Nodulation and root growth increase in lower soil layers of water-limited faba bean intercropped with wheat

Below-ground niche complementarity in legume-cereal intercrops may improve resource use efficiency and root adaptability to environmental constraints. However, the effect of water limitation on legume rooting and nodulation patterns in intercropping is poorly understood. To advance our knowledge of mechanisms involved in water-limitation response, faba bean (Vicia faba L.) and wheat (Triticum aestivum L.) were grown as mono- and intercrops in soil-filled plexiglass rhizoboxes under water sufficiency (80% of water-holding capacity) and water limitation (30% of water-holding capacity). We examined whether intercropping facilitates below-ground niche complementarity under water limitation via interspecific root stratification coupled with modified nodulation patterns. While no significant treatment effects were measured in intercropped wheat growth parameters, water limitation induced a decrease in shoot and root biomass of monocropped wheat. Likewise, shoot biomass and height, and root length of monocropped faba bean significantly decreased under water limitation. Conversely, water limitation stimulated root biomass of intercropped faba bean in the lower soil layer (15-30 cm soil depth). Similarly, total nodule number of faba bean roots as well as nodule number in the lower soil layer increased under intercropping regardless of water availability. Under water limitation, intercropping also led to a significant increased nodule biomass (48%) in the lower soil layer as compared to monocropping. The enhanced nodulation in the lower soil layer and the associated increase in root and shoot growth provides evidence for a shift in niche occupancy when intercropped with wheat, which improves water-limited faba bean performance.