Influence of freeze-thaw cycles on natural colloids release of a forest soil

Freeze-thaw events (FTE) induce stress in the soil matrix, leading to the disruption of soil aggregates and consequently causing the release and mobilization of natural soil colloids. Hence, it is fundamental to understand the effect of FTE on the formation of natural soil colloids and colloid-facilitated transport of elements, especially nutrients such as P. In this study, the influence of FTE on natural colloids mobilized after precipitation was investigated. Therefore, columns were packed with 17 cm of disturbed forest topsoil. The soil columns were exposed to either ambient temperature throughout the experiment (control) or to freeze-thaw (FT) conditions, which involved 2 days of freezing at -14 °C followed by 1 day of thawing at ambient temperatures. The FT cycles were repeated five times. Leachate was collected from the columns a day after precipitation (or irrigation using artificial rainwater) after each FTE. Size-resolved elemental composition of colloids in the leachates was determined using Asymmetrical Flow Field-Flow Fractionation (AF4). Findings showed that FTE resulted in higher colloidal organic C (+135%) and P (+85%) loads in the leachates than the control at first FTE, and Fe (+37%) and Al (+67%) at the second FTE. Moreover, higher loads of the smaller colloidal Fe and Al were observed with FT than with the control at first and second FTE. For larger colloids, FT showed higher organic C and P than the control from the first to fourth FTE, and Ca, Mg, Mn, and Zn at the fourth FTE. In terms of bulk elemental load, FT released lower Ca, Mg, Mn, and Zn than the control at the second and third FTE. At the last FTE, higher cumulative colloidal Al (+122%) and P (+114%) were observed with FT than with the control. FT resulted in lower cumulative bulk load of Ca, Mg, Mn, and Zn than the control after the second FTE. Furthermore, colloidal Fe, Al, Ca, Mg, Mn, and Zn mainly consisted of smaller colloids, while larger colloids dominated colloidal P. The findings from this study suggest that repeated freeze-thaw cycles can increase mobilization of colloids and colloid-associated elements in the soil.