Fire is an important disturbance factor in many terrestrial ecosystems and plays a key role in shaping many forest systems around the world. Fire influences both above-ground and below-ground nutrient cycles by burning some of the vegetation, litter and duff, and therefore leads to nutrient losses. It can also cause important changes in the physical and chemical properties of forest soils including increased bulk density, and altered physical structure, soil moisture, pH and microbial populations. The degree of severity of the fire is largely dependent on the depth, moisture content and flammability of the forest floor material. During burning, nutrients can be lost to the atmosphere through volatilization and particulate transport. Because of its low volatilization temperature this loss includes most of the N. Fly-ash losses of P and K may be significant, although their volatilization temperatures are often higher. After the fire, nutrients are usually deposited on the soil surface in the form of ash, which may then be lost by wind or water erosion or leached into the soil. Wildfires are an important component of the historic disturbance regime of Pinus massoniana stands in south China, which can lead to important changes in the physical, chemical and biogeochemical properties of the soils under these stands. The effects of a wildfire on soil physical, chemical and biogeochemical properties (0 - 0.20 m) were studied in a field-scale experiment on a P. massoniana stand in Heyuan, Guangdong Province, China. We compared soil bulk density, porosity, capillary water, pH, soil organic matter, N, P, K, microbial populations and enzyme activity in unburned plots and in plots affected by wildfires burned over a 4-yr period, to understand the mechanisms involved in soil degradation in the burned P. massoniana stand. Wildfire significantly increased soil bulk density by 11%; slightly increased capillary porosity by 6%; and significantly decreased non-capillary porosity, total porosity and clay content by 30%, 8% and 16%, respectively; whereas the soils’ capillary water content decreased by only 5%. Wildfire had no significant effect on soil pH. Lower soil organic matter, N, P and K contents were observed in the wildfire plots compared with the controls, and the wildfires significantly decreased the soil organic matter content, total N, P, and K, alkalized N, available P, and K with values of 43%, 29%, 23%, 36%, 43%, 20% and 47%, respectively. High temperature and environmental changes resulting from the wildfires affected the soil microbial populations. The bacterial counts were significantly lower, in the wildfire plots whereas numbers of fungi and actinomyces remained unchanged. The wildfires caused a large decrease in urease, acid phosphatase and catalase activities, reducing them by 36%, 61% and 19%, respectively. Burning also caused nutrients to be deposited on the soil surface in the form of ash which caused bulk density increases, whereas the size of the microbial populations and their enzyme activity tended to decrease because of the lower organic matter content present.