RAPID: Effects of changing wildfire regimes on soil carbon fluxes during and following fire

The soil of northern forests holds massive reservoirs of carbon (C). This carbon is at risk of being lost to the atmosphere and contributing to climate change, due to recent patterns of wildfire that are also related to climate change. Determining how wildfires affect C stored in the soil is difficult, because it is rare that researchers know where a wildfire will take place before it happens, so there are not often measurements of C content from before the wildfire.

In addition to assessing how much C is lost to the atmosphere when organic matter is combusted during a wildfire, fires can change how the remaining soil C is transformed, as remaining organic matter decomposes and returns C to the atmosphere. For this RAPID project, the extensive wildfire season in Canada in 2023 offers this team a unique chance to measure fire-induced soil C losses at their previously studied sites and to compare the size of C emissions derived from combustion during the fire vs. decomposition afterwards.

In this project, the researchers will test pre-fire samples and return to to obtain and test post-fire samples in order to determine how fires affected carbon cycling. This research offers a critical opportunity to leverage rare pre-fire data to investigate pressing questions about wildfire effects on soil C content in a globally-relevant ecosystem.

The findings will inform understanding of post-wildfire ecology and interactions between fire severity and C cycling. In addition, junior researchers will be trained, and the project team will develop new outreach materials to engage the public on the topic of fires and soils.

As part of sampling campaigns in 2019 and 2022 in Wood Buffalo National Park (WBNP), AB and NWT, Canada, and the surrounding region, the research group collected a large number of soil samples from sites that had not experienced fire for at least 30-years. They characterized pre-burn soil C and N concentrations, pH, microbial community composition, and respiration (decomposition).

The 2023 WBNP fire complex burned 32 out of 70 of the previously sampled sites, representing a range of soil types, including organic-rich peatlands and sandy acidic soils, and dominant tree species. In this project, researchers are pairing post-burn sampling with the existing pre-burn samples to answer two questions: (1) How does fire severity relate to post-burn soil C decomposition rates? (2) How does the relative importance of C losses due to combustion vs. changes in C decomposition rates change over time with increasing fire severity?

At each site, researchers will estimate C losses due to wildfire, collect soil cores and quantify soil C, and measure potential decomposition using soil incubations. They will use simple biogeochemical models to extrapolate potential net C losses through decomposition, to quantify the relative magnitude of these two major C loss pathways – direct losses through combustion vs. changes in C decomposition rates after the wildfires.

Further leveraging this dataset, researchers will collect and preserve subsamples of soil for future efforts to characterize microbial community composition. These samples will be compared with pre-burn (and lab-burned) microbial communities from previous sampling campaigns – another opportunity to take advantage of these unusual pre-fire datasets.

Samples will also be preserved for further chemical characterization supported by future proposals, in order to explore the potential role that organic matter chemistry plays in determining post-fire soil C decomposition rates. The research project will provide training for a graduate student, undergraduate field assistants, and a research technician, as well as enhance wildfire and climate change public education.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.