Mineral Soils Are an Important Intermediate Storage Pool of Black Carbon in Fennoscandian Boreal Forests.

Approximately 40% of earth's carbon (C) stored in land vegetation and soil is within the boreal region. This large C pool is subjected to substantial removals and transformations during periodic wildfire. Fire‐altered C, commonly known as pyrogenic carbon (PyC), plays a significant role in forest ecosystem functioning and composes a considerable fraction of C transport to limnic and oceanic sediments. While PyC stores are beginning to be quantified globally, knowledge is lacking regarding the drivers of their production and transport across ecosystems. This study used the chemo‐thermal oxidation at 375°C (CTO‐375) method to isolate a particularly refractory subset of PyC compounds, here called black carbon (BC), finding an average increase of 11.6 g BC m−2 at 1 year postfire in 50 separate wildfires occurring in Sweden during 2018. These increases could not be linked to proposed drivers, however BC storage in 50 additional nearby unburnt soils related strongly to soil mass while its proportion of the larger C pool related negatively to soil C:N. Fire approximately doubled BC stocks in the mineral layer but had no significant effect on BC in the organic layer where it was likely produced. Suppressed decomposition rates and low heating during fire in mineral subsoil relative to upper layers suggests potential removals of the doubled mineral layer BC are more likely transported out of the soil system than degraded in situ. Therefore, mineral soils are suggested to be an important storage pool for BC that can buffer short‐term (production in fire) and long‐term (cross‐ecosystem transport) BC cycling. Plain Language Summary: Northern forests store a substantial fraction of the earth's carbon. Monitoring these stores is crucial for understanding forest contribution to atmospheric CO2 levels as well as their health under a changing climate. Wildfire has a major impact on shifting forest carbon stores to the atmosphere via emissions due to burning. However, a fraction of burnt material is left in forests as pyrogenic carbon (PyC). PyC is known to have positive effects on forest health and is thought to have increased resistance to being emitted to the atmosphere. Little predictive knowledge exists regarding how much PyC is produced during wildfire and even less is known about how that PyC is transported into the larger environment. This study utilized a large area wildfire field sampling campaign to study PyC in northern forests. It found that the amount of PyC produced could not be predicted by forest or fire properties. However, it was discovered that a large fraction of wildfire produced PyC quickly moves downward in the soil profile where it will be protected from decomposition and future burning. This novel finding suggests that one of the first steps of PyC transport is also a protective one, conserving carbon and mitigating climate change. Key Points: Fifty separate summer 2018 boreal wildfires were sampled 1 year postfire for black carbon (BC) across the 57–67° latitudinal range within SwedenFire had an insignificant effect on total BC stocks in organic layers but approximately doubled stocks in mineral layersMineral soil may function to buffer short‐term (production) and long‐term (cross‐ecosystem transport) BC processes [ABSTRACT FROM AUTHOR]