Soybean Growth and Nodulation Under Mars-Like Magnesium Sulfate Concentrations

Colonising Mars requires sustainable food production systems that use local resources. However, the Martian regolith presents significant challenges for plant cultivation. We investigated whether soybean (Glycine max (L.) Merr.) could grow and form symbiotic nodules with Bradyrhizobium japonicum in MGS-1, a high-fidelity Martian regolith simulant. Soybean plants grown in an MGS-1:vermiculite mix (1:2) and supplemented with a nutrient solution were shorter, produced fewer leaves, and accumulated less biomass than sand-grown plants. More importantly, no nodulation occurred in MGS-1, despite successful nodulation in sand-grown controls. To identify factors inhibiting nodulation, we tested increasing concentrations of MgSO4; (50-150 mM), a major salt in MGS-1, and osmotic stress (300 mM Mannitol). While both treatments reduced nodule numbers, nodulation persisted across all MgSO4; concentrations and under osmotic stress, indicating that neither factor fully accounts for the absence of nodules in MGS-1. The shoot-to-root dry weight ratio increased under MgSO4; stress but decreased under mannitol, suggesting distinct stress responses. Our findings demonstrate that soybean can survive in MGS-1 with nutrient supplementation and an additional substrate to improve MGS-1 compactness and porosity. Still, soybean grown in MGS-1 exhibits severe developmental constraints and complete inhibition of biological nitrogen fixation. Overcoming these limitations will require strategies to mitigate salinity, alkalinity, and physical compaction, as well as engineering symbiotic conditions compatible with sustainable agriculture on Mars.