A Comparative Analysis of Soil Charcoal from Different Forest Types Along the Colorado Front Range.

Black charcoal (BC) is a form of soil C that can remain in situ for thousands of years.  Although research on BC is limited in western US forests it has been quantified for some boreal forests, a few tropical sites, and several agricultural settings.  Here we present results from BC comparisons for ponderosa (P. ponderosa) and lodgepole (P. contorta) pine stands located west of Denver, CO.  Mineral soil samples were collected beneath the O horizon at 0-10 cm (n=12 per stand).  We contrasted two methods that remove all soil C except BC.  The CTO-375 method uses 1.0 g dry soil placed in a muffle furnace for 24 hours at 375° C.  The second method is a chemical digest with 1.0 g dry soil and a 1.0 M nitric acid and 30% hydrogen peroxide solution that is heated in a block digester at 100° C for 16 hours.  For both methods BC is determined via dry combustion post-treatment with results extrapolated to an areal basis.  To estimate the BC proportion of total C in these soils we measured total soil C with a Carlo-Erba CHN analyzer.  Black carbon in ponderosa soils was 9 kg ha^-1 (SE± 0.7) versus 24 kg ha^-1 (SE± 3.7) for the combustion and digest methods respectively.  For lodgepole soils, BC was 5 kg ha^-1 (SE± 0.8) and 14 kg ha^-1 (SE± 1.5) respectively.  We expected lodgepole soils to have a larger BC pool, however regardless of method, BC is significantly greater in ponderosa soils than in lodgepole soils (one-way ANOVA p<0.05).  Finally, BC as a percentage of total soil C is 3.1% (muffle) or 8.1% (digest), and 3.5% (muffle) or 9.8% (digest) in ponderosa and lodgepole soils respectively.  Future work will include BC comparisons of all forest types from foothills treeline to alpine treeline.