Wildfire is a key factor regulating carbon (C) and nitrogen (N) retention in the Sierra Nevada. Therefore, as fire regimes shift in response to climate change and management, it is vital to understand how belowground C and N cycling will respond. However, studying wildfires is challenging. Fire timing and location are difficult to predict and as a result, researchers must often rely on space-for-time substitutions to evaluate fire effects. Unfortunately, wildfires rarely leave behind unburned areas that are representative of the areas that burned. To eliminate such problems, many studies have focused on prescribed fire which enables the researchers to conduct pre- and post-fire measurements at a known location. However, prescribed fires differ from wildfires in their severity, heterogeneity, and spatial scale. Thus, to understand the effects of wildfires on soils, we need to combine the benefits of sampling wildfires with pre- and post-fire sampling. Here, we directly assessed the effect of wildfire in the Sierra Nevada by collecting soil samples in the path of advancing wildfires. We then resampled the sites immediately post-fire and approximately one month later. Because some of our sampling sites did not burn, we were able to examine the role of wildfire alongside seasonal processes driving post-fire soil dynamics. We analyzed the organic and mineral soil horizons for N mineralization and nitrification rates, pH, microbial biomass, and total C and N.
Results/Conclusions
Preliminary results show pronounced spikes in pH following fire for both mineral and organic layers. The magnitude of these spikes increased with fire severity and were larger in organic than in mineral horizons (i.e., 4 and 0.7 pH unit increases in high severity plots, respectively). These pH increases were ephemeral and not seen in the plots that did not burn. Our results also suggest that microbial responses vary with fire severity: microbial biomass was higher in mineral soils that burned at high severity while plots that burned at low severity had similar biomass to unburned plots. Organic horizon microbial biomass was lower in the high severity plots than all other locations. Our future work includes soil temperature measurements to quantify the effects of wildfire intensity on belowground biogeochemical fluxes. These measurements are crucial for projecting how carbon and nitrogen retention will respond to future fire and climate conditions.