Dry soil layer forming and soil moisture restoration of alfalfa grassland in the semi-humid region of the Loess Plateau
黄土高原半湿润区苜蓿草地土壤干层形成及水分恢复

Soil water variations at the 0-1000 cm layers in alfalfa grassland with different years of growth (3a, 4a, 6a, 8a, 12a,14a,18a, 26a), dry soil layer formation and soil moisture restoration were investigated in the semi-humid region of the Loess Plateau. The study resealed that dry soil occurred in different soil layers of wild and alfalfa lands in a varying degree. Thus, slightly dry soil layer occurred in 80-100 cm soil of wild land; slightly dry soil layer appeared in 250-350 cm soil of the grassland with alfalfas growing for 8 years and less, and moderately dry soil layer appeared in the grassland with alfalfas growing for more than eight years and expanded beyond 500 cm deep below ground, indicating that as the alfalfa growth period prolonged, dry soil layer extended its lower boundary downward and got intensified. The grasslands with alfalfa growing for more than eighteen years began to recover their soil water contents in 0-200 cm upper soil at an average annual rate of 1.49%. The grasslands with alfalfa growing for eighteen and twenty-six years had soil water content of only 10.20% in 200-1000 cm deep soil showing the occurrence of soil drying through the soil layer.