Fire severity, nitrogen loss, and plant nitrogen availability in boreal black spruce forests

Nitrogen (N) accumulates slowly in boreal ecosystems and forest productivity is generally N-limited.  Increased fire frequency causes increased opportunity for N loss via combustion and post-fire losses to the atmosphere and aquatic systems.  Understanding controls over N loss during fire and cycling following fire is important for predicting the long-term consequences of an altered fire regime for forest productivity and carbon cycling. We examined the effects of fire severity on N loss (% of ecosystem N stocks lost) and post-fire N availability (resin available NH₄⁺ and NO₃^-) in 30 Interior Alaska black spruce forest sites that burned in 2004. These sites spanned three major fire complexes and covered a range of fire severity and site moisture conditions.  Burn severity and site moisture status were the best predictors of the % of ecosystem N lost during fire.  While well-drained sites that burned severely lost up to 30% of their N stocks, N loss was undetectable in poorly drained sites because of large and heterogeneous N stocks in organic and mineral soil.  Sites that lost the most N during fire had the highest levels of available N following fire, likely due to the effects of fire severity on C substrates for decomposition.  Soil organic matter C:N ratio was substantially lower in more severely burned stands, and SOM C:N ratio was negatively related to N availability. Our results suggest that increased fire severity in boreal black spruce forests will likely to lead to increased N loss during fire and increased N availability early in secondary succession, which appears to play a role in tree establishment and successional trajectory. Future research will explore the relationships among N availability, tree establishment and primary productivity across these sites.