COS 83-6 - Belowground effects of repeated, high severity fire in the Sierra Nevada of California, USA

Abstract: Fire behavior in the Sierra Nevada of California, USA is changing. Large areas of forest are experiencing repeated, high severity fire at sub-decadal return intervals. This has increased the proportion of forested land held in an early-successional state and has raised concerns regarding the resilience of forest ecosystems. In this study, we identified belowground effects of repeated, severe fire that could have an important role in forest recovery. This included changes to soil total carbon (C) and nitrogen (N) concentrations and C storage in mineral-associated organic matter (MAOM) and particulate organic matter (POM), inorganic N concentrations, and heterotrophic respiration (R_h). We compared soil biogeochemical properties from sites that had burned once in 2019 or in 2021 with sites that had burned in both 2007 and 2019 or in both 2007 and 2021. We also included a site that burned in 2007 alone and an “unburned” (no fire for 50+ years) control (6 sites total).





All sites that experienced severe fire had comparable soil total C and N concentrations that were lower than the unburned site despite differences in fire frequency or time since fire among the burned sites. Losses of soil C and N through fire included the loss of C and N in MAOM, suggesting MAOM is not protected from high severity fire and is slow to recover. Inorganic N concentrations and net transformations were comparable and relatively low in all sites that burned prior to 2021. The sites that burned most recently in 2021, regardless of whether they previously burned in 2007, showed elevated ammonium (NH₄⁺) and nitrate (NO₃^-) concentrations relative to all other sites. However, NH₄⁺ concentrations were higher in the single-burn 2021 site than the double-burn 2021 site, suggesting post-fire available N pulses may be attenuated by more frequent, severe fires. Interestingly, the 2007 site as well as the single- and double-burn 2019 sites had similar total soil C and N concentrations and fractions, inorganic N concentrations, and net N transformation rates. Heterotrophic respiration was comparable across all sites but more variable in recently burned sites. Overall, our study revealed that soil total C and N storage and fractions may differ more depending on time since fire, while inorganic N concentrations and net transformations also vary by fire frequency. Such aspects of soil biogeochemistry that are affected by repeated, severe fire may be particularly meaningful for forest recovery.