Soil N2O Emissions

Methods 5.0

on-farm
non-mechanical
Direct and indirect nitrous oxide emissions based on an IPCC Tier 1 method.
Published

September 22, 2025

Introduction

Nitrous oxide (N2O) is a potent greenhouse gas with a global warming potential approximately 273 times that of CO2. Much agronomic research has therefore been conducted to understand the factors that influence N2O emissions from cropland soils because factors that can be managed through farming practice changes could potentially lead to greater GHG emission reductions.

ARS soil scientists collect gas samples from chambers used to measure flux of nitrous oxide from experiments in a corn field in Minnesota. Photo: Stephen Ausmus, USDA ARS.

ARS soil scientists collect gas samples from chambers used to measure flux of nitrous oxide from experiments in a corn field in Minnesota. Photo: Stephen Ausmus, USDA ARS.

Methods

For v5 of the Fieldprint Calculator, we revised the soil N2O method with the method from Ogle et al. (2024). The method outlined here is based on an adaptation of the IPCC Tier 1 method with emission and scaling factors to address management factors, input type, and climate. The method includes direct and indirect emissions of nitrous oxide (N2O).

Direct emissions

According to Ogle et al. (2024), N2O is directly emitted from soils where there are nitrogen additions (such as mineral or organic fertilization) or management practices that influence nitrogen mineralization from soil organic matter.

\[ [N_2O]^{direct} = [N_2O]^{input} \times [N_2O]^{mw} \]

where:

  • \([N_2O]^{input}\) = annual soil N2O emissions from nitrogen inputs to the land parcel (kg N2O-N)
  • \([N_2O]^{mw}\) = ratio of molecular weights of N2O to N2O-N = 44/28

The \([N_2O]^{input}\) is calculated from the fertilizer application data and factors using these equations.

\[ \begin{align} [N_2O]^{synthetic} &= [ F_{sn} \times EF_{sn} \times (1 + S_{sr}) \times (1 + S_{inh}) ] \\ [N_2O]^{organic} &= [(F_{on} + F_{cr}) \times EF_{on} ] \\ [N_2O]^{animal} &= (F_{prp} \times EF_{prp}) \\ [N_2O]^{input} &= ([N_2O]^{synthetic} + [N_2O]^{organic} + [N_2O]^{animal}) \times (1 + S_{till}) \times (1 + S_{bc}) \end{align} \]

where:

  • \(F_{sn}\) = synthetic fertilizer nitrogen inputs to the land parcel (kg N)
  • \(EF_{sn}\) = emission factor for synthetic nitrogen input to soils (kg N2O-N /kg N)
  • \(S_{sr}\) = scaling factor for slow-release fertilizers, 0 where no effect (dimensionless)
  • \(S_{inh}\) = scaling factor for nitrification inhibitors, 0 where no effect (dimensionless)
  • \(F_{on}\) = organic fertilizer/manure nitrogen inputs to the land parcel (kg N)
  • \(F_{cr}\) = crop residue and forage renewal nitrogen inputs to the land parcel (kg N)
  • \(EF_{on}\) = emission factor for other nitrogen inputs, i.e., organic fertilizer/manure and crop/forage residue nitrogen input to soils (kg N2O-N [kg N]-1)
  • \(F_{prp}\) = manure nitrogen deposited directly onto the land parcel by livestock (kg N)
  • \(EF_{prp}\) = emission factor for manure deposited directly onto the land parcel by the livestock (kg N2O-N [kg N]-1)
  • \(S_{till}\) = scaling factor for no-tillage, 0 except for no-till (dimensionless)
  • \(S_{bc}\) = scaling factor for biochar addition—mineral soils only, 0 with no addition or organic soils (dimensionless)

The total amount of N from synthetic fertilizer \(F_{sn}\) is calculated by:

\[ F_{sn} = A \times \sum_i^n ( R^{sn}_i \times N^{prop}_i) \]

where:

  • \(R^{sf}_i\) = is the rate of synthetic ith fertilizer (kg fertilizer [ha-1])
  • \(N^{prop}_i\) = is the proportion of N in the fertilizer ith (kg N [kg fertilizer]-1)
  • \(A\) = is the area of the field (ha)

