Objective Reconstructing soil aggregate structure is a critical component of ecological restoration in rare earth mining wastelands. This study aimed to investigate the synergistic effects of sludge and different plant species on the improvement of soil aggregation in mine tailings and to elucidate the underlying mechanisms.

Method Using soil from an rare earth mining wastelands with poor aggregation properties, two primary treatments were established (with and without sludge application), under different planting configurations—monocultures of Neolamarckia cadamba or Alocasia longiloba, or their mixture, total of eight treatment groups. In a 150-day stratified rhizobox experiment, the distribution and stability of soil aggregates, soil cementing agents, and root morphological traits were analyzed, and a Mantel test for correlations was performed.

Result The synergistic application of sludge and plants significantly increased the proportion of water-stable macroaggregates (> 0.25 mm), mean weight diameter (MWD), and geometric mean diameter (GMD) in the soil layer directly in contact with the sludge. Among the treatments, 'sludge + A. longiloba' was the most effective, with the proportion of water-stable macroaggregates, MWD, and GMD being 1.27, 1.30 and 1.48 times than those of the unamended mine soil, respectively. The Mantel test indicated that aggregate composition and stability in the sludge-amended layer were significantly correlated with organic matter, polysaccharides, electrical conductivity, and pH; whereas in the layer not in direct contact with sludge, they were significantly correlated with total root length and the length of roots with a diameter < 2 mm. Subordinate function analysis further confirmed that the 'sludge + A. longiloba' treatment yielded the most effective aggregation.

Conclusion In conclusion, the synergistic application of sludge and plants could effectively promote soil aggregation in rare earth mining wastelands. In the soil layer directly amended with sludge, organic matter, polysaccharide content, and electrical conductivity were the key factors influencing aggregation. In the underlying, unamended layer, total root length and the length of fine roots (< 2 mm in diameter) were the primary influencing factors.