2.4 Soil aggregate separation
The wet sieving method (Elliott, 1986) was used to separate soil aggregates into three size fractions: (i) macro-aggregates (>0.25 mm) (ii) micro-aggregates (0.053–0.25 mm), (iii) silt plus clay (<0.053 mm). Briefly, four 50 g, air-dried soil subsamples were placed into a 0.25 mm sieve and slowly immersed with 5 mL of deionized water for 5 min prior to sieving. Then, the samples were placed into a shaker consisting of two sieves (0.25 and 0.053 mm) in series and vertically shaken (3 cm) 50 times over 5 min. After sieving, aggregates remaining in each sieve were flushed into separate beakers, and those remaining in the bucket (<0.053 mm) were precipitated in calcium chloride for 24 h. The fractionated samples were dried at 60 °C and weighed. The water stable macro-aggregate content with a diameter of >0.25 mm and mean weight diameter (MWD) were calculated according to Zhang et al. (2014). A portion of each aggregate sample was ground to pass through a 0.15 mm sieve and used for determination of organic C content.