Effects of Disturbance and Land Management on Water, Carbon, and Nitrogen Dynamics in the Terrestrial Ecosystems of the Southern United States.

摘要

Although the climate in the Southern United States (SUS) is warm and wet, with mild winters and high humidity, the terrestrial ecosystems in this region have been greatly disturbed. These disturbances include hurricanes, storms, fires, insect and diseases, floods, extreme droughts and land use and land cover change. Meanwhile, to alleviate these effects, land management practices such as irrigation, fertilization, tillage, forest harvest and thinning have been increasing applied to terrestrial ecosystems. It is still unclear how land disturbance and management have changed the terrestrial ecosystem structure and function. In this dissertation, a systematic approach of integrating field observational data, regional inventory data, remote sensing, and a process-based global biogeochemical model was used to evaluate the impacts of disturbance and management on terrestrial ecosystem carbon, water and nitrogen fluxes in the SUS. Results indicated that although the intensity and duration of drought disturbance in the SUS was not significantly increased, drought events over short periods (a year to a few years) could significantly reduce net primary production (NPP) and C storage (up to 40%). Climate change in the SUS has resulted in a net release of 0.33 Pg C (1 Pg = 1015 g) into the atmosphere, while changes in precipitation and temperature patterns induced C emissions of 0.035 and 0.14 Pg, respectively during 1895-2007. The interactions between precipitation and air temperature induced a C emission of ~0.15 Pg, suggesting that changes in air temperature could significantly enhance drought impacts in the SUS. In total, C emission from drought impacts induced both by precipitation and temperature could be ~0.19 Pg. The western SUS (dry region) was found to act as a C sink, while the east (water-rich region) acted as a C source due to changing precipitation patterns. C sources in the east were significantly enhanced by the interactions between precipitation and temperature changes. With changing climate, land use, and land management, both evapotranspiration (ET) and water yield were increased during 1895-2007, implying that available water in the terrestrial ecosystem of the SUS is decreased. N fertilization has greatly increased carbon storage by ~296 Tg (4.70 Tg yr -1) in the SUS cropland during 1945-2007, while N2O emissions were also significantly enhanced by 2.97 Tg N (0.047 Tg N yr-1). The ratio of N2O emission to fertilized N uses was 2.5% +/- 0.2%, indicating that about 2.5% of fertilized N was emitted as N2O. Combining the global warming potential (GWP) of these two gases, N fertilization was a net source that could enhance the GWP by 304.6 Tg CO2 equivalents during this period. The GWP induced by N fertilization increased after mid 1970s and N fertilization showed a saturation effect for increasing C storage, suggesting that further increases in N fertilizer use would not significantly stimulate C sequestration. To decrease GWP and maintain high crop productivity in the future, crop N use efficiency needs to be increased rather than increasing N fertilizer amounts. Forest disturbance in Mississippi and Alabama has resulted in a 1.3% annual mortality of forest trees during 1984-2007, resulting in a net C source of 0.23 Pg C. Most of this C source is due to the loss of the vegetation C pool since forest biomass accumulation requires a longer recovery time. Although small disturbance events may not significantly change forest structure, the legacy effects of forest disturbance on C storage could last over 100 years. To improve estimation accuracy of US C budget, impacts from small but continuous disturbance events should be taken into account. Combining the impacts of disturbances (Drought, land use change, and forest mortality) and management (N fertilization, site preparation, and forest plantation management) on C, N and water dynamics, this research suggested that disturbances could reduce C storage, NPP and available water resources and increase N2O emission in the SUS, while land management could increase C storage, NPP and N2O emission. Further research is needed to systematically explore the impacts of other major disturbance and management types on C, N and water dynamics in the SUS. The findings from this study could help policy makers and land managers to understand the potential consequences of various disturbance events and management practices, and thus taking precautions against these consequences through making appropriate policies.