The forest soil organic carbon pool is a major component of forest ecosystems, in recent years, due to climate change, the frequency of forest fires has increased, which significantly affects forest ecosystems. This study helps to further understand the effects of different fire intensities on the soil properties of Yunnan pine pure forests in subtropical southwestern China, and provides a scientific basis for the restoration of the vegetation on the burnt sites in the area, and for the management of nutrients in the renewal of the sites.

Yunnan pine forest burnt site in the middle of Yunnan was taken as the research sample site half a year after the fire, and soil samples from different soil layers (0-20 cm and 20-40 cm) were collected in the non-burnt, light, medium and severe burning, and then determined the soil physicochemical indexes, SOC WSOC, ROC, MBC in different soil layers, and analysed the soil properties under different burning intensities. To study the significant differences and trends of soil properties under different fire intensities.

1) Different fire intensities significantly affected some soil physicochemical properties, in which soil bulk density, AK and AN increased with the increase of burning intensity, while SWC, STP and TN decreased with the increase of fire intensity; 2) SOC decreased with the increase of burning intensity, and the three kinds of soil reactive organic carbon (ROC) decreased with the increase of burning intensity, in which the decrease of the soil layer from 0 to 20 cm was more significant; 3) soil organic carbon and its reactive components were significantly positively correlated with AN and TN (P < 0.05), and significantly negatively correlated with AK (P < 0.05).

The burning resulted in a significant decrease in the content of soil active organic carbon components, changing some of the physical and chemical properties of the soil, and the performance was more significant under heavy burning, which can provide a scientific basis for the management of soil nutrients and vegetation restoration of Pinus yunnanensis Forest burning sites in the southwestern subtropical region.