2.5 Analysis of soil C fractions
The soil water content was determined at 105 °C for 24 h with an oven to
a consistent weight. SOC in native soil and soil aggregates was
determined by the chemical oxidation using
K2Cr2O7 solution. MBC
concentration was measured using a modified chloroform fumigation
extraction method and calculated as difference between soluble C or N of
the fumigated and non-fumigated soils (Wu, Joergensen, Pommerening,
Chaussod, & Brookes, 1990). WSOC was extracted from solutions of 20 g
fresh soil with 40 ml distilled water according to Liang et al. (1997).
EOC was extracted from 10 g fresh soil with 50 ml 0.5 M
K2SO4 at room temperature, shaking for 1
h at 220 r min-1, filtered with a 0.45 μm membrane
filter (Jones & Willett, 2006). Total organic C concentrations in the
above filtrates were all analyzed with an automated TOC analyzer.
KMnO4-C concentration was measured with 333 mmol
L-1 KMnO4 based on the difference
between subsamples and the blank (only KMnO4) referred
to Blair et al. (1995). The BR was determined with an incubation method
(Carter, Kunelius, & Angers, 1994). Briefly, 100 g fresh soil subsample
was laid in an air tight flask of 500 ml at 37 °C, the evolved
CO2 was trapped in 50 ml of 0.2 M NaOH contained in a
vial placed inside the flask. The vials were removed 1 day after
initiating the incubation, and titrated with standardized 0.2 M HCl
after addition of 1 ml of 3 M BaCl2 using
phenolphthalein as a visual indicator. Each treatment was replicated 3
times and 2 flasks without soil were used as blank control.