Stable isotopes δ¹⁸O and δ²H reveal differential water uptake from intercropped maize and soybean soil profiles

The objective of this study was to investigate the effect of different intercropping modes on grain yield, crop water uptake, and water use efficiency in the rainfed agricultural regions. Conventional research methods had difficulties in accurately measuring water absorption soil profiles of grain crops under intercropping system. In this study, the seasonal variations in water uptake patterns of maize and soybean were identified using stable isotopes δ18O and δ2H in plant and soil water coupled with IsoSource model in monocrop and three intercropping modes: M2S2 (2 rows of maize and 2 rows of soybean), M2S4 (2 rows of maize and 4 rows of soybean), and M4S2 (4 rows of maize and 2 rows of soybean) on the Loess Plateau, China. The dominant water uptake depth of maize under monocrop contributed to 61.8 ± 10.3% in 0–20 cm, 40.3 ± 3.4% in 150–200 cm, and 65.3 ± 11.6% in 20–70 cm soil profiles of the total water uptake at the jointing, silking, and maturity stages, respectively. The dominant water uptake depth of intercropped maize contribute to 46.1 ± 1.1% in 0–20 cm, 31.8 ± 0.8% in 70–150 cm, and 36.9 ± 2.1% in 150–200 cm soil profiles of the total water uptake at the jointing, silking, and maturity stages, respectively. For soybean, the soil water at the 0–20 cm depth contributed 73.0 ± 2.4% (monocrop) and 78.4 ± 6.2% (intercrops) of the total water uptake across all three growth stages, respectively. Land equivalent ratio greater than one indicates yield advantages in intercropping systems compared to monocrop. Intercropped maize absorbed water from the soil layer deeper than that of soybean during the middle to late growth stages, indicating the intercropping advantages due to the differential water absorption depth. M4S2 mode had shallower main water absorption depth and higher water use efficiency than M2S2 and M2S4, respectively. The present study identified differential water absorption patterns between maize and soybean. The M4S2 intercrop mode was considered the optimal mode for rainfed intercropping systems due to its greater land and water use efficiency on the Loess Plateau.