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).