Soil carbon and nitrogen stocks in a semiarid urban ecosystem

The Earth's soils hold an estimated 1500 Pg of carbon and 100 Pg of nitrogen. Soils thus figure prominently into global C and N cycling. Human-related land cover changes have the potential to alter soil C and N pools, with implications for atmospheric CO2 concentrations, ecosystem nitrogen availability, and other global and regional characteristics. Land cover modifications associated with urbanization are occurring at high rates in the USA and elsewhere, yet studies of such changes are currently underrepresented in the ecological literature. This thesis examines the effects of urbanization on soil C and N stocks in a semiarid region of the US. Soil samples were collected and analyzed from 38 residential parcels ranging in age from 7 to 100 years. Along this urban age gradient in the Salt Lake Valley, UT metropolitan area, soil organic C (SOC) and total N (TN) stocks increased significantly as a function of yard age. The strongest trends and largest gains were observed in the upper 20 cm beneath lawns. At this depth yard age was the site characteristic exerting the greatest influence on lawn SOC and TN stocks, eclipsing other factors such soil texture, prior land use and elevation. SOC and TN were highly correlated at all depths and beneath all pervious ground covers, indicating a close association between C and N cycling in this urban ecosystem. Inorganic C content of soils was highly variable both within and between sites and showed no trend with regard to yard age. For the upper 20 cm of residential lawns, the results presented here suggest a more than three-fold increase in average SOC and more than two-fold increase in average TN following a century of urban land use. These gains exceed the average amount of C and N measured in aboveground tree biomass on residential study sites.