Lithium-induced alterations in soybean nodulation and nitrogen fixation through multifunctional mechanisms.

The increasing footprints of lithium (Li) in agroecosystems combined with limited recycling options have raised uncertain consequences for important crops. Nitrogen (N)-fixation by legumes is an important biological response process, but the cause and effect of Li exposure on plant root-nodule symbiosis and biological N-fixation (BNF) potential are still unclear. Soybean as a model plant was exposed to Li at low (25 mg kg), medium (50 mg kg), and high (100 mg kg) concentrations. We found that soybean growth and nodulation capacity had a concentration-dependent response to Li. Li at 100 mg kg reduced the nodule numbers, weight, and BNF potential of soybean in comparison to the low and medium levels. Significant shift in soybean growth and BNF after exposure to Li were associated with alteration in the nodule metabolic pathways involved in nitrogen uptake and metabolism (urea, glutamine and glutamate). Importantly, poor soybean nodulation after high Li exposure was due in part to a decreased abundance of bacterium Ensifer in the nodule bacterial community. Also, the dominant N-fixing bacterium Ensifer was significantly correlated with carbon and nitrogen metabolic pathways. The findings of our study offer mechanistic insights into the environmental and biological impacts of Li on soybean root-nodule symbiosis and N-acquisition and provide a pathway to develop strategies to mitigate the challenges posed by Li in agroecosystems.