The total amount of N from organic fertilizer \(F_{on}\) is calculated by:

\[ F_{on} = A \times \sum_i^n ( R^{of}_i \times N^{prop}_i) \]

where:

  • \(R^{of}_i\) = is the rate of organic fertilizer ith (kg fertilizer [ha-1])
  • \(N^{prop}_i\) = is the proportion of N in the fertilizer ith (kg N [kg fertilizer]-1)
  • \(A\) = is the area of the field (ha)

The total amount of N from crop residue \(F_{cr}\) calculated by:

\[ \begin{align} F_{cr} &= CRN_a + CRN_b \\ CRN_b &= CBa \times (1 + R) \times N_b \\ CRN_a &= (CB_a - Y \times A \times DM \times N_a) \times (1-R_m) \\ CB_a &= (Y \times HI^{-1}) \times A \times DM \end{align} \]

where:

  • \(CRN_a\) = aboveground crop and forage renewal residue inputs to the land parcel (kg N)
  • \(CRN_b\) = belowground crop and forage renewal residue inputs to the land parcel (kg N)
  • \(CB_a\) = aboveground crop and forage biomass in dry matter units (kg of dry matter)
  • \(R\) = aboveground biomass to belowground biomass (root-to-shoot) ratio (kg belowground dry matter [kg aboveground dry matter]-1)
  • \(N_b\) = N content in the belowground residue (kg N [kg dry matter]-1)
  • \(Y\) = fresh weight of crop harvest yield or peak grazing land forage amount (kg ha-1)
  • \(A\) = area of a parcel of land (ha)
  • \(N_a\) = N content in the aboveground residue (kg N [kg dry matter]-1)
  • \(R_m\) = proportion of crop or forage residue removed by burning, grazing, or harvesting residues (kg dry matter removed [kg dry matter produced]-1)
  • \(HI\) = harvest index: ratio of crop yield or forage removal to total aboveground biomass (kg biomass [kg yield]-1)
  • \(DM\) = dry matter content of harvested crop biomass or forage (kg dry matter [kg biomass]-1)

Indirect emissions

Indirect emissions occur when reactive nitrogen is volatilized as NH3 or NOx or transported via surface runoff or leaching in soluble forms from cropland or grazing lands where nitrogen additions are occurring, or management practices are influencing nitrogen mineralization from organic matter.

\[ [N_2O]^{indirect} = ([N_2O]^{vol} + [N_2O]^{leach} ) \times [N_2O]^{mw} \]

  • \([N_2O]^{vol}\) = N2O emitted by the ecosystem receiving volatilized nitrogen (kg N2O-N)
  • \([N_2O]^{leach}\) = N2O emitted by ecosystem receiving leached and runoff nitrogen (kg N2O-N)
  • \([N_2O]^{mw}\) = ratio of molecular weights of N2O to N2O-N = 44/28 (kg N2O [kg N2O-N]-1)

The \([N_2O]^{vol}\) is calculated from the fertilizer application data and factors using this equation:

\[ [N_2O]^{vol} = \{ (F_{sn} \times FR_{sn}) + [ ( F_{on} + F_{prp}) \times FR_{on} ] \} \times EF_{vol} \]

where:

  • \(F_{sn}\) = synthetic nitrogen fertilizer applied (kg N)
  • \(FR_{sn}\) = fraction of synthetic nitrogen (NSN) that volatilizes as NH3 and NOx (kg N [kg N in synthetic fertilizer]-1)
  • \(F_{on}\) = nitrogen fertilizer applied of organic origin, including manure, sewage sludge, compost, and other organic amendments (kg N)
  • \(F_{prp}\) = manure nitrogen deposited directly onto the land parcel by livestock (kg N)
  • \(FR_{on}\) = fraction or proportion of \(F_{on}\) that volatilizes as NH3 and NOx (kg N [kg N in organic fertilizer]-1)
  • \(EF_{vol}\) = emission factor for volatilized nitrogen or proportion of nitrogen volatilized as NH3 and NOx that is transformed to N2O in receiving ecosystem (kg N2O-N [kg N]-1)

And the \([N_2O]^{leach}\) is calculated from the fertilizer application data and factors using this equation:

\[ [N_2O]^{leach} = ([N_2O]^{input} \times FR_{leach}) \times EF_{leach} \]

where:

  • \([N_2O]^{input}\) = nitrogen inputs, including mineral fertilizer, organic amendments, manure nitrogen deposited by livestock, and residues (kg N)
  • \(FR_{leach}\) = fraction of nitrogen inputs (\([N_2O]^{input}\)) that is leached or runs off the land parcel (kg N [kg N in nitrogen inputs]-1)
  • \(EF_{leach}\) = proportion of leached and runoff nitrogen that is transformed to N2O in the receiving ecosystem (kg N2O-N [kg N]-1)

Total emissions

Finally, the total N2O emissions are the sum of direct and indirect emissions.

\[ [N_2O]^{total} = [N_2O]^{direct} + [N_2O]^{indirect} \]

  • \([N_2O]^{total}\) = the annual total \([N_2O]\) emissions (kg N2O)
  • \([N_2O]^{direct}\) = the annual total direct \([N_2O]\) emissions (kg N2O)
  • \([N_2O]^{indirect}\) = the annual total indirect \([N_2O]\) emissions (kg N2O)

Conversion N2O to CO2e

Finally, all N2O emissions can be converted to CO2e by applying the Global Warming Potential factor for N2O.

\[ \begin{align} [CO_2\text{e}]^{total} &= [N_2O]^{total} \times [N_2O]^{gwp} \\ [CO_2\text{e}]^{direct} &= [N_2O]^{direct} \times [N_2O]^{gwp} \\ [CO_2\text{e}]^{indirect} &= [N_2O]^{indirect} \times [N_2O]^{gwp} \\ \end{align} \]

where \([CO_2\text{e}]\) = annual \([N_2O]\) emissions expressed in (kg CO2e)

Constants and factors required for this method

The following values are considered constants for the N2O method.

symbol name value Units
N2O_MW ratio of molecular weight of N2O to N2O-N 44/28 kg N2O [kg N2O-N]-1
FR_on Fraction of nitrogen in organic amendments (excluding crop residues) and PRP nitrogen (FON,PRP) that volatilizes as NH3 and NOx 0.21 dimensionless
EF_leach Proportion of leached and runoff nitrogen that is transformed to N2O in the receiving ecosystem (metric tons N2O-N/metric tons N) 0.011 dimensionless
S_bc biochar scaling factor -0.23 dimensionless

Climate classifications by county

Climates are divided into the following categories.
class region condition
Wet/Mesic Temperate/Boreal ratio of mean annual precipitation to potential evapotranspiration is greater than 0.8
Wet/Mesic Tropical/Subtropical mean annual precipitation greater than 1000 mm
Arid/Semi-arid Temperate/Boreal ratio of mean annual precipitation to potential evapotranspiration is lower or equal than 0.8
Arid/Semi-arid Tropical/Subtropical mean annual precipitation lower or equal than 1000 mm

The U.S. counties from the 48 contiguous states were grouped into these two broad classes using the Koppen-Geiger climate classification map at the county level provided by Kottek et al. (2006).

Climate classifications by county. Sample of 5 records shown.
state county cls climate
Washington Grant BSk Arid/Semi-arid
Nevada Humboldt BSk Arid/Semi-arid
New Mexico Quay BSk Arid/Semi-arid
Texas Castro BSk Arid/Semi-arid
Texas Reagan BSh Arid/Semi-arid
California Contra Costa Csb Wet/Mesic
Minnesota Wabasha Dfb Wet/Mesic
Illinois Iroquois Dfa Wet/Mesic
Texas Bexar Cfa Wet/Mesic
Louisiana Natchitoches Cfa Wet/Mesic

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N content and volatilization fraction from fertilizers

N content and volatilization fraction from fertilizers. Sample of 10 records shown.
Source Detail Nprop FR_sn
Potassium nitrate 0.138 0.01
Calcium ammonium nitrate (green ammonia) 0.270 0.05
Ammonium nitrate 0.350 0.05
Ammonium sulfate 0.210 0.08
Ammonia (aqueous) 0.200 0.08
Calcium ammonium nitrate 0.270 0.05
Urea 0.460 0.15
Diammonium phosphate (green ammonia) 0.180 0.08
Ammonia (green) 0.820 0.08
US average nitrogen fertilizer 1.000 0.10

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Manure N content

Manure total N content for different regions, sources, and types. Sample of records shown.
region animal_category moisture_designation pct_as_applied
Midwest Dairy Semi-solid 0.330
Northeast Swine Slurry 0.550
Northern Plains Poultry Semi-solid 0.688
Pacific Northwest Beef Slurry 0.490
Southeast Swine Solid 0.640
Southern Plains Swine Slurry 0.166
Southwest Swine Slurry 0.650

Data derived from manureDB (Bormann et al. 2024).

N content from other organic fertilizers

fert_type Nprop
Green manure 0.0325
Compost 0.0125
Sewage sludge/Biosolids 0.0300

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N content from crop residues

N content from residues of 5 crops shown.
crop N_a N_b
Alfalfa 0.027 0.019
Barley 0.007 0.014
Chickpeas (garbanzos) 0.008 0.008
Corn (grain) 0.006 0.007
Corn (silage) 0.006 0.007

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Harvest indices and dry matter contents

Harvest indices and dry matter contents for a sample of 5 crops.
crop DM HI R
Alfalfa 0.880 0.95 0.87
Barley 0.855 0.46 0.11
Chickpeas (garbanzos) 0.840 0.46 0.08
Corn (grain) 0.845 0.53 0.18
Corn (silage) 0.350 0.95 0.18

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Leaching fraction factors

cover_crop FR_leach
Without cover crops 0.24
With leguminous cover crops 0.18
With non-leguminous cover crops 0.09

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N2O Global Warming Potential

Values for N2O global warming potential (GWP).
symbol Assessment Report (AR) Time Horizon value units
N2O_GWP AR6 100-yr 273 kg CO2-eq/kg N2O
N2O_GWP AR5 (with climate-carbon feedback) 100-yr 298 kg CO2-eq/kg N2O
N2O_GWP AR5 (without climate-carbon feedback) 100-yr 265 kg CO2-eq/kg N2O
N2O_GWP AR4 100-yr 298 kg CO2-eq/kg N2O
N2O_GWP AR6 20-yr 273 kg CO2-eq/kg N2O
N2O_GWP AR5 (with climate-carbon feedback) 20-yr 268 kg CO2-eq/kg N2O
N2O_GWP AR5 (without climate-carbon feedback) 20-yr 264 kg CO2-eq/kg N2O
N2O_GWP AR4 20-yr 289 kg CO2-eq/kg N2O

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References

Bormann, Nancy Bohl, Erin Cortus, Melissa Wilson, Kevin Silverstein, Larry Gunderson, and Kevin Janni. 2024. “ManureDB-National Database of Manure Nutrient Content and Other Characteristics: 1998-2023.”
Kottek, Markus, Jürgen Grieser, Christoph Beck, Bruno Rudolf, and Franz Rubel. 2006. “World Map of the Köppen-Geiger Climate Classification Updated.” Meteorologische Zeitschrift 15 (3): 259–63. https://doi.org/10.1127/0941-2948/2006/0130.
Ogle, Stephen M, Paul R Adler, Gary Bentrup, Justin Derner, Grant Domke, Stephen Del Grosso, Johannes Lehmann, Michele Reba, and Dominic Woolf. 2024. “Chapter 3: Quantifying Greenhouse Gas Sources and Sinks in Cropland and Grazing Land Systems.” In: Hanson, Wes L.; Itle, Cortney; Edquist, Kara, Eds. Quantifying Greenhouse Gas Fluxes in Agriculture and Forestry: Methods for Entity-Scale Inventory. Technical Bulletin Number 1939, 2nd Edition. Washington, DC: US Department of Agriculture, Office of the Chief Economist. 6-1-6-23. Chapter 3. 1939: 31.