Emissions from Managed Soils

The IPCC (2019) methodology for direct N₂O emissions from managed soils accounts for N₂O from three sources:

1. Nitrogen inputs, including synthetic fertilizers, organic fertilizers and manure applied to croplands; crop residues; and N mineralized from soil organic matter due to changes in land management.
2. Drained organic soils: organic soils (peatlands) are rich in N, some of which are lost when brought under cultivation.
3. Manure (urine and dung) is applied to grazing lands.

The basic equation is:

• Emissions of N₂O_Direct = annual direct N₂O–N emissions produced from managed soils, kg N₂O–N yr^-1
• N₂O–N _inputs = annual direct N₂O–N emissions from N inputs to managed soils, kg N₂O–N yr^-1 (see equation below)
• N₂O–N_OS = annual direct N₂O–N emissions from managed organic soils, kg N₂O–N yr^-1 (see equation below)
• N₂O–N_PRP = annual direct N₂O–N emissions from urine and dung inputs to grazed soils, kg N₂O–N yr^-1 (see equation below)

• F_SN = annual amount of synthetic fertilizer N applied to soils, kg N yr^-1
• F_ON = annual amount of animal manure, compost, sewage sludge and other organic N additions applied to soils, kg N yr^-1
• F_CR = annual amount of N in crop residues (above-ground and below-ground), including N-fixing crops, and from forage/pasture renewal, returned to soils, kg N yr^-1
• F_SOM = annual amount of N in mineral soils that is mineralized, in association with loss of soil C from soil organic matter as a result of changes to land use or management, kg N yr^-1

• F_OS = annual area of managed/drained organic soils, ha
  • CG: cropland and grassland
  • Temp: temperate
  • Trop: tropical
  • NR: nutrient-rich
  • NP: nutrient-poor
• EF₁ = emission factor for N₂O emissions from N inputs, kg N₂O–N (kg N input)^-1 EF_1FR is the emission factor for N₂O emissions from N inputs to flooded rice, kg N₂O–N (kg N input)^-1
• EF₂ = emission factor for N₂O emissions from drained/managed organic soils, kg N₂O–N (kg N input)^-1

• F_PRP = annual amount of urine and dung N deposited by grazing animals on pasture, range and paddock, kg N yr^-1
• EF_3PRP = emission factor for N₂O emissions from urine and dung N deposited on pasture, range and paddock by grazing animals, kg N₂O–N (kg N input)^-1
  • CPP: cattle, poultry and pigs
  • SO: sheep and other animals

All default emission factors are provided in Chapter 11 Table 11.1 of the 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories.

In addition to N₂O emissions directly from the soils to which N was applied, N₂O emissions can also occur via two indirect pathways;

1. Volatilization: N applied to soils can be volatilized as NH₃ and oxides of N (NO_x), which is then deposited on surface waters. Once deposited, these forms of N can then be transformed and emitted as N₂O, through the same nitrification and denitrification processes that occur in agricultural soils.
2. Leaching and runoff: N, in the form of NH₄ and NO₃^– can also be transported via leaching and runoff, and emitted as N₂O at other locations.

N₂O from volatilization

• Emissions of N₂O_{Indirect(ATD)}–N = annual indirect amount of N₂O–N emissions produced from atmospheric deposition of N volatilized from managed soils, kg N₂O–N yr^-1
• FS_N = annual amount of synthetic fertilizer N applied to soils, kg N yr^-1
• Frac_GASF = fraction of synthetic fertilizer N that volatilizes as NH₃ and NO_x, kg N volatilized (kg of N applied)^-1
• F_ON = annual amount of managed animal manure, compost, sewage sludge and other organic N additions applied to soils, kg N yr^-1
• F_PRP = annual amount of urine and dung N deposited by grazing animals on pasture, range and paddock, kg N yr^-1
• Frac_GASM = fraction of applied organic N fertilizer materials (F_ON) and of urine and dung N deposited by grazing animals (F_PRP) that volatilizes as NH3 and NO_x, kg N volatilized (kg of N applied or deposited)^-1
• EF₄ = emission factor for N₂O emissions from atmospheric deposition of N on soils and water surfaces, [kg N–N₂O (kg NH₃–N + NO_x–N volatilized)^-1]

