Agricultural Soil N 2 O Emissions in the US Greenhouse Gas Inventory: A Comparison of Methodologies Margaret Walsh 1, Steve Del Grosso 2, and Tom Wirth 3 1 ICF Consulting 2 USDA Agricultural Research Service 3 Non-CO 2 Gases and Sequestration Branch, US EPA (Formerly)

Overview ■ The US GHG Inventory ■ Ag N 2 O ■ 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: CenterPublicationsGHGEmissionsUSEmissionsInventory2005.html ■ Final available at the same URL on 15 April

2003 US GHG Inventory Sources Total Gross Emissions: 6,900.2 Tg CO 2 Eq. Note: Total does not sum to 100% due to independent rounding.

2003 Agriculture Sector Emissions Total Emissions: Tg CO 2 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 N 2 O Direct EmissionsIndirect Emissions Temperate: 8 Tg N 2 O/ha Subtopics: 12 Tg N 2 O/ha 1.25% NH 3 & NO x Volatilization (10% Synthetic N, 20% Manure N) NO 3 - 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 N 2 O Emissions 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 N 2 O Emissions IPCC DAYCENT Average Difference = 11.7%

Emission Factors IPCC EF: 1.25% DAYCENT EF wrt Total Added N Average EF = 0.95%

Nitrate Leaching & Runoff IPCC L&R Factor: 30% DAYCENT L&R Factor wrt Total N Avg = 20.9%

NO x & NH 3 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? VariableDAYCENT vs. IPCCNet Effect N Inputs Leaching & Runoff Volatilization Emission Factor

In Conclusion ■ Nationally, Ag Soil N 2 O is a Key Category. ■ IPCC Guidelines are being revised now. Revised drivers (indirect fractionation & EFs) should be considered. ■ For more info: