Choice of method

The level of detail and methods chosen for estimating N2O emissions from manure management systems will depend upon national circumstances and the decision tree in Figure 10.4 describes good practice in choosing a method accordingly. The following sections describe the different tiers referenced in the decision tree for calculating direct and indirect N2O emissions from manure management systems.

Direct N2O emissions from Manure Management

Tier 1

The Tier 1 method entails multiplying the total amount of N excretion (from all livestock species/categories) in each type of manure management system by an emission factor for that type of manure management system (see Equation 10.25). Emissions are then summed over all manure management systems. The Tier 1 method is applied using IPCC default N2O emission factors, default nitrogen excretion data, and default manure management system data (see Annex 10A.2, Tables 10A-4 to 10A-8 for default management system allocations).

Tier 2

A Tier 2 method follows the same calculation equation as Tier 1 but would include the use of country-specific data for some or all of these variables. For example, the use of country-specific nitrogen excretion rates for livestock categories would constitute a Tier 2 methodology.

Tier 3

A Tier 3 method utilizes alternative estimation procedures based on a country-specific methodology. For example, a process-based, mass balance approach which tracks nitrogen throughout the system starting with feed input through final use/disposal could be utilized as a Tier 3 procedure. Tier 3 methods should be well documented to clearly describe estimation procedures.

To estimate emissions from manure management systems, the livestock population must first be divided into categories that reflect the varying amounts of manure produced per animal as well as the manner in which the manure is handled. This division of manure by type of system should be the same as that used to characterize methane emissions from manure management (see Section 10.4). For example, if Tier 1 default emission factors are used for calculating CH4 emissions, then the manure management systems usage data from Tables 10A-4 to 10A-8 should be applied. Detailed information on how to characterise the livestock population for this source is provided in Section 10.2.

The following five steps are used to estimate direct N2O emissions from Manure Management: Step 1: Collect population data from the Livestock Population Characterisation;

Step 2: Use default values or develop the annual average nitrogen excretion rate per head (Nex(T)) for each defined livestock species/category T;

Step 3: Use default values or determine the fraction of total annual nitrogen excretion for each livestock species/category T that is managed in each manure management system S (MS(T,S));

Step 4: Use default values or develop N2O emission factors for each manure management system S (EF3(S)); and

Step 5: For each manure management system type S, multiply its emission factor (EF3(S)) by the total amount of nitrogen managed (from all livestock species/categories) in that system, to estimate N2O emissions from that manure management system. Then sum over all manure management systems.

In some cases, manure nitrogen may be managed in several types of manure management systems. For example, manure flushed from a dairy freestall barn to an anaerobic lagoon may first pass through a solids separation unit where some of the manure nitrogen is removed and managed as a solid. Therefore, it is important to consider carefully the fraction of manure nitrogen that is managed in each type of system.

The calculation of direct N2O emissions from manure management is based on the following equation:

Where:

N2OD(mm) = direct N2O emissions from Manure Management in the country, kg N2O yr-1 N(T) = number of head of livestock species/category T in the country

Nex(T) = annual average N excretion per head of species/category T in the country, kg N animal-1 yr-1

MS(T,S) = fraction of total annual nitrogen excretion for each livestock species/category T that is managed in manure management system S in the country, dimensionless

EF3(S) = emission factor for direct N2O emissions from manure management system S in the country, kg N2O-N/kg N in manure management system S

S = manure management system

T = species/category of livestock

44/28 = conversion of (N2O-N)(mm) emissions to N2O(mm) emissions

There may be losses of nitrogen in other forms (e.g., ammonia and NOx) as manure is managed on site. Nitrogen in the volatilized form of ammonia may be deposited at sites downwind from manure handling areas and contribute to indirect N2O emissions (see below). Countries are encouraged to consider using a mass balance approach (Tier 3) to track the manure nitrogen excreted, managed on site in manure management systems, and ultimately applied to managed soils. The estimation of the amount of manure nitrogen which is directly applied to managed soils or otherwise available for use as feed, fuel or construction purposes is described in the Section 10.5.4, Coordination with reporting for N2O emissions from managed soils. See Chapter 11, Section 11.2 for procedures to calculate N2O emissions from managed manure nitrogen applied to soils.

Indirect N2O emissions from Manure Management

Tier 1

The Tier 1 calculation of N volatilisation in forms of NH3 and NOx from manure management systems is based on multiplication of the amount of nitrogen excreted (from all livestock categories) and managed in each manure management system by a fraction of volatilised nitrogen (see Equation 10.26). N losses are then summed over all manure management systems. The Tier 1 method is applied using default nitrogen excretion data, default manure management system data (see Annex 10A.2, Tables 10A-4 to 10A-8) and default fractions of N losses from manure management systems due to volatilisation (see Table 10.22):

Equation 10.26 N LOSSES DUE TO VOLATILISATION FROM MANURE MANAGEMENT

Nvolatilization-MMS S S

FraCr

Where:

Nvolatilization- mms = amount of manure nitrogen that is lost due to volatilisation of NH3 and NOx, kg N yr-N(T) = number of head of livestock species/category T in the country

NeX(T) = annual average N excretion per head of species/category T in the country, kg N animal-1 yr-1

MS(TS) = fraction of total annual nitrogen excretion for each livestock species/category T that is managed in manure management system S in the country, dimensionless

FracGasMS = percent of managed manure nitrogen for livestock category T that volatilises as NH3 and NOX in the manure management system S, %

Figure 10.4 Decision tree for N2O emissions from Manure Management (Note 1)

Figure 10.4 Decision tree for N2O emissions from Manure Management (Note 1)

Policy Factors

Note:

1: N2O emissions from manure management systems include both direct and indirect sources

2: See Volume 1 Chapter 4, "Methodological Choice and Identification of Key Categories" (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

3: As a rule of thumb, a livestock species would be significant if it accounts for 25-30% or more of emissions from the source category.

