Soil ammonium (NH4+) toxicity thresholds for restoration grass species.

Ionic rare earth mining has resulted in large amounts of bare soils, and revegetation success plays an important role in mine site rehabilitation and environmental management. However, the mining soils still maintain high NH4+ concentrations that inhibit plant growth and NH4+ toxicity thresholds for restoration plants have not been established. Here we investigated the NH4+ toxicological effects and provided toxicity thresholds for grasses (Lolium perenne L. and Medicago sativa L.) commonly used in restoration. The results show that high NH4+ concentration not only reduces the plant biomass and soluble sugars in leaves but also increases the H2O2 and MDA content, and SOD, POD, and GPX activities in roots. The SOD activities and root biomass can be adopted as the most NH4+ sensitive biomarkers. Six ecotoxicological endpoints (root biomass, soluble sugars, proline, H2O2, MDA, and GSH) of ryegrass, eight ecotoxicological endpoints (root biomass, soluble sugars, proline, MDA, SOD, POD, GPX, and GSH) of alfalfa were selected to determine the threshold concentrations. The toxicity thresholds of NH4+ concentrations were proposed as 171.9 (EC5), 207.8 (EC10), 286.6 (EC25), 382.3 (EC50) mg kg-1 for ryegrass and 171.9 (EC5), 193.2 (EC10), 234.7 (EC25), 289.6 (EC50) mg kg-1 for alfalfa. The toxicity thresholds and the relation between plant physiological indicators and NH4+ concentrations can be used to assess the suitability of the investigated plants for ecological restoration strategies.