Murty, D., Kirschbaum, M.U.F., McMurtrie, R.E. and McGilvray, H. (2002):
Does forest conversion to agricultural land change soil organic carbon and
nitrogen? A review of the literature. Global
Change Biology 8:
105-123.
Abstract. Soil carbon is a large component of
the global carbon cycle and its management can significantly affect the
atmospheric CO2 concentration.
An important management issue is the extent of soil carbon (C) release
when forest is converted to agricultural land. We reviewed the literature to
assess changes in soil C upon conversion of forests to agricultural land.
Analyses are confounded by changes in soil bulk density upon land-use change,
with agricultural soils on average having 13% higher bulk density. Consistent
with earlier reviews, we found that conversion of forest to cultivated land led
to an average loss of approximately 30% of soil C. When we restricted our
analysis to studies that had used appropriate corrections for changes in bulk
density, soil C loss was 22%. When, from all the studies compiled, we
considered only studies reporting both soil C and nitrogen (N), average losses
of C and N were 24% and 15%, respectively, hence showing a decrease in the
average C:N ratio. The magnitude of these changes in the C:N ratio did not
correlate with either C or N changes.
When considering the transition from
forest to pasture, there was no significant change in either soil C or N, even
though reported changes in soil C ranged from -50% to +160%. Among studies that
reported changes in soil N as well as soil C, C:N ratios both increased and
decreased, with trends depending on changes in system N. Systems with
increasing soil N generally had decreased C:N ratios, whereas systems with
decreasing soil N had increased C:N ratios.
Our survey confirmed earlier
findings that conversion of forest to cropland generally leads to a loss of
soil carbon, although the magnitude of change might have been inflated in many
studies by the confounding influence of bulk-density changes. In contrast,
conversion of forest to uncultivated grazing land did not, on average, lead to
loss of soil carbon, although individual sites may lose or gain soil C,
depending on specific circumstances, such as application of fertiliser or
retention or removal of plant residues.