Possible Change of Water and Nitrate Cycles Associated with the Frost-Depth Decrease Under Climate Change

Abstract

Amounts of snowfall in early winter have increased in recent years because of climate change, thereby insulating soil from cold air and decreasing the soil frost depth during the last 20 years in the Tokachi region of northernmost Japan. To reveal soil water movement in recent winters, field observations were conducted for 5 years at an experimental field in this region. Soil froze to 0.2 m during two winters and a frozen soil layer was absent in three other periods. Substantial amounts of water infiltrated to the deep soil layer in both frozen and unfrozen winters, suggesting that the thin frozen layer did not impede the snowmelt infiltration in recent years. To simulate past soil frost conditions, we removed snow in early winter and replaced it after the soil frost depth reached the desired value. An approximately 0.4-m-thick frozen layer greatly impeded snowmelt infiltration during the spring snowmelt period. During the spring snowmelt period, the nitrate at the control plot, where natural snow conditions were maintained throughout winter and maximum soil frost depth was approximately 0.1 m, was carried deeper than that at the snow-removal treatment plot because of the greater amount of snowmelt infiltration at the control plot. These results imply a dramatic change of snowmelt infiltration and nitrate movement during the spring snowmelt period in this region, presumably resulting from climate change.