Seasonal patterns in water uptake for Medicago sativa grown along an elevation gradient with shallow groundwater table in Yanchi county of Ningxia, Northwest China

Abstract

Lucerne (Medicago sativa L.) is a deep-rooted perennial leguminous forage with high evapo-transpiration rate exceeding the annual precipitation in semi-arid areas of Northwest China. Groundwater might be the potential water sources of lucerne in the area with shallow groundwater table. In this study, stable isotopic compositions of oxygen and hydrogen (δ18O and δD) of different water sources and xylem sap were analyzed to determine the seasonal (April, June, July and August) and topographic (three slope positions) variations in water sources for lucerne growing in Ningxia eastern semi-arid area adjacent to Mu Us Desert characterized by shallow groundwater table. IsoSource software was used to calculate the probable contribution of different water sources to the total plant water uptake. Stomatal conductance, stem water potential, carbon isotope discrimination (Δ13C) of whole plant were also determined for evaluating the water status of lucerne growing at different slope positions. The results showed that soil water content increased as the elevation decreased. Oxygen compositions of soil water in the 0–40 cm profile fluctuated considerably. Soil water δ18O values in deep profile (>3.5 m) were similar to those in groundwater, implying the recharge of groundwater to this soil layer. Highest water utilization rate from deep soil profile (below 350 cm) was recorded for lucerne grown at the slope position 1 (groundwater table depth of 3.5–3.9 m) in April, June and July. The lucernes at slope position 2 (groundwater table depth of 5.8–6.4 m) and slope position 3 (groundwater table depth 7.1–8.3 m) mostly used water from deep soil layers (below 350 cm) during dry period, and turned to use water from superficial soil layer in wet period. Higher yield, Δ13C value of whole plant and stomatal conductance were observed for lucerne grown at the slope position 1 than those at other slope positions. These results indicated that groundwater is a significant water source for transpiration of lucerne grown in Ningxia semi-arid area with shallow groundwater table where lucerne grassland is suggested to be established so as to obtain better yield performance.