Effects of wildfire and topography on soil nutrients in a semiarid restored grassland

Wildfire and topography each have significant effects on soil biogeochemical cycles, but their interactive effects on soil nutrients remain largely unclear, hindering the precise prediction of the effects of fire on soil biogeochemical cycles in a larger spatial scale. We examined soil nutrient contents from restored grass slopes that had suffered wildfires and adjacent restored grass slopes without any wildfire. Topographic factors included slope aspects (north and south slopes) and positions (upper, middle and lower slopes). Fire significantly increased the contents of soil organic carbon (OC), total nitrogen (TN) (0-10 cm), ammonium (NH4 (+)), and extractable phosphorous (EP), decreased the contents of nitrate (NO3 (-)) and available potassium (AK), and had minimum influence on total phosphorus (TP) content. Slope aspect and position also affected soil nutrients, with higher contents in the north slope than the south slope and at the upper slope than the lower slope. The effects of fire on soil OC, TN, and NO3 (-) were consistent across north and south slopes, but the effects on soil NH4 (+), TP, EP and AK varied with slope aspect. However, the effects of fire on soil nutrients were not influenced by slope position. These results indicate that slope aspect should be considered in predicting the response of soil biogeochemical cycles to fire.