| Wetlands provide very important ecosystem services, such as removal of excess nutrients from uplands, and governance of fate and transport of various contaminants. However, there has been a steady deterioration of such ecosystem services due to interference from anthropogenic factors, primarily rapid urbanization and intensive agricultural practices. Effects of such individual disturbances on natural wetland biogeochemistry have been widely studied, but the combined influence of the disturbances is yet to be explored. This dissertation examines the effects of the combination of extensive urban development and intensive agricultural practices on wetland biogeochemistry. We analyzed the influence of vegetation removal, representative of extensive urban development, and excess urea application, a significant indicator of intensive agricultural practices, on soil iron and nitrogen cycles in wetlands. Iron (Fe) is an abundant redox species in wetlands which plays a key role in wetland biogeochemistry, and in contaminant removal. Similarly, nitrogen cycle regulates nutrient processing in these ecosystems. For investigation these processes, experiments were conducted at different scales ranging from laboratory analysis of soil collected from Assumpink wildlife management area, a riparian wetland in New Jersey, to in situ experiments.; Our studies showed urea-treated soils had elevated concentrations of bioavailable Fe(III), and a higher rate of Fe(III) production. Such results demonstrate that wetland soils with excess of urea runoff could potentially release contaminants bound to Fe(III) complexes due to their increased bioavailability for reduction. In terms of nitrogen species, when wetland soil was individually subjected to vegetation removal or urea application, only inorganic nitrogen pool was affected, whereas when applied together, the treatments affected the organic nitrogen pool as well. Our study conclusively demonstrates that depending on the type and intensity of disturbance, different components of nitrogen cycle are affected.; Seasonal flooding is an important characteristic of wetland behavior. However, working of the nitrogen cycle in anaerobic conditions is poorly understood. In order to develop a deeper understanding of nutrient processing in anaerobic conditions, the dissertation analyzes ammonium oxidation in such conditions, and the role of iron in the process. Contrary to commonly accepted notion, nitrite was observed in the anaerobic wetland soil without any initial nitrate concentrations, and ammonium was confirmed to be the source of nitrite in such conditions. Determination of in-situ utilization rates of ammonium and nitrite demonstrated the observed nitrite concentrations to be in a steady state. Absence of manganese oxides and other oxidized nitrogen compounds, led to the hypothesis that iron hydroxides could be the oxidant for ammonium in the soil. Previously, cycle of oxic and anaerobic conditions in wetlands was considered to be a pre-requisite for oxidation of ammonium into nitrite or nitrate and then reduction to dinitrogen. These new results demonstrate the potential for complete removal of excess ammonium even in inundated anaerobic conditions.; The dissertation establishes that urbanization and excessive fertilizer use alter the soil nitrogen and iron cycles, thereby changing the entire biogeochemistry of a wetland ecosystem. In addition, the thesis demonstrates the possibility of anaerobic ammonium oxidation in wetland soil. This enhances our knowledge of the nitrogen cycle, and reveals its previously unrealized potential for nitrogen removal in anoxic conditions. |
