Abstract: Oncodostigma hainanense, a wild plant species with extremely small population, is concentrated in Sanya and Baoting, Hainan Province. We compared the differences of soil nutrient contents in different habitats of O. hainanense in Ganshiling Provincial Nature Reserve, Yalong Bay Tropical Paradise Forest Park, and Qixianling Hot Springs National Forest Park in Baoting Li and Miao Autonomous County, and examined the effects of soil properties on the distribution characteristics of O. hainanense. The results showed that the structure of the three O. hainanense populations was unstable, indicating a declining trend, with the highest population density in Qixianling Forest Park (375 individuals per hectare). Soil in the distribution areas was characterized by acidity, with pH values ranging from 5.54 to 6.17. Organic matter content, available nitrogen content, and total nitrogen content were relatively high in the soils, while total potassium, total phosphorus, and available phosphorus contents were relatively low. Soil microbial biomass carbon ranged from 225.28 to 318.33 mg·kg^-1, and soil microbial biomass nitrogen ranged from 137.34 to 165.25 mg·kg^-1. Ganshiling Nature Reserve exhibited the highest soil catalase activity (4.53 mL·g^-1·h^-1), while Yalong Bay Forest Park had the highest soil acid phosphatase and urease activities (9.78 mg·g^-1·d^-1 and 0.84 mg·g^-1·d^-1, respectively). PCA analysis results showed that soil samples from each site could be well clustered into one group, and the response of each distribution site to soil factors was significant. Soil nutrients were significantly correlated with microbial biomass and enzyme activities, indicating that soil factors played a key role in driving population structure and growth of O. hainanense.

Key words: Oncodostigma hainanense, soil nutrient, soil microbial biomass, soil enzyme activity, extremely small population