Soil characteristics
One composite soil sample (18
sub-samples systematically distributed per plot) was made within each
plot at both 0-10 cm and 10-20 cm depth and analyzed for sum of bases
(cations were determined by spectrophotometry and P by colorimetry),
cation exchange capacity, and texture, which was analyzed using the
hydrometer method. Soil samples were additionally collected with a
cylindrical auger of known volume at 0-10 cm and 10-20 cm depth, in
three sampling points distributed
systematically within each plot (one at the center of the plot and one
10 m apart from each of its lateral edges), to measure soil texture and
total soil carbon. The soil bulk density of each plot (n = 3) and depth
was determined by the core method on a core of 0.5 cm diameter and 5.0
cm deep for the 0-10 and 10-20 cm depth (samples oven-dried at 105°C
until constant weight), using the methodology described by Donagema,
Campos, Calderano, Teixeira, and Viana (2011). Soil samples were
air-dried and sieved in 2 mm open mesh to obtain air-dried fine soil,
and then a portion of the sample was ground to pass through 100 mesh
sieve to measure the C content. Soil texture was measured according to
(EMBRAPA, 1997). Total C content was quantified by dry combustion method
(1000°C) using a LECO elemental analyzer (TruSpec CHNS, LECO). Due to
logistical problems, texture data were missing for one of the reference
forest. Soil carbon stock (SCS, Mg.ha-1) of a soil
layer i was then calculated as follows:
\begin{equation}
\text{SCS}_{i}=t_{i}{\times TCC}_{i}\times\text{BD}_{i}\ \ \ \ \ \ \ \ (Eq.\ 1)\nonumber \\
\end{equation}where t is layer thickness (cm), TCC is total carbon content
(gC.g-1), and BD is soil bulk density
(g.cm-3). We considered a soil layer thickness of 10
cm, allowing in this case the estimate of SCS in the 0-10 and 10-20 cm
horizons, which was subsequently summed to obtain SCS in the 0-20 cm
horizon.