Fire, Charcoal, and the Biogeochemistry of Carbon and Nitrogen in Pacific Northwest Forest Soils

Abstract

The rain shadow forests of the Olympic peninsula represent a unique, mixed-severity fire regime class in the midst of a highly productive landscape where spatial heterogeneity of fire severity may have significant implications for below and aboveground post-fire recovery. The purpose of this study was to quantify the impacts of wildfire on forest carbon (C) and nitrogen (N) pools and assess the influence of charcoal in a mixed-severity ecosystem on the Olympic Peninsula, Washington, USA. We established a fire chronosequence in forest stands ranging in time since fire (TSF) from 3 to 115 years prior to site establishment. At each site, we measured vegetation abundance, overstory composition, and attributes of surface mineral soil to a depth of 10 cm and forest floor organic matter that included pH, texture, bulk density, and C and N pools (dissolved organic C [DOC], phenol, ammonium, nitrate). Non-ionic resin lysimeters were buried at the interface of organic and mineral soil to measure the O-horizon leached DOC that would contact charcoal particles on the forest floor. Charcoal particles collected from the chronosequence sites were used in adsorption batch experimentation with phenol as a sorbate and measured an average 29.70 (± 6.23) μg phenol mg charcoal-1 adsorption capacity, which did not differ significantly between chronosequence sites. Wildfire-produced charcoal along the chronosequence showed high variability in adsorption capacity, which was partially explained by the thermogravimetric region of volatilized adsorbed compounds onto charcoal surfaces. The O-horizon leachate averaged 1.05 (SD ± 2.87) g DOC m-2 year-1 and increased significantly along the TSF gradient (Pearson’s r = 0.52; p < 0.0001). Multivariate, non-parametric analysis of soil and vegetation factors showed a significant relationship with the time since fire gradient between sites (p-value < 0.01) but not within sites. The TSF gradient was significantly correlated to charcoal mass in the O-horizon (r = -0.4), O-horizon C (r = 0.4), phenolic content in both O-horizon (r = 0.4) and mineral soils (r = 0.2), and potentially mineralizable N (r = 0.4). Recent sites contained higher mineral soil total N and inorganic available N, though not significantly correlated with the TSF gradient. Over time, soils appear to shift toward phenolic-rich organic and mineral soils, higher moss cover, and a higher potentially mineralizable nitrogen index. This study provides evidence of a multivariate, belowground soil response that is less sensitive to wildfire disturbances than the aboveground vegetation.