Introduction - Mathematics and Statistics for the Environment - page 35

land use (i.e., cropland) (IPCC, 2006). The carbon stock in annual cropland biomass after 1 year is 5
metric tons carbon per hectare, and the carbon content of dry above-ground biomass is 45% (IPCC,
2006). Therefore, the carbon stock in cropland after 1 year of growth is estimated to be 2.25 metric
tons carbon per hectare (or 0.91 metric tons carbon per acre).
The averaged reference soil carbon stock (for high-activity clay, low-activity clay, and sandy soils
for all climate regions in the United States) is 40.83 metric tons carbon per hectare (USEPA 2013a).
Carbon stock change in soils is time dependent, with a default time period for transition between
equilibrium soil organ carbon values of 20 years for mineral soils in cropland systems (IPCC,
2006). Consequently, it is assumed that the change in equilibrium mineral soil organic carbon will
be annualized over 20 years to represent the annual flux. The IPCC (2006) guidelines indicate that
there are insufficient data to provide a default approach or parameters to estimate carbon stocks in
perennial cropland.

1.7.11.1 Calculations for Converting U.S. Forests to U.S. Cropland
Annual change in biomass carbon stocks on land converted to other land-use category:

Δ C B = ΔC G + C Conversion - ΔC L

where
ΔC B = Annual change in carbon stocks in biomass due to growth on land converted to another
land-use category (i.e., 2.25 metric tons C per hectare).
ΔC G = Annual increase in carbon stocks in biomass due to growth on land converted to
another land-use category (i.e., 2.25 metric tons C per hectare).
C Conversion = Initial change in carbon stocks in biomass on land converted to another land-use
category; the sum of the carbon stocks in above-ground, below-ground, deadwood, and
litter biomass (-88.47 metric tons C per hectare). Immediately after conversion from for-
estland to cropland, biomass is assumed to be zero, as the land is cleared of all vegetation
before planting crops.
ΔC L = Annual decrease in biomass stocks due to losses from harvesting, fuel wood gathering,
and disturbances on land converted to other land-use category (assumed to be zero).

Therefore, ΔC B = ΔC G + C Conversion - ΔC L = -86.22 metric tons carbon per hectare per year of bio-
mass carbon stocks are lost when forestland is converted to cropland.

Annual change in organic carbon stocks in mineral soils

ΔC Mineral = (SOC O - SOC ( O-T ) ) ÷ D

where
ΔC Mineral = Annual change in carbon stocks in mineral soils.
SOC O = Soil organic carbon stock in last year of inventory time period (i.e., 40.83 mt C per
hecta re).
SOC ( O-T ) = Solid organic carbon stock at beginning of inventory time period (i.e., 62 mt C
per hectare).
D = Time dependence of stock change factors which is the default time period for transition
between equilibrium SOC values (i.e., 20 years for cropland systems).

Therefore, ΔC Mineral (SPC O - SOC ( O-T ) ) ÷ D = (40.83 - 62) ÷ 20 = -1.06 metric tons C per hectare per
year of soil organic C are lost. Consequently, the change in carbon density from converting forest-
land to cropland would be -86.22 metric tons of C per hectare per year of biomass plus -1.06 metric
tons C per hectare per year of soil organic C, equaling a total loss of 87.28 metric tons C per hectare