Micro- within macro: How micro-aggregation shapes the soil pore space and water-stability

Soil microaggregates are characterized by higher stability than macroaggregates; at the same time, they are their building units. The water-stability of soil structure is generally defined by architecture parameters, strength of linkages between the solids and is usually assessed by the size of the structural units into which the aggregates break down. Therefore, we hypothesized that within the shift of capillary to full saturation, macroaggregate water-stability is determined primarily by the content and the size of microaggregates, and number of contacts between the building units within macroaggregates. To test this hypothesis, two soil aggregate fractions (10–7 and 5–3 mm) were separated by the dry sieving from A horizons of the boreal forest and arable loamy Retisols contrasting in types of macroaggregates. Aggregate architecture was depicted by X-ray computed microtomography (µCT) with resolution 6.97 µm (15 aggregates of each fraction) and than the water-stability test of the individual aggregates with the subsequent analysis of fragment size distributions into which the aggregates had disintegrated by laser diffractometry was performed. Microaggregates are the prerequisites to the formation of complex and openwork aggregate pore space. For the studied soils, when the number of microaggregates is above 55%, a further increase of every one percent leads to 10 new contacts between the solids in the macroaggregates. A higher degree of intra-macroaggregate porosity is achieved with an increased number of micropores enclosed within the microaggregates and an increase of the mean diameter of the building units within the macroaggregate. Ultramicroporosity (< 7 µm) is characterized by an upper limit of 40%. Microporosity (7–30 µm) and the sum of pores > 30 µm strongly correlate to microaggregate content and the land use type. A 1.5-fold enhancement of microaggregation increases a microporosity by three times. Pores > 30 µm are shaped by two factors: nearly 5% of these pores are biopores attributed to the high activity of soil fauna and roots, and therefore, this percentage is reduced in arable aggregates. The second part of pores > 30 µm is linked to the content of microaggregates and is the packing porosity of micro- within macroaggregates. Therefore, we have shown that the aggregate architecture under air-dry conditions described by µCT determines the subsequent breakdown of the aggregates under the slow moistening.