Soil Quality Variation and Its Driving Factors in Rubber Plantations of Tropical China

Understanding the key factors that determine soil quality is essential for the recent and rapid expansion of rubber plantations in the tropics. Tropical China is an ideal experimental area because its rubber plantations are geographically dispersed and belong to different climatic zones. Soil quality in this study was comprehensively evaluated using a soil quality index (SQI) based on 28 soil physical, chemical, and biological properties. The factors driving variation in the SQI of rubber plantations were analysed utilising redundancy analysis and a structural equation model. The results indicated that no age effect of the plantation on soil quality was observed in the same rotation period, whereas the SQI of rubber plantations was found to be relatively low after long-term cultivation. There was an apparent discrepancy in the spatial distribution of the SQI of rubber plantations. The SQI demonstrated a declining trend with increasing soil depth, with the SQI of the surface soil layer (0–10 cm) exhibiting a significantly higher value than that of deeper soils. Soil condition was found to be predominant in the SQI variation of rubber plantations, particularly the seven soil properties including saturated water capacity, easily oxidisable organic carbon, ammonium nitrogen, available phosphorus, total potassium, manganese, and zinc. The direct effects of geographical location, vegetation cover, and climatic variables on rubber plantation SQI variation were relatively weak, while they indirectly affected the SQI variations by influencing soil condition. The SQI results are expected to provide a quantitative foundation for enhancing soil quality in rubber plantations in the tropics.