A comparative analysis of soil charcoal from different forest types along the Colorado Front Range

Background/Question/Methods

Black charcoal (BC) is a form of soil C that can remain in situ potentially for thousands of years. Soil BC has been quantified in some instances, mostly for boreal forests in northern latitudes, tropical ecosystems in Amazonia, and in some agricultural settings. In addition, limited experimental work has been conducted on soil BC in western US forests. Here we present results from comparisons of soil BC in different forest stand types along the Rocky Mountain Front Range. We analyzed soil samples from ponderosa pine (Pinus ponderosa) and lodgepole pine (Pinus contorta) stands located just west of Denver, CO at 2500 m elevation. Mineral soil samples were collected beneath the O horizon at 0-10 cm (n=12 per stand). Initially, we measured total soil C with a Carlo-Erba CHN analyzer. Subsequently, with other subsamples, we contrasted two methods that remove all forms of 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 after treatment and results are extrapolated to an areal basis.

Results/Conclusions

Black carbon in our ponderosa soil samples came to 9 kg ha^-1 (SE± 0.7) and 24 kg ha^-1 (SE± 3.7) for the combustion and digest methods respectively. For lodgepole pine soils we found soil BC at 5 kg ha^-1 (SE± 0.8) and 14 kg ha^-1 (SE± 1.5) respectively. Regardless of the method, the soil black carbon is significantly greater in ponderosa soils than in lodgepole soils where these are sampled at the same elevation (one-way ANOVA p<0.05). This is interesting because we expected lodgepole pine soils to have a larger soil BC pool. Finally, soil BC (which is rarely measured as a separate fraction) is 3.1% (CT0-375) or 8.1% (digest) and 3.5% (CTO-375) or 9.8% (digest) of total soil C in ponderosa and lodgepole soils respectively. Future work will expand these analyses to include all forest types from foothills treeline to alpine treeline. Using these data we expect to determine how much soil BC is present in forest soils at the landscape level from grassland to tundra.