Isotope-resolved root water uptake reveals dynamic water partitioning that underpins yield advantage in maize–soybean intercropping

Intercropping cereals with legumes is a promising strategy for sustainable intensification of global agriculture, yet the process-level mechanisms of its water-use advantages remain poorly quantified. Here, using dual stable isotopes (δ 18 O and δ 2 H) coupled with a Bayesian mixing model (MixSIAR), we quantified depth- and stage-resolved root water uptake (RWU) in maize–soybean intercropping under contrasting water regimes. Maize, the dominant competitor, shifted its RWU to deeper soil layers (40–100 cm) during reproductive development, peaking at silking when intercropped maize sourced up to 83% of its water from this zone. In contrast, soybean relied mainly on shallow soil water early in the season and showed partial mid-to-deep water uptake later, particularly under rainfed conditions, but remained more shallow-oriented than maize. This crop-specific partitioning minimized interspecific water-use overlap at silking (proportional similarity index, PSI: 0.42–0.50), indicating strong temporal hydrologic niche differentiation. Despite soybean yield trade-offs, intercropping improved system performance, with the 4:4 configuration consistently showing total land and water equivalent ratios exceeding 1.0; partial indices indicated that these gains were driven mainly by maize. Supplementary irrigation modified RWU partitioning and overlap dynamics across growth stages. Together, our findings provide process-level evidence that stage-dependent hydrologic niche complementarity underpins water-use efficiency gains in maize–soybean intercropping, supporting the design of water-saving cropping systems under contrasting water regimes.