reported that the SOC stock of timberland
and prairie soils declined with cultivation
in North America. The rate of decline was
25% ± 33% for southwestern, 36% ± 29%
for southeastern, 34% ± 24% for northwestern,
and 22% ± 10% for the northeastern
United States. Lal (1999) reported SOC
stock loss by cultivation of 30% in the
north-central United States. This compares
with a summary of historical studies in the
Great Plains by Cihacek and Ulmer (1995).
Depending on the length of time, cultivation
resulted in SOC losses of 7% to 51%
with longer periods of cultivation showing
more SOC loss than shorter terms of cultivation
(see table 1 in Cihacek and Ulmer
[1995]). They also summarized the effects
of tillage or residue management where
the latter showed positive increases in SOC
while various intensities of tillage reduced
SOC by 1% to 51% (see table 2 in Cihacek
and Ulmer [1995]). However these “classical”
studies do not often clearly define what
their baseline SOC stock was at the start of
the studies to permit verification of results.