Post Fire Forest Floor Fire Severity Index Relationships with Soil Carbon, Nitrogen and Mercury Pools.

Fire is a disturbance that has important implications on carbon (C), nitrogen (N), and mercury (Hg) dynamics.  Although we know fire leads to combustion of forest floor and mineral soil organic matter and losses of C, N and Hg, little research has assessed how fire severity influences post-fire pools of those elements. If we can relate fire severity indices with changes in soil elemental pools, fire severity can be used as a surrogate to evaluate C, N and Hg emissions post fire.  We used forest floor fire severity indices both aggregated at the plot scale and individual measurements at the subplot scale where soils were sampled, to assess our ability to predict C, N and Hg losses following the 2011 Pagami Creek Fire in northern Minnesota. We sampled forest floor and 0-10, and 10-20 cm mineral soil depths directly after fire and again after one growing season. During the first sampling we compare fire severity indices and forest floor and mineral soil C, N, and Hg aggregated at the plot scale.  During the second sampling we assessed forest floor fire severity and measured forest floor and mineral soil C, N, and Hg at the subplot scale.  Our results indicate that aggregating at the plot scale does not differentiate among forest floor fire severity categories.  Differences in mercury pools where only found between burned and unburned plots.  However, assessing forest floor fire severity at the subplot scale and relating it directly to the forest floor and mineral soil samples taken at that scale explain much more variability in the relationship between forest floor fire severity and post fire C, N and Hg pools.