Soil Carbon in Greenbelt Park

Jay S. Gregg

Wednesday, November 12, 2006

A meter-deep hole was dug in Greenbelt Park Maryland, and six 80-mL samples were collected at varying depths: 0 cm, 10 cm, 30 cm, 50 cm, 70 cm, and 90 cm. Each sample was weighed both wet and dry and the water content was computed. Also, the dry weight and the volume were used to calculate the soil dry bulk density and porosity. Each sample was then ground and analyzed for carbon content at the Geography Department at the University of Maryland, with special acknowledgement going to Dr. Alan Jay Kaufman, Chrissy France, and Nick Collins. The study was guided around two questions: 1) what is the total soil carbon storage of Greenbelt Park? and 2) is there evidence of past disturbance in the soil carbon record?

Greenbelt Park was chosen for a sample site because of its unique history. In the mid 1700s, around the founding of Bladensburg in 1742, European settlers appeared and began clearing the forest for agriculture. Tobacco was grown for about a century until the soil could no longer sustain that crop. Farmers converted to corn and other vegetables around 1850, eventually abandoning the fields around 1900. The land slowly returned to forest and was acquired by the state in 1947 with land for the B-W Parkway. The land attained National Park designation in 1950.

Results show a general decreasing increasing trend in density with depth. There was a decreasing trend in porosity and total carbon content with depth. Cubic spline fitting was used to interpolate data at the millimeter increments of depth, and the carbon weight per cubic meter was estimated by integrating across the spline function. Multiplying this by the area yielded a estimate of 5500 tonnes of carbon in the soil at Greenbelt Park.

Water content also decreased with depth, yet increased again at about 70 cm, where the soil was more clay. The ¹³C/¹²C ratio increased with depth as there was less organic material which tends to have higher concentrations of ¹²C carbon. However, as with mass and total carbon percent, the soil at 50 cm was anomalous to the trend. The soil at 50 cm had less density, greater porosity, greater carbon concentration, and a lower ¹³C/¹²C ratio than the soil at 30 cm and 70 cm. This suggests there may be evidence of past agricultural activities, but there is not enough data to be sure. Another possibility is organic matter from roots is contained in this layer.

The soil samples were collected in only one location, due to cost, permission, and time constraints. A more systematic sampling method across the entire park could answer these questions more thoroughly as well as give some understanding of the spatial variability of soils within the park.