SHIFTING INPUTS AND TRANSFORMATIONS OF NITROGEN IN FORESTED AND MIXED LAND USE BASINS: IMPLICATIONS FOR HYDROLOGIC NITROGEN LOSS
Sabo, Robert Daniel
Eshleman, Keith N.
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Increased N inputs along with changes in population, land use, and climate have globally altered the N cycle. This alteration has been associated with increased food, energy, and fiber availability, but has also contributed to the degradation of human health conditions and diminishment of expected ecosystem services in many regions throughout the world. In this context, my research explored the impact of shifting anthropogenic N inputs and other environmental drivers on terrestrial N surpluses and linked changes in terrestrial surpluses to observed changes in N loss to aquatic systems. Working in both forested and mixed land use catchments in the eastern USA, I hypothesized that processes that reduced terrestrial N surpluses in catchments by 1) reducing N inputs, 2) increasing plant uptake, and/or 3) increasing gaseous efflux would result in decreased hydrologic N export. Identification of potential processes was accomplished by first generating long-term atmospheric, remote sensing, terrestrial, and hydrologic datasets for individual catchments. The first two components of my dissertation highlighted potential interactions between atmospheric N deposition, acidic deposition, climate, and disturbance in influencing terrestrial N availability, as indicated by N isotopes in tree rings, in forested catchments. Leveraging trend analysis and statistical models, I identified continued long-term declines in terrestrial N availability in forests, but this decline was likely being modified by disturbance and long-term reductions in acidic deposition. The final component of my dissertation involved developing a lumped conceptual model to explain water quality trends in three mixed land use catchments within the Chesapeake Bay watershed. This study assessed the relative influence of point source N loading, agricultural practices, and atmospheric N deposition on long-term trends in riverine N loss. Insights from the simple N loading model strongly suggested that declines in atmospheric N deposition and point source loading were key drivers of historical water quality improvement. Whether relying on quasi-mass balances or dendroisotopic records, findings from this research emphasize the usefulness of constructing proxy datasets of terrestrial N surpluses in identifying likely processes driving changes in hydrologic N loss in forested and mixed land use catchments.