2.3. Soil carbon fractions
SOC was determined by the Walkley-Black dichromate oxidation method
(Nelson, 1982), using a mixture of potassium dichromate
(K2Cr2O7) and sulfuric
acid (H2SO4) to oxidize the organic
matter, after which it was titrated against ferrous sulfate
(FeSO4). The air-dried soil sample (0.1 g) was extracted
with 7.5 ml of K2Cr2O7and 7.5 ml of concentrated H2SO4 at
180°C for 30 min.
MBC was determined on fresh soil samples (sieving < 2 mm)
using the chloroform fumigation-extraction method (Vance, Brookes, &
Jenkinson, 1987). The fumigated and non-fumigated soil (5 g, accurate to
0.001 g) were extracted with 20 ml 0.5 M of
K2SO4 for 30 min on a shaker (180
r/min). 5 ml of supernatant was extracted and titrated according to the
method of organic carbon. MBC was calculated as (C fumigated-C
non-fumigated)/0.38 (McLatchey & Reddy, 1998).
POC was determined with necessary modifications based on the method
described by Yang (Yang et al., 2009). Twenty grams of the soil were
dispersed with 100 mL of a 5 g/L sodium hexametaphosphate solution by
hand shaking the mixture for 15 min and placing it on a reciprocal
shaker (100 r/min) for 16 h. The dispersed soil sample was then passed
through a 53 μm sieve and rinsed thoroughly with distilled water. The
remaining material was dried at 50°C, weighed and finely ground.
EOC was measured by slightly modifying the light group organic compound
separation method using KMnO4 oxidation (Janzen et
al.,1992).