SUMMARY: Drylands are intricately linked with wildfires, but land-use change, climate change, and expansion of invasive vegetation have altered the fire regime of these sensitive ecosystems. This study, located in a sub-catchment within the greater Reynolds Creek Experimental Watershed in SW Idaho (see Figure 1), begins to address a gap in our knowledge about dryland carbon cycling as impacted by fires. Specifically, the aim was to characterize the origin, fate, and impact of fire-induced soil carbonates (i.e., CaCO₃, calcite, or soil inorganic carbon – SIC). By installing instrumentation prior to a prescribed fire ( https://youtu.be/Oo4-U84tX1w?si=Jz7mwk2Rbq3wegNj) in October, 2023, and in a manner that allows monitoring during the burn, we were able to collect a unique dataset able to address the project objectives. These results will enable researchers and land managers to extrapolate our findings throughout dryland regions to better quantify and predict wildfire impacts on dryland carbon cycling and storage.

PROJECT FINDINGS: Soil conditions that promote carbonate formation varied by soil burn severity during the fire. Soils under dense vegetation that burned at high severity produced the most carbonates, owing to near complete combustion of plant material that yield extreme carbon dioxide (CO₂) concentrations and abundant calcium in ash deposits – two constituents needed to form calcium carbonate (CaCO₃). Combined with soil water vapor movement to the soil surface and high soil water pH values due to the ash, carbonates appear to form at or near the soil surface, during or very soon after the fire when the soil surface is hot (~450°C). Isotopic analysis of the soil carbonates supports these conclusions, as the carbon and oxygen isotopes are very distinct from those naturally occurring in the region, and in a way that denotes high temperatures and very rapid reaction rates. In total, there were approximately 0.33 Gigagrams ± 0.11 of carbon (as carbonates) deposited in the modest 1.83 km² (~450 acre) catchment, measured 1-week post-fire (see Figure 2). For context, this compares to 8.1 Gigagrams of inorganic carbon for the larger watershed (249 km²). Survey of the soils 8-months post-burn, and following snowmelt and the rainy season (~550 mm of precipitation annually), pools of fire-induced carbonates had dwindled to just 0.03 Gigagrams in the top 6 cm of soil, indicating leaching to lower depths, to the groundwater, and/or loss as CO₂ to the atmosphere. This significant change in storage of the fire-induced carbonates is important when considering the permanence of the sequestered carbon. In this study, storage of the fire-induced carbonates appears to be short-lived, because of the higher elevation and greater precipitation. However, it is likely that at lower elevations where soil carbonates accumulate naturally (i.e., <500 mm of precipitation annually), at least a portion of fire-induced carbonates would be retained in the soil, resulting in long-term carbon sequestration that mitigates a portion of the carbon losses due to the initial wildfire. The implications of excessive carbonate additions to soils post-fire, the offset of carbon losses, and the fate of these carbonates are yet to be studied thoroughly.

BROADER IMPACTS: In total, this 1-year study yielded or facilitated four scientific articles (https://doi.org/10.3390/s24186034; https://doi.org/10.48550/ARXIV.2407.02420) (two in prep) and four curated datasets (https://doi.org/10.15482/USDA.ADC/25684290; https://doi.org/10.5069/G9XW4H00) (two in prep). The majority of these datasets assist land managers in rangeland management and wildfire mitigation. Further, the research has been presented at 5 public seminars or conferences.

Another essential element of this project’s success includes the development of a custom soil CO₂ and greenhouse gas (GHG) sensor and logger system (see Figure 3). This system was used to monitor soil CO₂ concentrations before, during, and after the fire. A provisional patent application was submitted September 5, 2024, via Boise State University’s Office of Technology Transfer (US Serial No.: 63/690,910). These sensor/logger systems were developed due to the high risk of using them during the prescribed fire and because existing commercial sensors were unreliable and/or too expensive. Soil greenhouse gas sensors are used to measure soil pore-space gas concentration, essential data needed to study carbon cycling and emissions. Applications include calculating CO₂ and methane (CH₄) emissions from soils to the atmosphere, identifying the source of emissions, assessing the health and productivity of agricultural soils, monitoring fugitive gas emissions from decommissioned landfills or reclaimed mining sites, and validation of sequestered carbon permanency related to CO₂-injection and carbon commodities. The lack of affordable and reliable soil gas sensors, and lack of customization to environment and application, currently limits research on these important topics; developing and distributing our technology has begun to support these essential research topics and stimulate innovation in soil gas sensing. This study also helped supported two early career faculty researchers (Drs. Huber and Anderson), 5 undergraduate researchers, and 2 high school interns. Collaboration from this project directly led to an additional synergistic project led by co-PI Anderson on infrasound monitoring during wildfire.

Last Modified: 10/01/2024
Modified by: David P Huber