Human activities have caused changes in the earth's nutrient cycles and weather patterns. The combustion of fossil fuels has led to a large increase in the amount of carbon dioxide (CO2) in the atmosphere. Past studies have focused on potential storage of carbon within terrestrial plants. However, soils are a much larger and more long-term carbon pool. This thesis uses a 13C label to trace carbon through grassland soils in northern California. Soils were exposed to elevated CO2 (using FACE technology), water, nitrogen and temperature. New soil carbon inputs were determined by comparing plant and soil isotope values. Using information about NPP and new soil carbon, this study develops a model of soil carbon incorporation. After seven years of exposure to elevated carbon dioxide total new carbon present in the soils was 704.68+43.75 g/m2. There were no significant effects of treatment on new soil carbon incorporation and soils did not show any significant accumulation or loss of carbon over time. However, there is evidence for a priming effect during the first few growing seasons. Additionally, an examination of the differences between new carbon incorporation and production suggest that nitrogen and water may have important impacts on decomposition in this system. These findings agree with earlier studies conducted at the same location.