Rhizobium inoculation improves yield advantages and soil Olsen phosphorus by enhancing interspecific facilitation in intercropping

Background and aims: Maize/legume intercropping leads to overyielding and maintains soil nutrients. Rhizobium inoculation in maize/legume intercropping enhances soil nitrogen; however, its effects on overyielding and other soil nutrients in long-term intercropping systems is not well understood. Methods: We conducted a split-split-plot experiment with three factors in northwest China since 2009. The main plot treatments were without or with rhizobium inoculation in faba bean (-Rhizobium, +Rhizobium), while the sub-plot treatments were five nitrogen-application rates and the sub-sub-plot treatments were cropping system (monocultures of faba bean, maize and faba bean/maize intercropping). During 2018-2020, we measured the yield, soil nutrients in the 0-20 cm topsoil, calculated biodiversity effects, and quantified interspecific interaction of intercropping using the relative interaction index. Results: The grain yields in intercropping with treatments of -Rhizobium and + Rhizobium increased by 8.9% and 32%, respectively, compared with the corresponding weighted means of monocultures. Rhizobium inoculation increased the land-equivalent ratio at high nitrogen application. The combination of rhizobium inoculation and nitrogen application significantly enhanced the complementarity effect and relative interaction index of maize. With rhizobium inoculation, intercropping increased the soil Olsen P concentration by 13.9-59.9%, compared with the corresponding weighted means of monocultures which may be associated with interspecific facilitation, indicated by relative interaction index of maize. Conclusions: Our study shows that rhizobium inoculation increased yield advantages and soil Olsen P concentration via enhanced interspecific facilitation of faba bean on maize in the intercropping system. Rhizobium inoculation can be used as an efficient strategy to enhance the benefits of intercropping, especially in low-fertility soil.