Agriculture Reference
In-Depth Information
potential would, therefore, result in the same total loss of sequestered CO2
as lost from the soil under continuous perennial forage.
ULICEES assumes a new SOC equilibrium after 40 years. We assumed
that after 40 years SOC stabilizes to a new soil organic carbon level where
it no longer makes an appreciable contribution to the annual GHG emis-
sions budget. As well, 40 years from the present, 2050, is approximately
the time when GHG levels in the atmosphere were projected at the Nairobi
Climate Change Summit (COP 12) to double compared to pre-industrial
atmospheric levels [35]. For each livestock production system, yearly
GHG emissions of CH
4
, N
2
O and fossil CO
2
continue indefi nitely as long
as that production system operates. Carbon fl ow is, however, not perma-
nent or linear from a long term carbon balance perspective [11].
6.2.2 THE PAYBACK PERIOD
Based on the difference in GHG emissions from beef and pork, a period
can be calculated that is required to accumulate a multi-year quantity of
GHG emissions that equal the difference in carbon sequestration between
the two land uses. The time required to compensate for an amount of soil
carbon that is lost as a result of a land change is defined as the payback
period. This payback period relates the loss of soil carbon to the annual
GHG emissions. A precedent has been set for the payback period approach
in life cycle assessments of biofuels [36-39]. For example, if a tropical
forest is cut down to grow palm oil for biodiesel feedstock, it would take
many decades before the annual offset of fossil CO
2
emissions from the
biodiesel equals that loss in tropical soil carbon. The beef to pork conver-
sion can be treated in the same way. The change in crop areas for the beef
to pork redistribution would come from the forage area that supported the
displaced population of beef which would then be converted to feed grain
for additional hogs.
Since the complete decay curve for soil carbon between two steady
states is exponential, its slope approaches zero asymptotically near the
equilibrium. Hence, the decay period must be defi ned in terms of a toler-
able amount of residual soil carbon. In this analysis, the decay period was
set at 40 years which accounts for about 60% of the carbon stock [40]. The
