Background The long-term cultivation of rubber plantations can lead to soil nutrient imbalance, reduced fertility, and decreased biodiversity. Against this backdrop, the new concept of ecological rubber plantations has been proposed. To explore the improvement effects of different planting modes of ecological rubber plantations on soil quality. Five planting modes, namely pure rubber (Hevea brasiliensis) forest, rubber intercropping timber forest, rubber intercropping Noni (Morinda citrifolia) forest, rubber intercropping coffee (Coffea arabica) forest and rubber intercropping green manure forest, were selected as the research objects.

Methods The sampling points were selected by the "S" type random point layout method. The undisturbed soil was collected by the ring knife method for the determination of soil physical properties. Soil samples from a total of five soil layers ranging from 0 to 100 cm were taken for the determination of soil chemical properties. The differences in soil physical and chemical properties were analyzed. Based on the principal component analysis method and the comprehensive soil quality index method, the soil quality scores of each principal component were first obtained, and then the comprehensive soil quality index was calculated to conduct a comprehensive evaluation of soil quality.

Results 1) The soil moisture content, capillary water-holding capacity and field water-holding capacity of pure rubber forest were relatively low, which may be related to the relatively low root system richness. 2) The pH value of the soil in the five planting modes ranged from 4.18 to 4.66, with the rubber intercropping green manure forest having the highest value of 4.66 and the rubber intercropping Noni forest having the lowest value of 4.18. The contents of total phosphorus, total nitrogen, total potassium and organic matter in the soil were the highest in the rubber intercropping coffee forest and the lowest in the pure rubber forest (P < 0.05), which may be related to the different habits of each tree species and the amount of nutrients absorbed from the soil. 3) The comprehensive soil quality index Q value of the five planting modes was the highest in the rubber intercropping timber forest, at 0.477, and the lowest in the rubber intercropping Noni forest, at 0.373, indicating that the rubber intercropping timber forest had a significant effect on soil improvement. The soil quality in the intercropping patterns in this study was relatively low, which might be related to human disturbance.

Conclusions This study explored the soil quality improvement effects of five planting modes in the early stage of ecological rubber plantation construction and obtained corresponding results. Ultimately, the planting mode with the best soil quality improvement effect was identified as rubber intercropping timber forests. This conclusion may provide a theoretical basis for the restoration of soil ecological functions in rubber plantations and has certain guiding significance for the selection of rubber plantation understory planting models.