# Calculation steps for Tier 1

Mineral soils

The steps for estimating SOC0 and SOC(0-T) and net soil C stock change from Grassland Remaining Grassland are as follows:

Step 1: Organize data into inventory time periods based on the years in which activity data were collected (e.g., 1990 and 1995, 1995 and 2000, etc.)

Step 2: Determine the land-use and management by mineral soil type and climate region for land at the beginning of the inventory period, which can vary depending on the time step of the activity data (0-T; e.g., 5, 10 or 20 years ago).

Step 3: Select the native reference C stock value (SOC^), based on climate and soil type from Table 2.3, for each area of land being inventoried. The reference C stocks are the same for all land-use categories to ensure that erroneous changes in the C stocks are not computed due to differences in reference stock values among sectors.

Step 4: Select the land-use factor (FLU), management factor (Fmg) and C input levels (Fi) representing the land-use and management system present at the beginning of the inventory period. Values for FLU, FMG and FI are provided in Table 6.2.

Step 5: Multiply these values by the reference soil C stock to estimate the 'initial' soil organic C stock (SOC(0-T)) for the inventory time period.

Step 6: Estimate SOC0 by repeating Step 1 to 4 using the same native reference C stock (SOC^), but with land-use, management and input factors that represent conditions in the last (year 0) inventory year.

Step 7: Estimate the average annual change in soil organic C stock for the area over the inventory time period (AC ).

Mineral

Step 8: Repeat Steps 1 to 6 if there are additional inventory time periods (e.g., 1995 to 2000, 2001 to 2005, etc.).

A case example is given below for computing a change in grassland soil organic C stocks using Equation 2.25 (Chapter 2), default stock change factors and reference C stocks.

Example: The following example shows calculations for aggregate areas of grassland soil carbon stock change to a 30 cm depth. In a tropical moist climate on Ultisol soils, there are 1Mha of permanent grassland. The native reference carbon stock (SOCRef) for the climate/soil type is 47 tonnes C ha-1. At the beginning of the inventory time period (1990 in this example) the distribution of grassland systems was 500,000 ha of unmanaged native grassland; 400,000 ha of unimproved, moderately degraded grazing land; and 100,000 ha of heavily degraded grassland. Thus, initial soil carbon stocks for the area were: 500,000 ha • (47 tonnes C ha-1 • 1 • 1 • 1) + 400,000 ha • (47 tonnes C ha-1 • 1 • 0.97 • 1) + 100,000 • (47 tonnes C ha-1 • 1 • 0.7 • 1) = 45,026,000 tonnes C. In the last year of inventory time period (2010 in this example), there are: 300,000 ha of unmanaged native grassland; 300,000 ha of unimproved, moderately degraded grazing land; 200,000 ha of heavily degraded grassland; 100,000 ha of improved pasture receiving fertilizer; and 100,000 of highly improved pasture receiving fertiliser together with irrigation. Thus, total soil carbon stocks in the inventory year are: 300,000 ha • (47 tonnes C ha-1 • 1 • 1 • 1) + 300,000 ha • (47 tonnes C ha-1 • 1 • 0.97 • 1) + 200,000 • (47 tonnes C ha-1 1 • 0.7 • 1) + 100,000 • (47 tonnes C ha-1 1 • 1.17 • 1) + 100,000 • (47 tonnes C ha-1 • 1 • 1.17 • 1.11) = 45,959,890 tonnes C. The average annual stock change over the period for the entire area is: 45,959,890 - 45,026,000 = 933,890 tonnes/20 yr = 46,694.5 tonnes per year soil C stock increase. (Note: 20 years is the time dependence of the stock change factor, i.e., factor represents annual rate of change over 20 years).

Organic soils

The steps for estimating the loss of soil C from drained organic soils are as follows:

Step 1: Organize data into inventory time periods based on the years in which activity data were collected (e.g., 1990 and 1995, 1995 and 2000, etc.).

Step 2: Determine the amount of Grassland Remaining Grassland on drained organic soils in the last year of each inventory time period.

Step 3: Assign the appropriate emission factor (EF) for annual losses of CO2 based on climatic temperature regime (from Table 5.6).

Step 4: Estimate total emissions by summing the product of area (A) multiplied by the emission factor (EF) for all climate zones.

Step 5: Repeat for additional inventory time periods.

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