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Below-Ground Carbon

by Jennifer Levy

Spring 2008 

In terrestrial ecosystems, soils are the largest component of the terrestrial carbon sink. Carbon is stored in soil, in root systems of plants, and in forest floor litter (leaves, twigs, acorns). In the autumn and throughout the growing season, carbon is transferred from above ground to the below-ground system through falling leaves. It is continually lost from the below-ground system through erosion and soil respiration (the release from the ground of carbon dioxide (CO₂), a product of the bacterial breakdown of organic matter as well as plant and associated mycorrhizal processes).

Previous Black Rock Forest research  has produced measurements of above-ground carbon storage, but not one study has thoroughly examined the below-ground component. The focus of my research was to establish a baseline estimate of the below-ground carbon allocation in six of the oak forest study plots [Ed. Note: See “Oak Project Update,” p. 1].

Last summer, I measured growing season soil respiration, extracted soil cores to determine root biomass and soil carbon content, and collected forest floor litter layer samples.  I then created a model to estimate the yearly soil respiration rate based on a dataset from Harvard Forest . I combined my data with measurements of above-ground litter input from 2006 to estimate the maximum amount of carbon allocated below ground.

The results indicate that up to 1.38 kg carbon (C)/m²/yr could be distributed below ground. The two largest fluxes in the carbon budget are soil respiration and above-ground litter fall. Compared to other northeastern deciduous forests, Black Rock exhibited one of the highest growing season soil respiration rates, which led to an overestimate of annual soil CO₂ efflux. Consequently, this estimate describes the maximum amount of carbon allocated below ground. Research in a similar forest revealed a large interannual variability associated with soil respiration. The measured soil CO₂ efflux at Black Rock could represent interannual variation or a higher basal rate of soil respiration.  Above-ground litter fall measurements agree with those from other oak-dominated forests.

A recent publication examining long term changes in plant composition, structure, and biomass at Black Rock estimated the total ecosystem carbon storage in 2000 to be 17.5 kg C/m².  Assuming this estimate is representative of today’s processes, the equivalent of 7.9% of total ecosystem carbon is being pumped underground each year. The below-ground distribution is important in understanding the carbon budget of this forest and similar forests in the northeast. Shifts in allocation patterns have the potential to determine a forest’s standing as a carbon source or sink, thus impacting the global carbon cycle.

Jennifer Levy is a doctoral student in Columbia University’s Department of Earth and Environmental Science.  She conducted this study for her master’s research.