New insights into the swelling of black soil aggregates

Soil aggregates swell when infiltrated by water, and their size can increase in two ways. First, aggregates can attach to one another with water acting as a bridge (i.e., adsorption), and water can enter the aggregate pores (i.e., swelling). This is defined as the "adsorption-swelling" effect. Second, clay can fill the pores of macroaggregates and microaggregates (i.e., filling) when adsorption occurs. This is defined as the "adsorption-filling" effect. However, the size range of aggregates affected by these effects and the extent of their influence on aggregate swelling are still unclear. Therefore, different initial size fractions (5 similar to 2, 2 similar to 1, 1 similar to 0.5, 0.5 similar to 0.25 and 0.25 similar to 0.053 mm) of soil aggregates from the black soil zone of Northeast China were studied. The size range of swollen aggregates, the "adsorption-swelling" rate (V) of initial size fractions, and the "adsorption-filling" rate (E) of size fractions < 0.053 mm were measured and calculated in three experimental treatments that involved the following procedures: i) wet-sieving of each initial size fraction in deionized water (WS); ii) wet-sieving of each initial size fraction of air-dried aggregates after they were soaked in absolute ethanol (WSas); and iii) the size fraction < 0.053 mm air-dried aggregates were mixed with each initial size fraction of air-dried aggregates in absolute ethanol and then wet-sieved (WSaf). The results were as follows: i) the size fraction 2 similar to 0.053 mm were swollen. ii) V decreased exponentially with decreasing initial particle size, with a maximum value of 32.30% at a size fraction of 2 similar to 1 mm; and iii) the "adsorption-filling" effect of size fraction < 0.053 mm was obvious in the size fraction < 2 mm swelling aggregates with a maximum of 29.54%. The "adsorption-swelling" and "adsorption-filling" effects had greater impacts on soils with high contents of the size fractions 2 similar to 1 and < 0.053 mm. This study provides a theoretical basis for understanding the swelling mechanisms of soil aggregates.