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Year

N2O

Em

issi

on

s(T

g C

O2

Eq

)

IPCC

DAYCENT

Average Difference = 11.7%

0.6%

0.7%

0.8%

0.9%

1.0%

1.1%

1.2%

1.3%

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Year

Imp

lied

EF

(g N

2O/g

Fe

rtili

ze

r N

)Emission Factors

IPCC EF: 1.25%

DAYCENT EF wrt Total Added N Average EF = 0.95%

10%

15%

20%

25%

30%

35%

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Year

Lea

chin

g &

Ru

no

ff P

rop

ort

ion

(g N

lea

ch

ed

/g a

dd

ed

N)

Nitrate Leaching & Runoff

IPCC L&R Factor: 30%

DAYCENT L&R Factor wrt Total N Avg = 20.9%

0%

5%

10%

15%

20%

25%

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Year

Vo

latil

izat

ion

(g N

Ox-

& N

H3-

N/g

ad

de

d N

)NOx & NH3 Volatilization

IPCC Volatilization Factor for Manure N (20%)

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%)

What do these Differences Mean?

Variable DAYCENT vs. IPCC

Net Effect

N Inputs

Leaching & Runoff

Volatilization

Emission Factor

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: mwalsh@icfconsulting.com