Fertilizer associated soil N2O emissions

Summary

This methodology represents nitrous oxide (N₂O) emissions associated with the application of synthetic and organic 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 application of N to managed soils via the organic and synthetic fertilizers.

Both direct and indirect emission pathways of fertilizer-N (i.e. fertilizer-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 fertilizer-N added to the soil. Indirect emissions occur through volatilisation and/or leaching of fertilizer-N from the soil. These effects are each represented by (1) a factor describing the typical fraction of fertilizer-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 fertilizer-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

Conversion factors for 5 fertilizer types are provided within this methodology: synthetic fertilizer and 4 types of organic fertilizer (e.g., compost, manure, sewage). Each fertilizer type is represented by a general direct N₂O-N conversion rate (kg / kg), as well as an alternative direct conversion factor representing the specific context of rice cultivation. Also included for each scenario are the common factors for indirect (volatilised/leached) N 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 fertilizer-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 fertilizer-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.

Additional information

Nomenclature

Following IPCC advice, the term manure is used collectively to include both dung and urine (i.e., the solids and the liquids) produced by livestock.

Related methodologies

A number of other methodologies focus on emissions from managed soils. These include those arising from the addition of urine and dung, urea and carbonates/lime, as well as drained/managed organic soils.