Abiotic legacies mediate plant-soil feedback during early vegetation succession on rare earth element mine tailings

An increasing number of studies have shown how feedback interactions between plants and soil can influence primary and secondary succession. However, very little is known about the patterns and mechanisms of such plant-soil feedbacks on stressed mine tailings ecosystem, which can be severely contaminated by a range of toxic elements. 

In a two-phase plant-soil feedback experiment based on the rare earth element (REE) mine tailing soil, we investigated biotic (changes in bacterial and fungal community) and abiotic legacies (changes in chemical properties) of three pioneer grass species, and examined feedback effects of three grasses, two legumes and two woody plants with different root traits.

Positive plant-soil feedback was found in Miscanthus sinensis, Paspalum thunbergii and Tephrosia candida, and neutral feedback was observed in other four plants. These effects corresponded with an increase of nutrients and total organic carbon, as well as a decrease of acidity and extractable aluminum and REEs. There were less signs of biotic changes in the conditioned tailings. 

The correlation analysis suggested a relationship between responses to soil legacies and root traits, as well as root economics spectrum. On the mine tailings, acquisitive species with higher specific root length appeared to have greater potential for positive feedback. 

Synthesis and application: Our study shows that early succession on contaminated REE mine tailings may lead to more positive plant-soil feedback than predicted based on results of non-contaminated soils, mainly due to the alleviation of abiotic stress in tailings. Therefore, the improvement of specific abiotic soil stress and the trait-based selection of acquisitive plants should be preferentially considered to promote the primary restoration of degraded land.

Methods
In a two-phase plant-soil feedback experiment based on the rare earth element (REE) mine tailing soil, we investigated biotic (changes in bacterial and fungal community) and abiotic legacies (changes in chemical properties) of three pioneer grass species, and examined feedback effects of three grasses, two legumes and two woody plants with different root traits. 

The soils for the experiment were collected from several patches of REE mine tailings that had been abandoned since 2007. The grass species, comprising M. sinensis, Paspalum thunbergii and Digitaria sanguinalis, were planted in both the conditioning and feedback phases.Besides grasses, the legume species, comprising of Tephrosia candida and Cajanus cajan, and the woody species, comprising of P. massoniana and C. lanceolate, were planted in the feedback phase. 

The conditioning phase lasted for 18 weeks. At the end of this phase, the shoots and coarse roots were removed, soil samples were collected for chemical properties (abiotic legacies) and microbial communities(biotic legacies) measurement. [The collection of data in "SoilAbioticData" and "SoilBioticData"] 

The feedback phase lasted for 16 weeks. At the end of this phase, the plant shoots and roots were harvested for plant functional trait analyses. [The collection of data in "PlantTraitData"]

More details about materials and methods can be found in the paper.