The indirect N2O emissions from volatilisation of N in forms of NH3 and NOx (N2OG(mm)) are estimated using Equation 10.27:

EQUATION 10.27

INDIRECT N2O EMISSIONS DUE TO VOLATILISATION OF N FROM MANURE MANAGEMENT

N2OG(mm) = \Nvolatilization-MMS • EF4 )• "28

Where:

N2OG(mm) = indirect N2O emissions due to volatilization of N from Manure Management in the country, kg N2O yr"1

EF4 = emission factor for N2O emissions from atmospheric deposition of nitrogen on soils and water surfaces, kg N2O-N (kg NH3-N + NOx-N volatilised)"1 ; default value is 0.01 kg N2O-N (kg NH3-N + NOx-N volatilised)-1 , given in Chapter 11, Table 11.3

Tier 2

Countries may wish to develop a Tier 2 methodology for better consideration of national circumstances and to reduce uncertainty of estimates as much as possible. As for direct N2O emission from manure management, a Tier 2 method would follow the same calculation equation as Tier 1 but include the use of country-specific data for some or all of these variables. For example, the use of country-specific nitrogen excretion rates for livestock categories would constitute a Tier 2 method. National NH3 emission inventories developed by some countries could be used for Tier 2 estimation of nitrogen volatilisation from manure management systems. A Tier 2 method would require more detailed characterisation of the flow of nitrogen throughout the animal housing and manure management systems used in the country. Double counting of emissions associated with the application of managed manure should be avoided, as well as manure associated with pasture and grazing operations, which should be calculated and reported under Chapter 11, Section 11.2 (N2O emissions from managed soils).

There are extremely limited measurement data on leaching and runoff losses from various manure management systems. The greatest N losses due to runoff and leaching typically occur where animals are on a drylot. In drier climates, runoff losses are smaller than in high rainfall areas and have been estimated in the range from 3 to 6% of N excreted (Eghball and Power, 1994). Studies by Bierman et al. (1999) found nitrogen lost in runoff was 5 to 19% of N excreted and 10 to 16% leached into soil, while other data show relatively low loss of nitrogen through leaching in solid storage (less than 5% of N excreted) but greater loss could also occur (Rotz, 2004). Further research is needed in this area to improve the estimated losses and the conditions and practices under which such losses occur. Equation 10.28 should only be used where there is country-specific information on the fraction of nitrogen loss due to leaching and runoff from manure management systems available. Therefore, estimation of N losses from leaching and runoff from manure management should be considered part of a Tier 2 or Tier 3method.

Nitrogen that leaches into soil and/or runs off during solid storage of manure at outdoor areas or in feedlots is derived as follows:

Equation 10.28

N LOSSES DUE TO LEACHING FROM MANURE MANAGEMENT SYSTEMS

Where:

Nieaching-MMS = amount of manure nitrogen that leached from manure management systems, kg N yr-1 N(T) = number of head of livestock species/category T in the country

Nex(T) = annual average N excretion per head of species/category T in the country, kg N animal-1 yr-1

MS(T,S) = fraction of total annual nitrogen excretion for each livestock species/category T that is managed in manure management system S in the country, dimensionless

FracleachMS = percent of managed manure nitrogen losses for livestock category T due to runoff and leaching during solid and liquid storage of manure (typical range 1-20%)

Nleaching-MMS = S

The indirect N2O emissions from leaching and runoff of nitrogen from manure management systems (N2OL(mm)) are estimated using Equation 10.29:

Equation 10.29

Indirect N2O emissions due to leaching from manure management

Where:

N2OL(mm) = indirect N2O emissions due to leaching and runoff from Manure Management in the country, kg N2O yr"1

EF5 = emission factor for N2O emissions from nitrogen leaching and runoff, kg N2O-N/kg N leached and runoff (default value 0.0075 kg N2O-N (kg N leaching/runoff)-1, given in Chapter 11, Table 11.3

Tier 3

To reduce uncertainty of the estimates, a Tier 3 method could be developed with country-specific emission factors for volatilisation and nitrogen leaching and runoff based on actual measurements.

All losses of N through manure management systems (both direct and indirect) need to be excluded from the amount of manure N that is available for application to soils and which is reported in Chapter 11, Section 11.2 N2O Emissions from Managed Soils. Refer to Section 10.5.4, Coordination with reporting for N2O emissions from managed soils, for guidance on calculating total N losses from manure management systems.

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