Post-wildfire wind and water erosion could accelerate toxic metals and nutrients movements on subtropical karst hillslopes

Postfire nutrients and toxic heavy metals movements threaten the clean water supply and ecosystem maintenance in the karst region. However, the complexity of these processes and spatial heterogeneities of burning severity and karst landscape limit the study of these processes. In this study, we disentangled the complexities by dividing the processes into wind-dominated and water-dominated periods. Field sampling, artificial burning and overland flow detachment, and in-situ observations were conducted on both burn and unburned sites. The results exhibited that wildfire induced ash produced P, As, and Cd. Approximately 70 % of ash was lost driven by the wind in only 1.5 months after the wildfire. The loss was significant two years after the fires. The combustion-induced ash had twice more P than the unburned soil. However, it lost fast via ash wind erosion after the fire. The combustion promoted rock surface weathering, which increased soil nutrients and heavy metals after rainfall occurred. The effect was offsetted by water erosion in the wet season. In the water-dominated period, 4.26 mm of land surface soil was eroded by the raindrops and overland flow due to the reduced soil resistance to erosion and land surface cover. Water erosion could promote soil particle-absorbed nutrients and toxic metal migration on the land surface and through the subsurface fractures. This study calls for controls of the wind and water coupled processes on soil erosion, heavy metals, and nutrient movements for post-wildfire land management in the subtropical karst region.