Background A large number of mine tailings have been generated by the excessive mining of rare earth ores. These tailings are characterized by low acidity, poor water storage capacity, low nutrient content, and high contents of rare earth and heavy metal elements, which seriously restrict the growth of plants. Biological soil crusts (BSCs) (associations of soil particles with mosses, fungi, algae, cyanobacteria, or lichens) are distributed extensively in South China, which can adapt to the arid and barren environment. Therefore, it may be used as an option for ecological restoration of mine tailings. However, limited information is available about the growth and influencing factors of artificial cultured BSCs in rare earth tailings.

Methods Indoor incubation experiments of BSCs in the rare earth tailings were conducted to investigate the development characteristics of BSCs artificially cultivated in the rare earth tailings. Nine experimental treatments with 5 replications, including two factors (e.g. light intensity and soil water content) and three levels were randomly arranged. During the cultivation period (from November 2019 to July 2020), the coverage, plant density, and biomass of BSCs were dynamically monitored. The coverage, plant density, and biomass of cultivated BSCs were further statistically analyzed by univariate multivariate analysis of variance of a general linear model.

Results 1) The indoor incubation experiment showed that the coverage of BSCs reached 100% in 80 d, and the plant density was more than 54.8 plants/cm² in 140 d, under the condition of high moisture (28%-30%). No significant differences in plant coverage and density existed among different light intensities.2) Also under the condition of high moisture, significant differences in moss biomass were observed among different light intensities in a decreasing order as follows: 86.79 mg/cm² (at a medium light intensity of 5 900 lx), 70.15 mg/cm² (at a high light intensity of 5 900 lx), and 36.18 mg/cm² (at a low light intensity of 1 900 lx) (P < 0.05). Under the conditions of medium soil moisture (13%-15%) plus medium light intensity, the coverage of BSCs was 54.07%, which was significantly higher than those under the conditions of medium soil moisture plus high light intensity and medium soil moisture plus low light intensity (P < 0.05). Under the conditions of low soil moisture (5%-8%), BSCs could not grow and develop normally under all light intensities.

Conclusions The obtained results indicate that BSCs could be used as a measure of ecological restoration in the rare earth tailings mining areas. Several moss species such as Trichostomum brachydontium, Trichostomum involutum, Anoectangium stracheyanum, and Brachymenium exile have good adaptability in vegetation restoration of rare earth tailings. Soil moisture is the most important factor affecting the development of BSCs. The optimal conditions for the growth of BSCs in rare earth tailings are soil water content of 28%-30% and light intensity of 5 900 lx.