Impacts of nitrogen on physiological interactions of the hemiparasitic Santalum album and its N 2 -fixing host Dalbergia odorifera

External N inputs can significantly affect the physiological traits and resource budget to both hemiparasite S. album and its N2-fixing host rosewood. Hemiparasitism is a mixotrophy nutritional strategy that combines autotrophy (photosynthesis) and heterotrophy (parasitic extraction), and hemiparasitic plants constitute ~ 90% of parasitic plants due to their evolutionary success and stability. Understanding the interactions between hemiparasite Santalum album and its N2-fixing hosts is of practical significance for S. album plantations, yet the impact of nitrogen (N) availability on the plants’ physiological traits is unclear. Under glasshouse conditions, N2-fixing host rosewood (Dalbergia odorifera), either infected or uninfected with parasitic, and unparasitic S. album (in the absence of its host D. odorifera) were fertilized under 0, 150, 300, 450, 600 mg N kg−1 growth media. Variables including haustorium formation, nodulation and N2 fixation, biomass, leaf chlorophyll concentration, carbon (C), N, phosphorus (P) and potassium (K) concentrations, 13C and 15N stable isotope composition, were measured. Nitrogen and parasitism had significantly interactive effects on total biomass, leaf δ15N and haustorium formation of S. album. At N450, the parasite had significantly higher total biomass when attached than unattached to rosewood. When grown with N2-fixing rosewood under N450, plant biomass production, C and N accumulation were maximized in S. album, indicating hemiparasitic S. album could shift between autotrophy and heterotrophy strategies that might be balanced according to the physiological requirements. With the exception of N600 treatment, leaf δ15N values of parasitic S. album for the same N rate were significantly lower than those of unparasitic plants, suggesting that the S. album directly derived N from the N2 fixation of rosewood. Regarding comparisons between respective N treatments, infection by S. album significantly suppressed host total and shoot biomass, N and P acquisition under N300, N450 and N600, but not under N0 or N150. These results demonstrate that the S. album-rosewood association is an example of highly variable hemiparasitism, in which host and nutrient availability affect the parasitic proportions of autotrophic and heterotrophic assimilates within the resource budget. This information can be used in the field to help host selection and N management for S. album plantations.