| Summary: | Wildfire activity in the western United States has increased with widespread consequences for soil biogeochemistry and forest ecosystem resilience. As wildfires become more severe and frequent, the recurrence of high-severity wildfires also increases. However, studies that examine the impacts of repeated high-severity wildfires on forest soil properties are limited. Here, we investigate the effects of single and repeated high-severity wildfires on soil carbon (C) and nitrogen (N) dynamics as well as soil nutrients, across two critical forests in California: the Coast Range redwood forest and the Sierra Nevada mixed conifer forest. Each of these ecoregions experienced sequential, overlapping high-severity fires within an 11-year period: Sierra Nevada (2014, 2020) and Coast Range (2009, 2020). Soil samples were collected from combinations of soil burn severity (SBS) classes (high-high, high-unburned, unburned-high, and unburned-unburned), at two depths (0–5 cm and 5–10 cm) from each ecoregion, 3 years post-fire. Results suggest that the effects of repeated and single wildfire were ecoregion, soil property, and SBS combination dependent. Coast Range soils (generally Mollisols), with initial higher total organic C (TOC), total N (TN), permanganate oxidizable C (POXC) and mineralizable C (Min C) were more susceptible to both single and repeated wildfire. For example, in the Coast Range a single initial high-severity fire significantly reduced TOC in surface soils, while repeated wildfire significantly reduced POXC and Min C. Sierra Nevada soils (generally Inceptisols) had lower initial POXC, TOC, and Min C and were less impacted by single and repeated wildfire, although TOC declined and Min C trended lower (p = 0.07) in reburned soils. Total N declined from repeat wildfire in the Coast Range, reflecting overall consumption of soil organic matter and volatilization of N, while repeat wildfire in Sierra Nevada soils lead to an increase in ammonium (NH4+) reflecting greater N pyro-mineralization relative to volatilization. Available phosphorus (P) decreased with single and repeated wildfires in both ecoregions. Results suggest that reburning led to the repeated consumption of surface litter, standing vegetation, fire-killed biomass, and coarse woody debris, reducing post-fire soil organic matter inputs, and therefore soil recovery. This resulted in significant impacts on the C, N, and P cycles in forest soils that experienced repeated high-severity fire. Our findings suggest that repeated high-severity wildfires may exacerbate negative impacts on soils from single wildfires in some contexts and could hinder long-term forest soil recovery and resilience.
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