Free soil colloids and colloidal building units of soil aggregates

Soil colloids < 220 nm including nanoparticles (1-100 nm), mainly composed of mineral particles and organic matter (OM) as well as their associations, have been gradually recognized as primary building units of the hierarchically organized soil aggregate system. As these colloidal building units are normally occluded inside soil aggregates, we refer to them as occluded colloids. Meanwhile, a large proportion of soil colloids is free from aggregate occlusion and mobile in the soil matrix. These free colloids can potentially serve as carriers for adsorbed nutrients and contaminants, mediating their translocation in the subsurface. However, the differences between free and occluded colloids remain unclear.

Here, both occluded and free colloids were isolated from soil samples of an arable field with different clay contents. The occluded colloids were released from soil macroaggregates (>250 µm) with ultrasonic treatments. The free and occluded colloids were sequentially characterized for their size-resolved elemental composition using flow field-flow fractionation inductively coupled plasma mass spectrometry and organic carbon detector (FFF-ICP-MS/OCD). Besides, selected samples were also subjected to transmission electron microscopy (TEM) and pyrolysis field ionization mass spectrometry (Py-FIMS).

Both free and occluded colloids mainly consisted of three size fractions: the first size fraction (0.6–60 nm), the second sized fraction (60–170 nm), and the third size fraction (>170 nm). The first size fraction was dominated by organic carbon and Ca, which were likely to be present as Ca-bridged OM associations. The elemental composition of the second and third size fractions was similar, which mainly consisted of organic carbon, Al, Si, and Fe, indicating the presence of mineral-mineral or mineral-organic associations. However, the ratios of organic to inorganic components in each size fractions varied among colloidal samples. TEM-EDX revealed that particles from free colloids were mainly present as mineral-mineral associations, while particles from occluded colloids tended to be mineral-organic associations. The C and N analysis showed higher N contents and narrower C/N ratios in free colloids when compared with occluded ones, suggesting different OM compositions in free and occluded colloids. The Py-FIMS results suggested that alkyl aromatics, phenols, lignin monomers, and lipids were the major OM compound classes in both free and occluded colloids. The relative abundance of carbohydrates, amides, heterocyclic nitrogen, and nitriles were higher in occluded colloids, whereas suberin and free fatty acids were relatively abundant in free colloids. Moreover, thermograms of OM compounds showed that occluded colloids possessed a higher proportion of thermal stable fractions of OM compounds, while the proportion of thermal liable fractions of OM compounds was relatively higher in free colloids. Overall, shedding light on the differences between free and occluded colloids may help us to gain insight into soil aggregate formation.