Lithium Uptake, Physiological Responses, and Root-Scale Stabilization Mechanisms in Perennial Ryegrass

Mining activities are a major driver of long-term geo-environmental and eco-environmental degradation, particularly during the post-mining stage when residual contamination and ecological risks often persist or intensify. The rapid expansion of lithium mining and processing linked to the battery and energy-transition industries has increased Li accumulation in soils and waters around mine sites, posing emerging risks to terrestrial ecosystems and land reclamation. Owing to its high mobility and weak natural attenuation, lithium presents particular challenges for post-mining soil remediation, highlighting the need for effective, low-cost, and ecologically compatible remediation strategies.

Here we evaluate perennial ryegrass (Lolium perenne L.) for lithium phytoremediation using a 42-day hydroponic experiment with six LiCl treatments (0, 7, 140, 280, 560, and 700 mg L^-1). Plant performance was assessed through seed germination, growth traits, photosynthetic pigment contents, oxidative stress indicators, and antioxidant enzyme activities. Lithium removal and accumulation were evaluated to assess plant tolerance and remediation efficiency. Lithium exposure induced clear dose-dependent inhibitory effects on germination and seedling development, with pronounced suppression occurring at concentrations ≥280 mg L^-1. Under high Li stress (560–700 mg L^-1), ryegrass exhibited significantly reduced growth and photosynthetic pigment contents, accompanied by enhanced oxidative damage, indicating that prolonged and intense Li stress can disrupt physiological homeostasis. Correlation analysis further demonstrated a stress-threshold–dependent physiological shift, whereby Li accumulation was positively associated with oxidative stress indicators (MDA) and antioxidant enzyme activities (SOD, POD, and CAT) under moderate Li stress, but became negatively correlated with growth and photosynthetic parameters at higher Li levels, reflecting a transition from adaptive defense responses to toxicity-dominated inhibition. Despite these adverse effects, ryegrass maintained substantial remediation capacity across a wide concentration range. Lithium removal increased consistently with exposure time, and after 42 days, removal efficiencies exceeded 60% for all moderate-to-high treatments (280–700 mg L^-1). At the highest Li concentration, maximum Li accumulation reached 38.26 mg g^-1, which is significantly higher than Li accumulation levels reported for maize and sunflower under comparable conditions. Tissue partitioning indicated root-dominated Li retention, limiting translocation to shoots.

To elucidate the microscale mechanisms of Li stabilization, time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging was conducted on root cross-sections, and an ion–ion spatial correlation matrix was constructed. Lithium signals were concentrated in organic-rich microdomains near the cortex and endodermis, showing strong co-localization with oxygen-rich fragments derived from polysaccharides and phenolic structures (indicative of carboxyl and hydroxyl functional groups), as well as phosphate-related fragments. In contrast, negative spatial associations with Si-rich mineralized regions were observed, highlighting the dominance of biochemical domains rather than silicified structures in Li sequestration.

Overall, effective Li removal by perennial ryegrass is supported by root-dominated uptake and coordinated physiological regulation, with Li primarily associated with oxygen-rich organic functional groups and phosphate domains in roots, and a remediation threshold around 280 mg L^-1. These findings provide both quantitative performance metrics and mechanistic evidence supporting the application of ryegrass-based phytoremediation for Li-mining and Li-industrial soil pollution, and they offer practical guidance for developing scalable remediation strategies at post-mining sites.