The sowing ratio in legume intercropping regulates the facilitation of phosphorus uptake in wheat

The intercropping of legumes with wheat is an interesting method to optimize the use of resources such as phosphorus (P) and nitrogen (N). This occurs when the roots of wheat and legumes interact generating positive (facilitation and complementarity) rather than negative interactions (competition). This research aims to assess how the sowing ratio, which affects root interactions, drives the shift from competition to complementarity and facilitation in legume-wheat intercropping, hypothesizing that each legume species exhibits different behaviour. Under climate-controlled conditions, wheat was intercropped with pea, faba bean, and lupin, varying the number of legume plants per pot (legume density). The effects of legume density on competition/complementarity/facilitation were investigated by monitoring root exudation of carboxylates, acid phosphatase, and nutrient acquisition in both wheat and legumes. Legumes enhanced P uptake of wheat, while N was more strongly influenced by the increase in wheat dry matter observed under intercropping. Faba bean and lupin facilitated wheat P uptake at higher density, while pea facilitated P uptake at lower density. The legume species and plant number affected the concentration and composition of carboxylates in the wheat rhizosphere. Shoot P concentration of wheat was positively correlated with oxalate, malate, citrate, and succinate. In the case of lupin intercropping, phosphatase activity contributed more to facilitation than carboxylates. Intercropping with legumes is a valuable strategy for enhancing the use of resources such as P and N when the shift from competition to complementarity and facilitation is triggered by an appropriate sowing ratio, which depends on the legume species.