Wildfire and Extreme Rainfall Reduce Soil Carbon and Nitrogen Pools in a Semiarid Shrubland Ecosystem

Abstract Due to a combination of anthropogenic causes, semi‐arid shrublands like those in California and other Mediterranean‐type ecosystems are increasingly subject to wildfires and variable rainfall, including drought and atmospheric rivers. Fire changes soil structure, leaving soils vulnerable to erosion and loss of organic matter (OM), especially when followed by intense rain events. However, interactions of fire and extreme rainfall are challenging to study due to their unpredictability and the complex interacting factors such as fire intensity, vegetation structure, topography, and soil type. This study took advantage of a wildfire, natural variation in rainfall, and a controlled rain‐shelter experiment to examine the interactions of fire and rain on soil structure and OM storage. Fire reduced organic carbon and nitrogen associated with particulate organic matter and water stable aggregates, and changed soil hydrology, slowing infiltration of water to deeper layers. The combination of rain shelters and the extreme rain fall year (2023) allowed us to evaluate threshold effects of rain in post‐fire soils: in 2023, there was a loss of total soil OM from burned relative to unburned plots; rain shelter effects on soil organic matter pools were only detectable in 2023; and the interactive effects of fire and rain shelters in this year reduced infiltration of rain into soil in burned areas exposed to full rain conditions. These results suggest a feedback mechanism in which fire causes loss of soil OM, damages soil structure, and decreases rainfall infiltration, increasing erosion through increased runoff and slowing recovery of soil structure.