The biogeochemical consequences of late Holocene wildfires in three subalpine lakes from northern Colorado

Wildfire activity has been increasing in forests of western North America over the past several decades. However, the biogeochemical effects of changing fire regimes are poorly understood. Here, we utilize sediment records from three subalpine lakes in northern Colorado (Hinman, Gold Creek, and Summit) to investigate the biogeochemical consequences of charcoal-inferred fire events over the past ∼2500 years. We measured element concentrations and stable isotope ratios (δ¹³C, δ¹⁵N) in lake sediments to track past biogeochemical processes. On average, fires were followed by increases in carbon (C), nitrogen (N), and sulfur concentrations in lake sediments, which lasted ∼20 years, while titanium and other metals found in terrestrial mineral material decreased. These changes were only statistically significant (p < 0.10) for nitrogen, titanium, and δ¹³C at Gold Creek Lake, and for sulfur and δ¹⁵N at Hinman Lake, suggesting either that the biogeochemical response to fire is variable through time or that the low temporal resolution of the proxy records (i.e., ∼19.2 years/sample) limited the ability to detect short-term impacts. Measurements of C/N, δ¹³C, and δ¹⁵N suggest that the sources of post-fire C and N differed among the study lakes. Sources of sedimentary organic matter include both erosion of soil organic matter and increased in-lake primary productivity, depending on site-specific watershed characteristics (e.g., vegetation, hydrology, elevation). These results suggest that if fire frequencies increase in the future, soil C and N stocks may not have adequate time to recover after fires, potentially jeopardizing the long-term biogeochemical resilience of these ecosystems.