Highlights

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  Fall prescribed burns combusted more surface soil carbon than spring burns.

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  Fall burns stimulated microbial respiration, slowing soil carbon recovery.

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  Microbial respiration was lower after spring burns, allowing soil C to accumulate.

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  Soil MAOC was more resistant to prescribed burning than POC.

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  Soil PyOM did not respond to spring or fall burning.

Abstract

Wildfires burn as much as ∼6 % of Earth’s ice-free land annually and remove carbon (C) stored in surface soils. Prescribed burns may offer an opportunity to preserve soil C stocks by limiting the extent and severity of wildfires. However, prescribed burns in western North America are mostly conducted during short windows in the early fall season. Expanding burning to other seasons, like spring, is key to increasing effectiveness. Yet, it is not clear how burning stands in the spring versus the fall affects soil C stocks, a key consideration for managers seeking to preserve soil C. To assess how burn season affects soil C dynamics, we measured bulk soil C, mineral associated organic C (MAOC), particulate organic C (POC), pyrogenic organic matter (PyOM), microbial respiration, and extracellular enzyme activity in surface soils for two years after burning young mixed-conifer forest stands in either the fall or spring. We found that fall burning, which consumed more surface fuels, immediately combusted soil organic matter, lowering POC from 37 ± 14–14 ± 8.3 g C kg⁻¹ three days after burning. Furthermore, fall burning stimulated microbial respiration, preventing bulk C from increasing above 43 ± 14 g C kg⁻¹ for two years. Spring burning did not immediately decrease soil C, and increased C by 64 % two years after the fire, potentially due to suppressed microbial respiration allowing C to accumulate. Taken together, our results suggest that spring prescribed burns preserved soil C stocks relative to fall burns in young mixed-conifer forest stands.