Burn-induced decreases in soil microbial carbon use efficiency vary across soil types and substrates

Wildfires cause immediate changes in above and belowground carbon (C) stocks in boreal forest ecosystems with long-term repercussions for C cycling. Understanding the role of soil microbes in mediating post-fire C cycling and recovery is an important step to predicting how these ecosystems will respond to novel wildfire regimes caused by climate change. Wildfires can cause large shifts in soil bacterial and fungal community composition that can persist for years post-fire. Less is known about the effects of fire on soil microbial community function, such as C use efficiency (CUE). In this study, we measured the effects of burning on substrate-specific CUE using a laboratory incubation of boreal forest soils. We amended burned and unburned soils with either 13C-labelled ground pine roots or glucose and measured the amount of added substrate C that was incorporated into microbial biomass C versus respired as CO2 in order to calculate CUE. Burning caused a decrease in the amount of soil microbial biomass and respiration derived from soil organic C. Glucose-specific CUE declined with burning, driven by a decrease in glucose-derived microbial biomass. This decrease in glucose-specific CUE following burning correlated with an increase in weighted mean predicted 16S rRNA gene copy number, raising the possibility of using copy number as a proxy for post-fire CUE in boreal forest soils. Overall, pine-specific CUE was lower than glucose-specific CUE, likely reflecting the difference in chemical complexity between the two substrates; burning had a much smaller effect on pine-specific CUE, highlighting the variability of CUE between substrates in burned soils.