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Abstract Boreal forests harbor as much carbon (C) as the atmosphere and significant amounts of organic nitrogen (N), the nutrient most likely to limit plant productivity in high‐latitude ecosystems. In the boreal biome, the primary disturbance is wildfire, which consumes plant biomass and soil material, emits greenhouse gasses, and influences long‐term C and N cycling. Climate warming and drying is increasing wildfire severity and frequency and is combusting more soil organic matter (SOM). Combustion of surface SOM exposes deeper older layers of accumulated soil material that previously escaped combustion during past fires, here termed legacy SOM. Postfire SOM decomposition and nutrient availability are determined by these layers, but the drivers of legacy SOM decomposition are unknown. We collected soils from plots after the largest fire year on record in the Northwest Territories, Canada, in 2014. We used radiocarbon dating to measure Δ14C (soil age index), soil extractions to quantify N pools and microbial biomass, and a 90‐day laboratory incubation to measure the potential rate of element mineralization and understand patterns and drivers of legacy SOM C decomposition and N availability. We discovered that bulk soil C age predicted C decomposition, where cumulatively, older soil (approximately −450.0‰) produced 230% less C during the incubation than younger soil (~0.0‰). Soil age also predicted C turnover times, with old soil turnover 10 times slower than young soil. We found respired C was younger than bulk soil C, indicating most C enters and leaves relatively quickly, while the older portion remains a stable C sink. Soil age and other indices were unrelated to N availability, but microbial biomass influenced N availability, with more microbial biomass immobilizing soil N pools. Our results stress the importance of legacy SOM as a stable C sink and highlight that soil age drives the pace and magnitude of soil C contributions to the atmosphere between wildfires.
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This content will become publicly available on August 5, 2026
Divergent Roles of Clay versus Iron (Hydr)oxide Minerals in Preserving Soil Organic Matter during Wildfire Heating
The interaction between soil minerals and soil organic matter (SOM) plays an important role in governing carbon release and sequestration in soil, yet understanding their behavior during wildfires remains poorly understood. This study examined the evolution of humic acid (HA, a representative of SOM) under simulated wildfire heating conditions (30–900 °C) in the presence of two representative soil minerals: montmorillonite (Mnt) and ferrihydrite (Fhy). Whereas Fhy accelerated the mineralization of HA, Mnt enhanced its preservation. These disparities stemmed from variations in the surface reactivity, structure, and transformations of Fhy and Mnt. Lewis acid sites, more abundant on Fhy surfaces than on Mnt surfaces, enhanced the decarboxylation of HA and caused carbon losses as CO2. However, Brønsted acid sites, which are more abundant on Mnt surfaces than on Fhy surfaces, enhanced carbon preservation by promoting HA isomerization and aromatization. Above 350 °C, lattice oxygen release from Fhy promoted the oxidative decomposition of HA, while Fhy itself underwent reduction to form magnetite, wüstite, and zero-valent iron. The confinement of HA within the micro/mesopores created by Mnt’s inert nanolayers prevented the thermal degradation of HA, enhancing carbon preservation. These findings advance our understanding of the specific roles of soil minerals in the decomposition, transformation, and preservation of SOM during wildfires.
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- Award ID(s):
- 2347694
- PAR ID:
- 10651692
- Publisher / Repository:
- ACS publications
- Date Published:
- Journal Name:
- Environmental Science & Technology
- Volume:
- 59
- Issue:
- 30
- ISSN:
- 0013-936X
- Page Range / eLocation ID:
- 15803 to 15815
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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