Changes in the mineral element compositions of soil colloidal matter caused by a controlled freeze-thaw event

This study investigated the effects of a controlled freeze-thaw event on the mineral element compositions of colloidal matter in soil interstitial water. The experiment was designed to decouple the freeze-thaw effect from the warming effect by conducting sequential treatments. The temperate soil samples were incubated in a refrigerator at 2 °C for four weeks and compared with those frozen at − 20 °C in the second week and thawed at 2 °C for the following two weeks; this was done in order to study the freeze-thaw effect while minimizing the influence caused by warming. The soil samples incubated at 25 °C in the fourth week after two weeks of the freeze-thaw treatment were compared with the freeze-thaw treatment group to investigate the warming effect. The laser-induced breakdown spectroscopy (LIBS) technique was used to analyze the relative elemental concentrations in the soil colloidal matter (SCM), which was collected on nylon membrane filters. The six predominant mineral elements were classified into two groups based on their distinct trends in response to the treatment conditions. The type 1 element (Si, Mg, Al) contents were decreased both by the warming condition and by the freeze-thaw treatment, whereas the type 2 element (Fe, Ca, Ba) contents were decreased by the freeze-thaw treatment but increased by the warming condition. Only type 1 elements showed positive correlations with the amount of organic carbon. From the results of the elemental compositions in SCM, the effects of a freeze-thaw event can be contradictory depending on the thawing temperatures.