Irrigation during Flowering Improves Subsoil Water Uptake and Grain Yield in Rainfed Soybean

Water is the main factor limiting soybean yield and the timely supply of supplemental irrigation could increase the grain yield, but the effects of a supplemental water supply on soybean yields have not been well studied. Field and pot experiments were conducted to compare the grain yield, yield components, water use efficiency for grain yield (WUEG), flower number, filled-pod number, soil water content, and root dry weight at different depths with and without supplemental irrigation at flowering. Field experiments showed that compared to rainfed conditions, 40 mm of water applied during flowering significantly increased grain yield by 26%, WUEG by 12%, filled-pod number by 16%, grain number by 13.3%, and water uptake from soil by 11% in 2011, and increased grain yield by 22%, WUEG by 7%, filled-pod number by 26%, grain number by 27%, and water uptake by 21% in 2012. The soil water content in the subsoil (1.2–2.0 m) layers under the irrigated treatment was lower, indicating greater water extraction, than in the rainfed treatment and water uptake was significantly and positively correlated with yield in both years. In a pot experiment, flower and filled-pod number, water use during flowering and podding were significantly higher in the well-watered (WW) treatment than cyclic water stress (WS) treatment. Flower number and filled-pod number were significantly and positively correlated with water use during flowering and podding, respectively, under both the WW and WS treatments. The root dry weight was higher in the 0.2–0.8 m soil layer in the WW treatment than the WS treatment. We conclude that supplementary water at flowering increased the water uptake from deeper soil layers by increasing the distribution of roots in the subsoil layers that resulted in the production of more flowers and filled pods and increased the WUEG and grain yield.