Abstract: To assess the feasibility of utilizing oil shale residue to increase Medicago sativa yield and ensure edible safety, we measured the biomass, carbon (C), nitrogen (N), phosphorus (P) stoichiometry, and heavy metal enrichment in different organs (leaf, stem, and root) of M. sativa under four levels of oil shale residue addition (0, 3, 6, and 9 kg·m^-2). The results showed that, with increasing oil shale residue addition level, a pattern of initial increase and then decrease was found in plant height, basal diameter, number of roots and nodules, biomasses and total N concentrations in each organ, and root N∶P at seedling and flowering stages, and C concentrations in leaves and roots at mature stage. Moreover, a pattern of first decrease and subsequent increase was found in specific root length and specific root area, and the concentrations of copper and chromium in various organs increased, being higher in roots than in leaves and stems. Compared with the control, plant height increased by 36%, 23% and 26%, basal diameter increased by 56%, 16% and 20%, and total biomass increased by 130%, 135% and 30% at seedling, flowering, and mature stages, respectively, while heavy metal concentrations in the aboveground parts did not exceed the standard under oil shale residue addition of 6 kg·m^-2. Therefore, 6 kg·m^-2 oil shale residue addition was optimal for M. sativa, which effectively promoted plant growth and alleviated N limitation.

Key words: oil shale residue, Medicago sativa, root growth, heavy metal accumulation, Horqin Sandy Land