PARENT SESSION
Organized Oral Session 36: Role of microbial communities in mediating biogeochemical response to disturbance
Organizer(s): EAH Smithwick and MG Turner
Wednesday, August 10, 1:30 PM - 5:00 PM, Meeting Room 511 B, Level 5, Palais des congrès de Montréal

The role of fire and fire exclusion in forest nitrogen cycling.

DeLuca, Thomas^*,1, ¹ Department of Ecosystem and Conservation Sciences, Missoula, MT, USA

ABSTRACT- Timber management and active fire suppression have led to the exclusion of fire from both boreal and temperate pine forests. It is well understood that fire exclusion results in a change in fuel loading and stand structure making the forest more susceptible to high severity stand replacing wildfires. Far less is known regarding the influence of long-term fire exclusion and ultimately the reintroduction of fire on soil microbial processes and in particular those that influence N turnover. Studies conducted on a variety of sites in the interior Northwestern US and boreal Sweden serve as the basis for this discussion. Fire exclusion has placed pine ecosystems in both regions in late secondary succession resulting in significant changes in vegetative composition and microbial processes. Nitrogen capital increases with fire exclusion, but N availability declines. The reintroduction of fire to fire excluded ecosystems results in a significant loss of organic N and a short-term increase in N availability. Sites exposed to natural fire return intervals maintain the presence and activity of nitrifying organisms, perhaps related to the deposition of char deposited during fire events. Dominant N forms in late succession likely influence species success. Continued fire exclusion will clearly alter N dynamics in forest ecosystems that directly and indirectly influence the potential for recovery of natural ecosystem function.

Key words: Nitrification, Nitrogen-fixation, Pinus spp., Fire