N₂O from leaching and runoff

• Emissions of N₂O_Indirect(L)–N = annual indirect amount of N₂O–N emissions produced from leaching and runoff of N additions to managed soils in regions where leaching/runoff occurs, kg N₂O–N yr^-1
• F_SN = annual amount of synthetic fertilizer N applied to soils in regions where leaching/runoff occurs, kg N yr^-1
• F_ON = annual amount of managed animal manure, compost, sewage sludge and other organic N additions applied to soils in regions where leaching/runoff occurs, kg N yr^-1
• F_PRP = annual amount of urine and dung N deposited by grazing animals in regions where leaching/runoff occurs, kg N yr^-1
• F_CR = amount of N in crop residues (above- and below-ground), including N-fixing crops, and from forage/pasture renewal, returned to soils annually in regions where leaching/runoff occurs, kg N yr^-1
• F_SOM = annual amount of N mineralized in mineral soils associated with loss of soil C from soil organic matter as a result of changes to land use or management in regions where leaching/runoff occurs, kg N yr^-1
• FRAC_LEACH-(H) = fraction of all N added to/mineralized in managed soils in regions where leaching/runoff occurs that is lost through leaching and runoff, kg N (kg of N additions)^-1
• EF₅ = emission factor for N₂O emissions from N leaching and runoff, kg N₂O–N (kg N leached and runoff)^-1

Default values for FRAC_GASF, F_RACGASM, FRAC_LEACH-(H), EF₄ and EF₄ are provided in Chapter 11 Table 11.3 of the 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories.

Equations for estimating indirect N₂O emissions make use of the same activity data as those for direct N₂O emissions, with the additional requirement of identifying soils where runoff and leaching likely occur.

Conversion of N₂O–N emissions to N₂O emissions

For reporting purposes, the conversion of N₂O–N emissions to N₂O emissions is performed by using the following equation:

CO₂ emissions from liming

Liming is the application of a calcium-containing inorganic material used to reduce soil acidity and improve plant growth in agricultural systems. Common compounds used to reduce acidity include lime (e.g., calcium carbonate (CaCO₃), or dolomite (CaMg(CO₃)₂). Both of these products are carbonates, which lead to CO₂ emissions as the carbonate dissolves and releases bicarbonate (2HCO₃^–), which evolves into CO₂ and water (H₂O).

• CO₂–C emissions = annual C emissions from lime application, tons C yr^-1
• M = annual amount of calcium carbonate (CaCO₃) or dolomite (CaMg(CO₃)₂), tonnes yr^-1
• EF = emission factor, ton of C (e.g. calcium limestone or dolomite) ^-1

Default emission factors (EF) are 0.12 for limestone and 0.13 for dolomite (IPCC, 2006; Chapter 11.3).

Therefore, the only activity data required are the amounts of calcium carbonate or dolomite applied annually to soils in the country or area under consideration.

CO₂ emissions from urea fertilization

Besides emissions of N₂O, adding urea fertilizer to soils also releases CO₂ that was fixed during the production of the urea product. Urea (CO(NH₂)₂) is converted into ammonium (NH₄⁺), hydroxyl ion (OH^–), and bicarbonate (HCO₃^–), in the presence of water and urease enzymes. Similar to the soil reaction following the addition of lime, bicarbonate that is formed evolves into CO₂ and water.

As with liming, the only activity data required is the amount of urea applied annually to soils in the country or area under consideration. Default emission factors (EF) are 0.20 for urea (IPCC, 2006; Chapter 11.4).

Conversion of CO₂–C emissions to CO₂ emissions

For reporting purposes, the conversion of CO₂–C emissions to CO₂ emissions is performed by using the following equation:

The Tier 2 methodology for estimating direct N₂O emissions is identical to Tier 1, but a Tier 2 approach should use country-specific emission factors and disaggregate the N inputs according to different types and conditions under which they are used. For example, rather than using a single estimate of synthetic N fertilizer used in the country, a Tier 2 approach might divide the country into zones according to soil types and climate, estimate the N applied to cropland in each zone, and sum the emissions from each zone. This requires similarly disaggregated emission factors.

As with direct N₂O emissions, the methodology is identical to Tier 1, with disaggregation by climate or soil conditions.

CO₂ emissions from liming

The Tier 2 approach for estimating CO₂ emissions from liming uses country-specific emissions factors, but the activity data are identical to Tier 1.

CO₂ emissions from urea fertilization

The Tier 2 approach for estimating CO₂ emissions from urea fertilization uses country-specific emissions factors, but the activity data are identical to Tier 1.