Soil water distribution on different number of growing years of alfalfa pasture in the Loess Plateau of Northwest China

The alfalfa pastureland in the semiarid Loess Plateau region of Northwest China usually has dry soil layers. We studied the soil water variations on alfalfa (Medicago sativa L.) grassland with different number of growing years. Seven growing years of alfalfa grassland were chosen in this study: (1) 4-year-old, (2) 6-year-old, (3) 8-year-old, (4) 12-year-old, (5) 14-year-old, (6) 18-year-old and (7) 26-year-old. The results showed that the highest soil water content, 12.0 - 17.15%, with different number of growing years of alfalfa grassland occurred in 80 - 100 cm soil layer, but gradually became stable below 300 cm soil layer. The soil water content with 4, 6 and 8 year alfalfa grassland was in the range of 13.66 - 14.76%, with 12 and 14 year ranged within 11.76 -1 1.87% and with 18 and 26 year within 10.5%. We also found that in 0 - 1000 cm soil layer, the soil water content with different number of growing years of alfalfa grassland had differences due to different soil water conditions and water-supplying capability. The soil water content with 4 and 6 year were 13.85 and 14.22%, respectively, with 8, 12 and 14 year were 12.98, 11.25 and 11.22%, respectively, and with 18 and 26 year were 10.27 and 10.76%, respectively. After alfalfa grew for >18 years, the annual recovery of its soil water at 0 - 200 cm soil depth was 1.49%, whereas the soil water with 18 and 26 year alfalfa pastureland at 200 - 1000 cm soil depth was only 10.10%, dry soil layers occurring in alfalfa pasture. The dry soil layer of alfalfa grassland appears at 160 - 600 cm soil depth in the Loess Plateau. We found that at 250 - 350 cm soil depth, the soil water content with 4 and 6 year alfalfa grassland was in the range of 10.23 - 10.48%, presenting slightly dry soil layer and for more than 8 year, alfalfa was in the range of 7.78 - 8.48%, presenting moderately dry soil layer. In summary, the soil water use of alfalfa would grow with the number of growing years and the depth of desiccated layers will become intensified and thicker.