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.