Supplementary material from "Resilience of lake biogeochemistry to boreal-forest wildfires during the late Holocene"

Novel fire regimes are expected in many boreal regions, and it is unclear how biogeochemical cycles will respond. We leverage fire and vegetation records from a highly flammable ecoregion in Alaska and present new lake-sediment analyses to examine biogeochemical responses to fire over the past 5300 years. No significant difference exists in δ¹³C, %C, %N, C : N, or magnetic susceptibility between pre-fire, post-fire and fire samples. However, δ¹⁵N is related to the timing relative to fire (χ² = 19.73, p < 0.0001), with higher values for fire-decade samples (3.2 ± 0.3‰) than pre-fire (2.4 ± 0.2‰) and post-fire (2.2 ± 0.1‰) samples. Sediment δ¹⁵N increased gradually from 1.8 ± 0.6 to 3.2 ± 0.2‰ over the late Holocene, probably as a result of terrestrial-ecosystem development. Elevated δ¹⁵N in fire decades likely reflects enhanced terrestrial nitrification and/or deeper permafrost thaw depths immediately following fire. Similar δ¹⁵N values before and after fire decades suggest that N cycling in this lowland-boreal watershed was resilient to fire disturbance. However, this resilience may diminish as boreal ecosystems approach climate-driven thresholds of vegetation structure, permafrost thaw and fire.