Animal associated soil N2O emissions

Summary

This methodology represents nitrous oxide (N₂O) emissions associated with the deposition of urine and dung-sourced nitrogen to managed soils. The data and calculation methodology is sourced from the IPCC, as published in Volume 4, Chapter 11 - N2O Emissions from Managed Soils, and CO2 Emissions from Lime and Urea Application of their 2006 IPCC Guidelines for National Greenhouse Gas Inventories.

The methodology

Emissions model

Nitrous oxide (N₂O) is produced naturally in soils through the processes of nitrification and denitrification. Nitrification is the aerobic (microbially-mediated) oxidation of ammonia to nitrate, and denitrification is the anaerobic microbial reduction of nitrate to nitrogen gas (N₂). N₂O is a gaseous intermediate in the reaction sequence of denitrification and a by-product of nitrification. One of the main controlling factors in this reaction is the availability of inorganic nitrogen in the soil. Human-attributable N₂O emissions therefore arise where net nitrogen (N) additions are made to managed soils in the form of synthetic or organic fertilizers, manure, crop residues, or sewage sludge, all of which contain forms of nitrogen. This methodology describes N₂O emissions associated with the addition of N to managed soils via the urine and dung of grazing animals.

Both direct and indirect emission pathways of urine and dung-N (i.e. urine and dung-associated nitrogen) are represented in the methodology. Direct N₂O-N (nitrous oxide associated nitrogen) production is calculated on the basis of conversion factors which describe the rate at which N₂O-N is produced in relation to the quantity of urine and dung-N added to the soil. Indirect emissions occur through volatilisation and/or leaching of urine and dung-N from the soil. These effects are each represented by (1) a factor describing the typical fraction of urine and dung-N which is volatilised/leached; and (2) a factor describing the rate at which this N is converted into N₂O-N.

The total production of N₂O-N attributable to a particular quantity of added N is calculated by adding together all direct and indirect quantities. This is then converted into an N₂O emission quantity on the basis of the relative molecular masses of N₂O and N.

This methodology represents the IPCC Tier 1 approach.

Model data

The rate at which N₂O is emitted from urine and dung-N varies with livestock type (in relation to factors such as the concentration of urine in patches, and soil compaction). As such, conversion factors for 5 livestock types are provided within this methodology (e.g., cattle, poultry, sheep).

Each livestock type is represented by a direct urine and dung-N to N₂O-N conversion rate (kg / kg), as well as the common factors for indirect (volatilised/leached) conversion rates. In addition, this methodology uses two general constants for deriving emissions quantities from N₂O-N:

• molecular mass ratio of N₂O and N: the factor for converting a N₂O-N quantity into N₂O (44/28)

• global warming potential of N₂O: the factor for converting a N₂O emissions quantity into CO₂e - the quantity of CO₂ which would exert the same atmospheric warming effect.

Activity data required

N₂O emissions are directly proportionate to the quantity of urine and dung-N added, which therefore needs to be specified in order to make an emissions calculation. Indirect emissions associated with leaching can be incorporated optionally, if appropriate to the scenario under consideration.

Calculation and results

This emissions calculated by this methodology represent those attributable to the specified quanity of urine and dung-N added to managed soils.

The methodology calculates two emissions quantities: (1) the absolute quantity of N₂O; and (2) N₂O emissions expressed in terms of CO₂e.

Related methodologies

N₂O emissions associated with livestock manure are also covered in the IPCC manure-management methodologies for cattle and swine, poultry, fur-bearing and other livestock. These methodologies focus on a large variety of specific practices for storing and treating manure, but include manure freely deposited on rangelands (the 'pasture/range/paddock' option) which is directly analogous to this methodology. The main difference is that the manure-management methodology calculates emissions on the basis of livestock population or liveweight and thus may be useful in those cases where the mass of added manure-N is not known explicitly.

In addition, a number of other methodologies focus on emissions from managed soils. These include those arising from the addition of urea, carbonates/lime and synthetic and organic fertilizers, as well as organically-managed soils.