Influence of an arbuscular mycorrhizal fungus on competition for phosphorus between sweet orange and a leguminous herb

Grass or herb intercropping with trees is widely practiced as an orchard-floor management strategy, but nutrient competition from grass species can inhibit the growth of intercropped fruit trees. Two experiments were conducted to investigate whether inoculation with the arbuscular mycorrhizal (AM) fungus Glomus versiforme can alleviate such competition and thus promote the growth of intercropped fruit trees by increasing soil nutrient exploitation. In the first experiment, intercropping was established in rhizoboxes containing sweet orange ( Citrus sinensis ) and the leguminous herb Stylosanthes gracilis inoculated with the AM fungus. Mycorrhizal inoculation did not appear to decrease competition, but increased the biomass of the herb much more than that of sweet orange. Inoculation had little effect on phosphorus (P) content of sweet orange, but significantly increased that of the legume roots. The AM fungal contribution to P uptake of the herb was twice that of sweet orange. Lateral roots of the herb tended to branch horizontally, with a large proportion entering the soil volume occupied by sweet orange; AM inoculation enhanced this effect. In Experiment 2, growth of the plants in monoculture revealed that the mycorrhizal dependency of the legume was much higher than that of sweet orange. It is suggested that mycorrhizal dependency can have a large influence on the role of the AM fungus in mediating competition in an intercropping system, and that fruit trees with high mycorrhizal dependency, together with a grass or herb with low mycorrhizal dependency, may be the optimum intercrop combination in orchards.