CARBON SEQUESTRATION IN THE DRAKE PRAIRIE
Carol Kim, Matthew Jurysta, Kathryn Szramek, David Courard-Hauri; Environmental Science & Policy Program, College of Arts & Sciences
Abstract
While land use alterations currently result in the addition of about 2 gigatons of
carbon to the atmosphere annually, changes in bioproductivity and soil storage
have the potential to serve as an important managed sink as well. Reforestation
and afforestation have received the most attention in this regard, but native
grasslands, with high short-term production of belowground biomass, may also
provide significant sequestration opportunities when compared with agricultural
systems and managed turfgrass, although this claim is controversial in the
literature. In order to determine whether significant differences in soil carbon
content could be observed between a historical turfgrass and restored prairie
system, we measured soil carbon levels at sixteen sites in and around Drake’s
restored prairie fragment north of Meredith Hall. Soil samples were taken in
roughly 15 cm increments to a depth of one meter (where possible), and carbon
content was determined through destructive heating.
Hypothesis
Because of their deep roots, we hypothesize that prairie grasses will lead to soil
carbon enrichment, and that this enrichment will be particularly pronounced at
depths of 0.5 – 1 m.
Results
We found that Drake prairie soil and turfgrass do differ significantly in SOC,
but that the difference is only significant in the first 30 cm of soil. Assuming
differences only in the first 30 cm of soil, we estimate that the prairie soil
contains approximately 1.3% more SOM than turfgrass soil in this region
(integral of the difference between the curves divided by the distance).
The density of dry soil is approximately 1.3 g/cm3, giving an excess SOM in
prairie soils of 5.2 kg/m2, and an excess SOC of 3 kg/m2. This suggests that the
Drake prairie has most likely sequestered nearly two metric tons of carbon, or
about 350 kgC/yr. This annual sequestration is the equivalent to the amount of
carbon dioxide emitted by a typical car driving about 5000 miles.
The calculation above includes only soil carbon, and not the carbon content of
the plants and roots themelves, or other organic debris. A future project would
be to measure these values to include in the calculation.
Introduction
Understanding the sequestration of carbon in various soils is of importance
because soil ecosystems are a major reservoir of organic carbon. Because carbon
sequestration in soils and biomass is seen as a low-cost mechanism for carbon
dioxide mitigation, significant effort has gone into determining the sequestration
potential for agricultural and forest lands. However, comparatively little has gone
into determining whether heavily managed land, such as lawns, might serve as
sinks or sources for carbon dioxide depending upon what is grown there. Although
there is some work suggesting that turfgrass can lead to carbon sequestration1, we
hypothesize that this work may simply measure the mineralization of roots from
the former ecosystem, and not new sequestration.
Our research consists of determining the amount of total organic carbon present in
soil growing turfgrass in comparison to soil covered by native prairie grass.
Materials and Methods
We collected soil cores along two transects perpendicular to the slope of the northfacing hill that contains the prairie, as well as one sample above the prairie and
one below. There were a total of 16 sample sites and 141 soil cores samples: 8
within the prairie and 9 around the surrounding turf grass. Soil cores (2 cm
diameter) were taken from each sample site at an approximate total depth of 1
meter. Soil color recorded, and samples were placed in sealed plastic bags.
After collection, soil was air dried then stored in a freezer for a period of one to
five months. Prior to analysis, the contents of each bag was thawed, and large
clumps were broken with a hammer. Organic matter was removed along with
pebbles, and the contents of were divided into eight separate samples, of which
three were randomly chosen and stored for future analysis. Each sample was then
dried overnight at 250°C to remove any remaining water, and then sifted through
a 2 mm sieve. The remaining soil was then weighed and placed into a kiln for
eight hours at 400°C. This second heating oxidized and volatilized Soil Organic
Matter (SOM), the total mass of which was equal to the difference between the
pre-processing and post-processing weights. Soil Organic Carbon (SOC) can be
estimated as SOC = SOM x 0.58. (see 2, 3 for information on carbon
measurement)
Logarithmic fits to SOM content of both prairie (circles) and turfgrass (crosses) at various depths. Center lines are best
fits, blue regions indicate estimated standard deviations.
Acknowledgements
We thank Robert Craig and Edward of the Drake University Art Department for
their extreme generosity in watching the kiln to keep its temperature stable
while samples were having organic carbon driven off. The kiln needed to be
checked with great frequency (every 15 minutes or so) during the entire 8-hour
heating period, and we processed samples nearly weekly during the spring
semester. Their assistance was invaluable to the success of this project.
References
1. Qian, Y. & Follett, R.F. (2009) Assessing soil carbon sequestration in turfgrass systems using long-term soil testing data.
Agronomy Journal, 94, 930-935.
2. Blanco-Canqui, H. & Lai, R. (2004) Mechanisms of carbon sequestration in soil aggregates. Critical Reviews in Plant
Sciences. 23, 481-504.
3. Hesse, P.R. (1972) A Textbook of Soil Chemical Analysis. Chemical Publishing Company, London.