Rhizosphere properties in monocropping and intercropping systems between faba bean (Vicia faba L.) and maize (Zea mays L.) grown in a calcareous soil

The processes involving pH modification, carboxylate exudation and phosphorus (P) dynamics in the rhizosphere of crops grown in intercropping are poorly understood. Two groups of maize (Zea mays L.) or faba bean (Vicia faba L.) plants (monocropping) or one group of plant of each species (intercropping) were grown between three 1-mm-thick soil layers; the central soil layer is referred to as inter-rhizosphere, and the two outer soil layers are designated sole-rhizosphere. Faba bean intercropped with maize had an 11% increase in shoot biomass and a 15% increase in P uptake compared with monocropped faba bean. The cropping pattern did not significantly influence maize growth. After 4 weeks of growth, faba bean significantly decreased soil pH in both the sole- and inter-rhizosphere in monocropping, but no effects were apparent for the intercropping rhizosphere. The major carboxylates in the rhizosphere of faba bean were malate (18–45 nmol g–1 soil) and maleate (1.2–2.4 nmol g–1 soil). Only trace amounts of carboxylates were measured in the rhizosphere of monocropped maize. However, intercropped maize had a high concentration of malate (~11 nmol g–1 soil) in both sole- and inter-rhizosphere; the malate was likely exuded by faba bean and was then diffused to the sole-rhizosphere of intercropped maize. The amount of malate exuded by intercropped faba bean was 19% higher than with monocropped plants. The results indicate that diffusion of protons and carboxylates extended the interaction zone between maize and faba bean, and may have contributed to enhancements of P uptake in the intercropping system.