Pore system characteristics of soil aggregates and their relevance to aggregate stability

Aggregates are the structural units of soils, and the physical stability is considered to be a keystone parameter of soil quality. However, little is known about the evolution of the pore system in aggregates and its importance in defining aggregate stability. In this paper, we investigated the pore system and stability of three dominant macroaggregate sizes (1-2, 2-5 5-10 mm) obtained from a fine sand-loamy Chernozem under three distinct land uses (arable, grassland and forest). We used non-invasive X-ray microtomography (XMT) in combination with pore network extraction to characterise PSD (pore size distribution) of aggregates and their potential changes upon continued submergence in water. We showed that smaller aggregates (1-2 mm) have significantly higher total X-ray resolvable porosity than the medium (2-5 mm) and large (5-10 mm) aggregates. Also, using imaging tools, we demonstrated for the first time, that the pore system of stable aggregates from grassland and forest does not undergo significant changes upon continued submergence in water. It can be hypothesised that a physically stable pore structure allows the storage and transmission of water without a structural collapse, thereby contributing to aggregate stability. We found statistically significant positive correlations between different pore groups (closed pores, water holding pores and air space spores) and water stability of aggregates from all three land uses suggesting that pore system characteristics play a significant role in aggregate stability. Our results suggest that PSD is an important factor that determines the stability of soil aggregates.