Post-fire nitrogen translocation by wood-decaying fungi in southwestern ponderosa pine forests

Background/Question/Methods

Stand-replacing wildfires alter the ecosystem carbon (C) balance of southwestern ponderosa pine forests by leaving vast amounts of dead wood on the landscape. The subsequent decay of this wood releases carbon dioxide (CO₂) and shifts the ecosystem from a net sink (pre-fire) to a net source (post-fire) of C to the atmosphere. Fungi are responsible for the majority of wood decay; however, little is known about the effects of wildfire on fungal communities and the related impacts on wood decomposition rates. Fires may cause heat-related fungal mortality and induce changes in soil biogeochemistry, particularly nitrogen (N) availability to fungi. We conducted a laboratory experiment to assess the effects of stand-replacing wildfire and N availability on wood decomposition. We predicted that fungi would translocate soil N to their active mycelia within the wood, thereby stimulating decomposition. We incubated sterile wooden pine dowels with soil collected from paired burned and unburned sites along a wildfire chronosequence (burn years: 1977, 1984, 1996, and 2001) and added N fertilizer to the soil as ¹⁵N ammonium sulfate. We measured microbial CO₂ respiration (as an indicator of decomposition) and the amount of ¹⁵N translocated into the wood tissue.

Results/Conclusions

CO₂ respiration did not vary between burned and unburned sites, nor was it affected by the addition of N. However, more ¹⁵N was translocated to wood at the two recent burns than the two older burns and the unburned sites. When coupled with associated data on fungal community composition, our findings suggest that distinctions in fungal communities as influenced by time since wildfire may lead to functional differences in ecosystem-level processes. Specifically, decaying wood may serve as an important post-fire N sink, and this may ultimately influence wood decomposition rates over the long-term.