The subhumid Chaco region of Argentina, originally covered by dry
sclerophyll forest, has been subjected to clearing since the end of the 1970s
and replacement of the forest by no-till farming. Land use changes produced a
decrease in aboveground carbon (C) stored in forests, but little is known
about the impact on soil organic C stocks. The aim of this study was to
evaluate soil C stocks and C fractions up to 1 m depth in soils under
different land use: < 10-year continuous cropping, > 20-year continuous
cropping, warm-season grass pasture and native forest in 32 sites distributed
over the Chaco region. The organic C stock content up to 1 m depth expressed
as equivalent mass varied as follows: forest
(119.3 Mg ha−1)  >  pasture (87.9 Mg ha−1)  >  continuous
cropping (71.9 and 77.3 Mg ha−1), with no impact of the number of
years under cropping. The coarse particle fraction (2000–212 µm) at
0–5 and 5–20 cm depth layers was the most sensitive organic carbon
fraction to land use change. Resistant carbon ( < 53 µm) was the
main organic matter fraction in all sample categories except in the forest.
Organic C stock, its quality and its distribution in the profile were responsive
to land use change. The conversion of the Chaco forest to crops was
associated with a decrease of organic C stock up to 1 m depth and with the
decrease of the labile fraction. The permanent pastures of warm-season
grasses allowed higher C stocks to be sustained than cropping systems and so could
be considered a sustainable land use system in terms of soil C preservation.
As soil organic C losses were not restricted to the first few centimetres of the soil,
the development of models that would allow the estimation of soil organic C
changes in depth would be useful to evaluate the
impact of land use change on C stocks with greater precision.</p
