Choice of stock change and emission factors

Mineral soils Tier 1

For unmanaged land, as well as for managed Forest Land, Settlements and nominally managed Grassland with low disturbance regimes, soil C stocks are assumed equal to the reference values (i.e., land use, disturbance (forests only), management and input factors equal 1), while it will be necessary to apply the appropriate stock change factors to represent other systems such as improved and degraded grasslands, as well as all cropland systems. Default reference C stocks are given in Chapter 2, Table 2.3. See the Choice of Stock Change and Emission Factors in the appropriate land-use chapter for default stock change factors (Forest Land in Section 4.2.3.2, Cropland in 5.2.3.2, Grassland in 6.2.3.2, Settlements in 8.2.3.2, and Other land in 9.3.3.2).

Note that it is good practice to use the management factor (FLU) for set-asides (Table 5.5) if dealing with cultivated annual Cropland converted into Grassland (i.e., until the land is re-classified as Grassland Remaining Grassland) because recently converted annual cropland systems will typically gain C at a rate similar to set-aside lands. Moreover, the Tier 1 set-aside factors were derived from empirical data to explicitly represent the expected gain during the first 20 years for lands removed from cultivation. If countries decide to assume a faster increase in C that raises levels to native conditions within 20 years, a justification should be provided in the documentation.

Tier 2

Estimation of country-specific stock change factors is probably the most important development for the Tier 2 approach. Differences in soil organic C stocks among land uses are computed relative to a reference condition, using land-use factor (FLU). Input factor (FI) and management factor (FMG) are then used to further refine the C stocks of the new grassland system. Additional guidance on how to derive these stock change factors is given in Grassland Remaining Grassland, Section 6.2.3.2 as well as other general guidance in Section 2.3.3.1 (Chapter 2). See the appropriate section for specific information regarding the derivation of stock change factors for other land-use sectors (Forest Land in Section 4.2.3.2, Cropland in 5.2.3.2, Settlements in 8.2.3.2, and Other land in 9.3.3.2).

Reference C stocks can also be derived from country-specific data in a Tier 2 approach. However, reference values must be consistent across land-use sectors (i.e., Forest Land, Cropland, Grassland, Settlements, Other land), which requires coordination among the various teams conducting soil C inventories for AFOLU.

Tier 3

Constant stock change rate factors per se are less likely to be estimated in favor of variable rates that more accurately capture land-use and management effects. See Section 2.3.3.1 in Chapter 2 for further discussion.

Organic soils

Tier 1 and Tier 2

Land Converted to Grassland on organic soils within the inventory time period is treated the same as Grassland Remaining Grassland on organic soils. Tier 1 emission factors are given in Table 6.3, while Tier 2 emission factors are derived from country-specific or region-specific data.

Tier 3

Constant emission rate factors per se are less likely to be estimated in favor of variable rates that more accurately capture land-use and management effects. See Section 2.3.3 in Chapter 2 for further discussion.

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