2.6. Carbon and nitrogen analysis
Organic C and N concentrations in litter, bulk soil, aggregate
fractions (large macroaggregates, small macroaggregates, micro-
aggregates and aggregates <53 mm) and POM fractions (fPOM,
iPOMc and iPOMf in large- and small macroaggregates and fPOM
and iPOMf in microaggregates) were analyzed with an Elemental
Combustion System SS4010 (Costech). For sugarcane litter, a total
of 20 samples from the nine plots (two samples from seven plots
and three samples from the remaining two plots) were used for the
determination of C and N concentrations. Since the variation in C
and N concentrations in sugarcane litter samples was low (CV = 3%
and 9% for C and N, respectively), the mean concentrations of each
element (C = 430 mg g1; N = 6.6 mg g1) were used to determine
litter C and N contents (g C or N m2) for each sugarcane plot. Prior
to elemental analysis in samples of bulk soil, aggregate fractions
and POM fractions, potential carbonates were removed with the
HCl fumigation method (Harris et al., 2001). Aliquots (300 mg) of
the ground material were exposed to HCl vapor of a concentrated
HCl solution (12 M) in a vacuum desiccator during 12 h. This time
was determined experimentally by exposing the samples to HCl
vapor for periods increasing in 4 h increments to 48 h (data not
shown). Soil organic C and total N stocks (Mg ha1) were calculated
from bulk SOC and soil N concentrations (g kg suelo1) and soil
bulk density (g cm3) for a given soil depth. Since soil bulk density
did not vary among plots (see Section 3), average values of each
depth (1.30 and 1.44 g cm3 for 0–10 and 10–20 cm depths,
respectively) were used for calculations. Soil organic C and N
stocks at 0–20 cm depth were determined by summing C and N
stocks from 0–10 and 10–20 cm depths in each sample. Aggregate C
and N concentrations were corrected for fPOM C and N
concentrations according to Elliott et al. (1991). Aggregate, fPOM,
iPOMc and iPOMf C and N concentrations were normalized on a
sand-free basis (g kg1 sand-free aggregate) according to Six et al.
(1998). Weights of sand plus iPOM were used to sand-correct
aggregate C and iPOM C concentrations, (Degryze et al., 2004). For
a given aggregate sample, the concentration of organic C and N
associated with the silt plus clay fraction (<53 mm) (i.e., mineral
associated C and N) were estimated by means of mass balance
equation (Six et al., 2000); where, for example, the concentration
of the mineral associated C of a small macroaggregate is equal to
the aggregate C concentration minus the concentrations of iPOMc
C and iPOMf C fractions of the same aggregate. Total concentrations
of fPOM, iPOMc, iPOMf and mineral associated organic C and N
were expressed on a per weight soil basis (g kg1 soil) (Degryze
et al., 2004). For this, in each soil sample, total fPOM C and N
concentrations were determined by summing the concentrations
of fPOM C and N from small macroaggregates (250–2000 mm) and
microaggregates (53–250 mm). Similarly, total iPOMc C and N
concentrations were determined by summing the concentrations
of iPOMc C and N from large macroaggregates and small
macroaggregates, and total iPOMf and total mineral associated C
and N concentrations were determined by summing their
concentrations in large macroaggregates, small macroaggregates
and microaggregates.
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