t Conversion of native ecosystems to
agro-ecosystems influences the amount, quality and
turnover of soil organic carbon (SOC). As most agroecosystems
are not in a steady state in terms of the
content of SOC, the time scale and feedback mechanisms
of changes in SOC are highly relevant for
predicting future soil fertility and potential rates of
soil carbon losses or sequestration. This paper focuses
on changes in land use linked to measured changes in
the distribution of total stocks of SOC and the d13C
signature in the upper 0.5 m of cultivated soils in the
semi-arid parts of Tanzania. Based on documented
land use changes since 1950s using remote sensing
data, 12 sampling sites along two transects were
selected to represent semi-natural/natural savannah
and maize fields cultivated for up to five decades.
Comparisons between sites representing a chronosequence
of well-drained soils showed that soils
cultivated the last 50 years have in average less than
50% SOC compared to soils which have never been
cultivated. Variations between sites were significant
and a reduction in SOC could not be established at
sites near present or former villages which have
received substantial manure despite a long cultivation
history or along a chronosequence representing
wetter and more fine-grained soils. Spatial variations
in land use changes were parameterized based on
remote sensing data and successfully validated
against sampling sites. Site-specific rates of soil
element loss following cultivation were extrapolated
to the study area and uncertainties related to scaling
up were discussed.