Use of the Stable Oxygen Isotope Method to Evaluate the Difference in Water Consumption and Utilization Strategy Between Alfalfa and Maize Fields in an Arid Shallow Groundwater Area

Returning farmland to grasslands (RFG)to restore soil fertility and prevent land degradation in overused agricultural lands is an important measure. However, the hydrological processes, water consumption and utilization in field systems are undoubtedly changed in shallow groundwater areas owing to RFG. Experiments in the Hetao irrigation district, Inner Mongolia, China, during 2017 (2-yr for alfalfa) and 2018 (3-yr for alfalfa) were conducted to evaluate the difference in soil water distribution, percolation, groundwater recharge, water consumption and water productivity between alfalfa (Medicago sativa L.) and maize (Zea mays L.) fields. The stable oxygen isotope method was used to reveal a strategy to utilize different water sources for alfalfa and maize in an arid shallow groundwater area. Compared with maize, the increase in water consumption by alfalfa increased the soil water consumption of the 0-100 cm soil layer and the groundwater recharge, which led to an increase of 9.44 mm and 65.32 mm, respectively. However, the leakage below 100 cm layer decreased by 44.91% compared with those of the maize field. The yield and WP, of alfalfa were 29.27% and 6.28% greater than that of maize in 2018, respectively. Percentages of 75.43%, 18.73% and 5.84% of irrigation water, groundwater and precipitation were taken up by alfalfa during the irrigation period, respectively, while it was 81.27%, 12.90% and 5.83% in the maize field, respectively. After irrigation, the crops primarily absorbed water from upper soil layer (0-40 cm), comprising 72.93% (alfalfa) and 69.8% (maize). However, more water from the deeper layers was used by alfalfa before irrigation, and the average contribution of 40-100 cm soil layer was 63.0%, an increase of 11.2% compared with maize. In addition, even though maize absorbed water by roots that could reach a soil depth of 80 cm, it primarily drew water from the 40-60 cm soil layer.