Soil Carbon

The Tier 1 method is based on changes in soil C stocks over a finite period of time. The change is computed based on C stock after the management change relative to the carbon stock in a reference condition (i.e., native vegetation that is not degraded or improved). Calculation of soil C stock changes rests on two assumptions:

1. Over time, soil organic C reaches a spatially-averaged, stable value specific to the soil, climate, land-use and management practices; and
2. SOC stock changes during the transition to a new equilibrium SOC are linear.

• ∆C_mineral = annual change in carbon stocks in mineral soils, tonnes C yr^-1
• SOC₀ = soil organic carbon stock in the last year of an inventory time period, tonnes C
• SOC_(0-T) = soil organic carbon stock at the beginning of the inventory time period, tonnes C
• SOC₀ and SOC_(0-T) are calculated using the SOC equation where the reference carbon stocks and stock change factors are assigned according to the land-use and management activities and corresponding areas at each of the points in time (time = 0 and time = 0-T)
• T = number of years over a single inventory time period, yr
• D = Time dependence of stock change factors which is the default time period for the transition between equilibrium SOC values, yr. Commonly 20 years, but depends on assumptions made in computing the factors FLU, FMG and FI. If T exceeds D, use the value for T to obtain an annual rate of change over the inventory time period (0-T years).
• c,s,i = c represents climate zones, s represents soil types, and i represents the set of management systems that are present in a country
• SOC_REF = the reference carbon stock, tonnes C ha^-1 (Table 2.3; IPCC, 2019)
• F_LU = stock change factor for land-use systems or sub-system for a particular land-use, dimensionless (Table 5.5; IPCC, 2019 for croplands and Table 6.2; IPCC 2019 for grasslands)
• F_MG = stock change factor for management regime, dimensionless (Table 5.5; IPCC, 2019 for croplands and Table 6.2; IPCC 2019 for grasslands)
• F_I = stock change factor for input of organic matter, dimensionless (Table 5.5; IPCC, 2019 for croplands and Table 6.2; IPCC 2019 for grasslands)
• A = land area of the stratum being estimated, ha. All land in the stratum should have common biophysical conditions (i.e., climate and soil type) and management history over the inventory time period to be treated as one unit for analysis.

Estimating soil carbon stock changes in mineral soils thus requires data on the land area under each land use and management category in the first year (time 0-T) and last year (time 0) of the inventory time period. IPCC (2006) guidelines recognize three approaches for representing land-use areas.

Approach 1. Represents land areas within a defined spatial unit. Only the net changes in land-use area can be tracked through time. Consequently, the exact location or pattern of the land uses is not known within the spatial unit, and the exact changes in land-use categories cannot be ascertained. This approach does not allow for analysis of soil C stock changes for land remaining within a land use category.

Table 1. Net changes in land use within a defined special unit

Using this approach, changes in mineral soil C stocks are calculated as:

Approach 2 & 3. In contrast, activity data may be collected based on surveys, remote sensing imagery, or other data providing the total areas for each land management system and the specific transitions in land use and management overtime on individual parcels of land. These are considered Approach 2 and 3 activity data in Chapter 3, and soil C stock changes are computed using formulation B of Equation 2.25 (IPCC, 2019):

• p = a parcel of land representing an individual unit of area over which the inventory calculations are performed

Table 2. Illustrative example of approach 2 data in a land-use conversion matrix with category stratification (based on Table 3.5; IPCC, 2019)*

For croplands, IPCC (2006) provides default Tier 1 land-use factors (F_LU) for land under long-term cultivation of annual (non-paddy) crops, paddy rice, perennial tree crops, and set aside. Tier 1 management (F_MG) and input (F_I) factors are also available.

Table 3. Relative carbon stock change factors (F_LU, F_MG, and F_i) (over 20 years) for management activities on cropland

For grasslands, IPCC (2006) uses a single land-use factor (F_LU) of 1.0. Management factors (F_MG) vary according to the level of degradation or improvement of the grassland. Input factors (F_I) are used only for improved, managed grassland.

Table 4. Relative carbon stock change factors (F_LU, F_MG, and Fi) (over 20 years) for grassland management

The basic methodology for estimating C emissions from organic (e.g., peat-derived) soils is to assign an annual emission factor that estimates C losses following drainage.

• L_organic = annual carbon loss from drained organic soils, tonnes C yr^-1
• A = land area of drained organic soils in climate type c, ha
  • Note: A is the same area (F_os) used to estimate N₂O emissions from drained organic soils (above). See Emissions from Managed Soils
• EF = emission factor for climate type c, tonnes C ha^-1 yr^-1

Therefore, the activity data required to estimate C emissions from organic soils are the area of drained or managed organic soils, categorized by climate (temperate and tropical). In addition, for temperate forest land, areas should be further stratified by soil fertility (nutrient-rich and nutrient-poor). In case no stratification by soil fertility is possible, countries may rely on expert judgment. These data will also be needed to estimate N₂O emissions from drained organic soils. IPCC (2006) provides Tier 1 emission factors for drained grassland organic soils and cropland organic soils, found in Table 5 below.

Table 5. Tier 1 emission factors (EFs) for drained grassland and cultivated cropland organic soils

The Tier 2 method for calculating soil C stocks changes uses the same calculation steps as for Tier 1. Still, it incorporates country-specific data to improve four components of the Tier 1 approach:

1. Type of management system: default systems (e.g., improved and unimproved grassland) can be disaggregated into a finer characterization that better represents the impact of management on soil organic C stocks in the land area inventory.
2. Climate regions and soil types: more detailed soil and climate classifications, specific to the region or country of the inventory, could be used.
3. Reference C stocks: identifying country- or region-specific reference levels of C stocks (SOC_ref) is another option for improving an inventory of soil C stock changes.
4. Stock change factors: country- or region-specific soil C stock change factors developed from measurement data and model simulations.

The Tier 2 steady-state method is an intermediate complexity approach between Tier 1 and Tier 3 methods and is based on a steady-state solution to the three-soil organic C sub-pools in the Century ecosystem model. It provides, therefore, an optional alternative method for estimating soil C stock changes in the 0-30 cm layer of mineral soils in Cropland Remaining Cropland.

The Tier 2 steady-state method incorporates spatial and temporal variation in climate, organic carbon inputs to soils, soil properties, and management practices. However, compilers can further develop and/or parameterize this model given appropriate datasets (Tier 3). The method is not appropriate for rice cultivation and is not parameterized to estimate the change in soil organic C stocks due to biochar C amendments.

Table 6. Globally calibrated model parameters to be used to estimate soc changes for mineral soils with the tier 2 steady-state method (Table 5.5a, Chapter 5, IPCC, 2019)

Other default values for nitrogen and lignin contents in crops for carbon to nitrogen ratios, nitrogen, and lignin contents in livestock manure for the steady-state method can be found in tables 5.5b and 5.5c in Chapter 5, IPCC, 2019, respectively.

Tier 2 methods for biochar C amendments utilize a top-down approach. The total amount of biochar generated and added to mineral soil is used to estimate soil organic C stocks’ change with country-specific Factors (see Section 2.3.3.1, Chapter 2, IPCC, 2019).