CARBON BALANCES ACROSS VIRGINIA: A COMPARATIVE STUDY OF PIEDMONT AND SHENANDOAH VALLEY FARMS
We compared 4 farms, 2 in the Shenandoah Valley (SV: Blandy Experimental Farm, Bell’s Lane Farm) and 2 in the Piedmont (P: Glade Creek Farm, Charlton Farm), with 2 SV forests (Paul State Forest, Glick Farm). Parcels ranged between 40 and 280 ha, and are underlain by a diversity of rock types, from calcareous shale to granites and muscovite schists. Soils data (USDA Web Soil Survey) were compiled for each parcel; we compared soil organic matter (SOM), bulk density, and rock fragment data for map units with field-collected data. Family farm records of land use and agricultural practices were also compiled; soil organic carbon (SOC) was related to SOM via a SV-specific algorithm (%SOC=0.5*%SOM+0.3, R2=0.92, n=34).
Mapped SOM underestimated measured SOM at Bell’s Lane Farm, in some cases by a factor of 6. Expressed volumetrically, the topsoil (0.1 m) of Bell’s Lane Farm contains either ~400 (mapped) or ~1900 (measured) kg SOM/m2∙0.1 m (=0.7 or 3.0 Tg SOM across the 162-ha farm). Farms were sampled by catena to avoid topographic artifacts in map comparisons.
Surprisingly, mapped carbon densities (expressed as Mg SOM/m2) varied less between SV and P soil map units (range: 0.2-0.7) than between mapped and measured densities (range: 0.4-1.9). These mismatches highlight the need for more detailed soil carbon mapping as Virginia farmers explore ways of maintaining profitability in an evolving political, cultural, and environmental landscape. Forested parcels can contain up to 2.2 Mg SOM/m2, so there is potential for order-of-magnitude increases in carbon storage. More detailed baseline measurements of soil carbon will clarify some of the environmental consequences of land use and land cover change for this region.