Freeze-thaw cycles shape building units for soil microaggregate formation: Experiments with mineral and organic model substances

The observation of platy-shaped composite building units in soil microaggregates of temperate soils suggests exposure to repeated freeze-thaw cycles. Upon soil freezing, soil solution components escape from growing ice by Brownian motion. When the remaining liquid freezes, particle shapes are defined by the sub-grain boundaries of the ice crystals. The role of solution chemistry and the number of freeze-thaw cycles (FTCs) on size, shape, and stability of composite particles formed is poorly understood. Illite, goethite, cell envelopes, and tannic acid were used as model substances and individually exposed to up to 20 FTCs. Model compounds were used at concentrations of 0.005 to 10 g L⁻¹ with and without background electrolytes (NaCl, CaCl₂, AlCl₃); freezing was delayed (0 °C was reached after 1.5 h) for slow growth of ice crystals. After freeze-drying of ice columns, size and shape of the composite particles formed by ice exclusion were analyzed by confocal laser scanning microscopy. Particles were sized according to the equivalent circle diameter (ECD) and their shape classified into different categories. In the thawed suspensions, particle size was determined by the hydrodynamic diameter (HD) obtained with dynamic light scattering. Shapes of the composite particles formed in the freezing experiments were similar for all model substances, with a morphology resembling ice surfaces, typically with layers and veins from two- and three-grain boundaries, respectively. At high concentrations, larger particles (ECDs >10 µm) with platy morphology were formed, due to thicker and more filled sub-grain boundaries. The smaller HD values in thawed suspensions, especially for cell envelopes, revealed that composite particles were prone to dispersion. Sizes of illite and tannin composite particles formed by freezing at low concentrations were smaller (ECD <6 µm) than at high concentrations, but in the thawed suspended state, the HD of particles was larger than of those formed at high concentrations. Obviously, the freeze-concentration effect is most intense at low particle concentrations, likely due to formation of larger ice crystals and higher crystallization pressures. An increasing number of FTCs amplified this effect. Low pH values of 3 and the presence of electrolytes resulted in a considerable increase in the ECD of tannin particles. In contrast, this effect was not traceable in suspension after thawing and HDs were very similar for pH 3 and 6. The observed effects on particle formation upon freezing are potentially stronger under natural soil conditions as freezing is slower, favoring a more intense freeze-concentration effect. We conclude that freeze-thaw cycles can significantly modify the architecture of soil microaggregates by shaping their building units, with possible consequences for other soil functions like C retention and availability.