Initial soil moisture prewinter affects the freeze-thaw profile dynamics of a Mollisol in Northeast China

The freeze-thaw characteristics are of great hydrological significance in seasonally frozen agricultural soil and may vary for different initial moisture conditions because of variability in land use, topography, tillage and climate change. However, the initial soil moisture regimes before winter freezing that control the dynamics of profile freeze-thaw cycles remain unclear. This study explored the soil profile freeze-thaw dynamics under different prewinter moisture regimes located in a seasonally frozen Mollisol in China. The high, medium and low soil moisture regimes were simulated by infiltrating different water into the soil profiles before winter freezing. We investigated the soil temperature and water content at five depths from October 2017 to May 2018. Our results revealed that the variation in soil water storage occurred after infiltration before winter freezing and lasted throughout the thawing period. The highest soil water occurred above a depth of 20 cm in the early days of the thawing period under all treatments. During the thawing period, the soil water content was highest above depths of 0-20 cm, 20-40 cm and 40-60 cm under high, medium and low infiltration, respectively. Freezing took longer than thawing from 0 cm to 100 cm soil depths. Freezing started later and thawing ended later under higher soil moisture conditions prewinter. The top 10 cm of soil experienced intense soil temperature rise and fall cycles with an average of 20 cycles during the thawing, and higher moisture conditions reduced the freeze-thaw cycles. The depth of zero amplitude in soil temperature under high, medium and low soil water was estimated as approximately 140, 134 and 160 cm, respectively. Our findings suggest that more attention should be given to the top layers and sidewall layers under the lower prewinter soil moisture conditions. This study provides a very detailed understanding of soil temperature and moisture dynamics over a full winter freeze-thaw cycle and would makes a worthy contribution in seasonally frozen areas where few such comprehensive datasets exist.