Abstract:The worst drought in 100 years in southwest China occurred in the winter-spring period in the year of 2009-2010, causing forest fires as a secondary disaster. The effect of fire on the physicochemical properties of soil in a artificial secondary forest of Pinus massoniana in the center of Guizhou Province, China was investigated. The trees had an average diameter at breast height (DBH) of 5.6 to 19.4 cm, an average height of 4.11 to 18.60 m, and average density of 500 to 2400 clumps/hm2, and were studied using a comparison of burnt and unburnt plots in a karst mountain area covered by a Quaternary clay. In the surface soil (depth determined by the influence of the fire) of the burnt plots, capillary porosity and total porosity increased, and the soil bulk density and non-capillary porosity decreased, becoming 104.0, 102.2, 96.0 and 79.9% of their previous values, respectively. The water content of the soil quality and soil volume, and the maximum and minimum values of the soil's water holding capacity were 92.5, 86.9, 110.0, and 111.4% respectively. Also, the relative amount of organic matter, total nitrogen, total phosphorus, and total potassium and the pH increased to 130.8, 138.0, 148.7, 108.3, and 101.6% of their previous values, respectively, while the cation exchange capacity was reduced to 74.2%. In contrast, the relative amount of hydrophilic dissolved organic nitrogen, effective phosphorus, available potassium and exchangeable bases were increased, being 185.7, 301.7, 201.3 and 109.7%, respectively. As a result of the forest fire, the organic matter carbonized, soil biota was reduced, the soil aggregates collapsed and the soil water stability was reduced with changes in the osmotic potential; hence, the texture was degraded in the surface soil. Because of the carbonization of the organic matter of the different litter layers of the burnt forest, a considerable amount of ash, small carbon particles and organic debris covered the surface soil, and infiltrated into the soil through the action of gravity and rain, and as a result the fertility of the surface soil increased. There were two changing trends for soil physicochemical properties: one of them was the influencing factors of the surface soil (IFS) which were much greater than those for the profile soil (IFP), and included variables such as soil bulk density, capillary porosity, total porosity, water contents of soil quality or soil volume, the maximum or minimum water holding capacities, the amount of soil organic matter, total phosphorus, total potassium, hydrophilic dissolved organic nitrogen, available potassium and pH value; the other was when soil properties representing IFS were less than the IFP, such as non-capillary porosity, the amount of total nitrogen, and effective phosphorus, cation exchange capacity and exchangeable bases in the burnt amd non-burnt forest soil. Physicochemical indices of the burnt and non-burnt forest soil rose or fell in the soil profile, and trends simulated either the power or logarithmic curve well. The changes in the physicochemical indices between the burnt and unburnt forest in the surface soil reflected mainly the impact of the fire, and mirrored chiefly the difference of the natural soil properties and the effect of the biological community on the regolith. The coefficients for the relationships between the plant dead ratio of tree layer and the surface soil bulk density (R=-0.8250*, r0.05=0.7545, the same was as follows), the maximum and minimum water holding capacity, (R=0.7615* and R=0.7689*, respectively), the amount of organic matter, (R=0.9035* *, r0.01=0.8745, the same was as follows) and total nitrogen (R=0.7558*) were remarkable. The dependence coefficients of the plant dead ratio of shrub layer to the surface soil bulk density (R=0.8547*), capillary porosity (R=-0.7597*), total porosity (R=-0.7629*), water content of soil quality (R=-0.7593*), maximum and minimum water holding capacities (R=-0.9573* * and R=-0.9124* *, respectively), the amount of organic matter (R=-0.9436* *), total nitrogen (R=-0.8335*), total phosphorus (R=-0.7599*), and available potassium (R=-0.7995*) were also notable. The correlation coefficients for the shrub biomass loss ratio to surface soil bulk density, amount of organic matter and available potassium were R=0.7684*, R=0.7763* and R=-0.7600*, respectively. Coefficients for the litter biomass loss to the pH value of surface soil indicated a very strong relationship existed between these two variables (R=0.7550*). In addition, the correlation coefficients for the average DBH of the burnt forest to the surface soil bulk density, capillary porosity, total porosity, water content of soil quality, maximum and minimum water holding capacities, amount of organic matter, were notable, being R=0.8085*, R=-0.8162*, R=-0.8077* and R=-0.9556* *, R=-0.9153* * and R=-0.9049* * and R=-0.8120*, respectively.