The dynamics of SOM is closely related to soil physical properties, such as particle sizes distribution and soil aggregation (Tisdall and Oades 1982). Organic matter components are involved in the different steps of soil aggregate hierarchy, from the formation of organo-mineral complexes up to stabilization of aggregate-size classes (Oades and Waters 1991; Six et al. 2004). Since the role of OM in soil physical aggregation determines the processes of OC decomposition and accumulation (Plante and McGill 2002a), an evaluation of aggregate distribution in soil and their stability is useful to follow SOC dynamics (Kasper et al. 2009).
The classical procedure described by Kemper and Rosenau (1986) was used to separate water-stable aggregates. Briefly, 20 g of <4.75 mm air-dried soil samples was put on the topmost of a nest of three sieves with 1.00, 0.50, and 0.25 mm mesh size and pre-soaked in distilled water for 30 min. Then, the nest of sieves was oscillated vertically in water 20 times, using a 4 cm amplitude at the rate of one oscillation per second. Care was taken to ensure that soil particles on the topmost sieve were always below the water surface during each oscillation.
After wet-sieving, the water-stable soil materials left on each sieve and the unstable (<0.25 mm) aggregates were quantitatively transferred into beakers, dried in the oven at 50°C for 48 h, weighed and stored for analysis. The percentage ratio of aggregates in each sieve represents the water-stable aggregates for size classes: 4.75-1.00, 1.00-0.50, 0.50-0.25, and <0.25 mm. Mean-weight diameter (MWD) of water-stable aggregates was calculated by the following equation:
where Xi is the mean diameter of the ith sieve, and Wi is the amount of total aggregates in the ith fraction.
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