Phosphorus modifies the effects of nitrogen on nodulation in split‐root systems of Hippophaë rhamnoides

Summary The effects of N (ammonium nitrate), P (phosphate) and their interactions on nodulation were studied in the intercellularly infected actinorhizal plant Hippophaë rhamnoides. A split‐root design, with pots receiving different concentrations of N and P, was used to determine whether the effects of N and P are local or systemic, if they are specific to nodulation or general, and whether P could counteract N inhibition. H. rhamnoides plants were grown for 6–10 wk after inoculation with Frankia. Inhibition of nodulation by N was systemic for both nodule number and nodule biomass in H. rhamnoides. By contrast, high P had a systemic stimulation on nodule number and biomass and P prevented systemic, but not local, N inhibition. Stimulation by P was specific to nodulation and not simply mediated via plant growth. Whether N and P alter not only nodulation but also N2‐fixation in the nodules requires further investigation.