Quick Freeze-Thaw Cycles enable Rapid Solute Movement in Vertical Soil Columns

Quick and frequent freeze-thaw cycles (FTCs) are expected to increase due to climate change-induced warming in mid- and high- latitude regions. Warming trends during winter can impact biogeochemical cycles in land and water bodies. Bioaccumulation of soil nitrogen (N) products (nitrate, ammonium, and total nitrogen) on the soil surface during early spring and elevated N levels of streams hint at N movement within soil during winter. Natural field observations may not capture changes occurring during quick FTCs, and therefore, we developed a laboratory experiment to observe the movement of soil N products and unfrozen soil water during quick FTCs. Active solute transport occurs within a soil column during winter, as not all soil water undergoes freezing. Winter soil warming has been found to influence biogeochemical reactions within the top 100 cm, with high impact on solute movement in the top 30 cm depth. A 100-cm soil column filled uniformly with freely draining sandy loam (3.35 mm or finer grain size) was used for successive freezing and thawing for 4 days. Soil freezing was enabled using a 30-cm long freezing jacket with 10-cm wide detachable layers to adjust freezing depths over each 10-cm depth. Soil freezing for the top 10-cm, 20-cm, and 30-cm depths were enabled for three scenarios to observe the effects of freezing depth on solute movement during a 4-day FTC. An intensity-controlled infrared lamp above the soil column was used to thaw the soil. Soil moisture and temperature were monitored at the surface and at column depths of 15 cm, 30 cm, 45 cm and 60 cm. Soil water samplers collected porewater samples from 5 cm, 15 cm, 25 cm, 55 cm, and 80 cm depth. The depth below 60 cm was considered for the movement of solute towards or away from the freezing front during a FTC. There was an upward N migration observed during the 10-cm freezing depth scenario. N migration was the highest in the 10-cm freezing depth scenario. The observations obtained during FTCs were compared with a control scenario (no soil freezing) for the same duration. This experiment identified the direction of migration of solutes during FTCs. These results can help in soil nutrient management by controlling the availability of excess soil nitrogen, thus mitigating the impact of climate-warming on soil and water resources at a catchment scale.