Kirschbaum, M.U.F.
(2000). Will changes in soil organic matter act as a positive or negative
feedback on global warming? Biogeochemistry 48: 21-51.
The world’s soils
contain about 1500 Gt of organic carbon to a depth of 1m and a further 900 Gt
from 1-2m. A change of total soil organic carbon by just 10% would thus be
equivalent to all the anthropogenic CO2 emitted over 30 years.
Warming is likely to increase both the rate of decomposition and net primary
production (NPP), with a fraction of NPP forming new organic carbon. Evidence
from various sources can be used to assess whether NPP or the rate of
decomposition has the greater temperature sensitivity, and, hence, whether
warming is likely to lead to an increase or decrease in soil organic carbon.
Evidence is reviewed
from laboratory-based incubations, field measurements of organic carbon
storage, carbon isotope ratios and soil respiration with either naturally
varying temperatures or after experimentally increasing soil temperatures.
Estimates of terrestrial carbon stored at the Last Glacial Maximum are also
reviewed. The review concludes that the temperature dependence of organic
matter decomposition can be best described as:
d(T) = exp[3.36 (T -
40) / (T + 31.79)]
where d(T) is the
normalised decomposition rate at temperature T (in ºC). In this equation,
decomposition rate is normalised to ‘1’ at 40ºC.
The review concludes
by simulating the likely changes in soil organic carbon with warming. In
summary, it appears likely that warming will have the effect of reducing soil
organic carbon by stimulating decomposition rates more than NPP. However,
increasing CO2 is likely to simultaneously have the effect of
increasing soil organic carbon through increases in NPP. Any changes are also
likely to be very slow. The net effect of changes in soil organic carbon on
atmospheric CO2 loading over the next decades to centuries is,
therefore, likely to be small.