Forest Soil Black Carbon Retention a Decade after a Fire in Northwestern Colorado

Rebecca Elkus Poore, University of Colorado Boulder

Abstract

Soil carbon is the largest pool of terrestrial organic carbon. Black carbon (BC) is the most recalcitrant fraction of that pool. During a forest fire, while large amounts of carbon are released as carbon dioxide, a small fraction of the biomass consumed by the fire is only partially combusted, yielding soot and charcoal. These products make up only 1-5% of the biomass burnt, yet they can have a disproportionate effect on both the atmosphere and fluxes in long-term terrestrial carbon pools.

Although debated, most studies indicate a BC turnover time on the order of 500-1,000 years, so charcoal may function as a long-term carbon sink. However, its overall significance depends on its rates of formation and loss. At the landscape level, fire characteristics are one of the major factors controlling BC production. This study examines the effects of both fire severity and stand age on soil BC pools.

In northern Colorado in 2002, a large fire swept through a subalpine spruce, fir and lodgepole pine forest. From 2010-2013, I sampled BC pools in 43 sites across a range of fire severities from unburned to severe crown fire. To measure BC, I used a hydrogen peroxide and nitric-acid digestion method, which reliably measures a portion of the BC continuum. I found no difference in overall soil BC across fire severity, or between burned and unburned plots. Stand age did not explain variation in soil BC in unburned plots, indicating that soil BC pools remain relatively stable through time. However, older plots that burned with high severity showed significantly less soil BC, likely due to different fire characteristics either directly changing BC production or indirectly effecting subsequent erosion.