Coupling between the carbon and nitrogen cycles in a forested environment
Abstract
Over the past century humans have dramatically altered the global nitrogen cycle via the combustion of fossil fuels, agricultural activities, and many industrial processes. The amount of atmospheric nitrogen deposition is predicted to more than double in the next 40 years due to a continued increase in these activities. Consequently, these changes will dramatically alter the global carbon cycle affecting the rate of increase of carbon dioxide and global climate change. It is likely that the perturbations to these cycles will result in a cascade of environmental and human health problems. Thus, determining the fate of atmospheric nitrogen is of fundamental importance in understanding these consequences. This is especially true in environments with significant plant life, such as forests, which act as important sinks for anthropogenic pollutants. This work addresses the fate of atmospheric nitrogen in three parts; (1) investigating the production of alkyl nitrates in the aqueous phase, (2) determining the fraction of soil nitrogen that is from atmospheric deposition, and (3) identifying and quantifying the evolution of gas-phase organic nitrates above a forest canopy. (1) Chapter two discusses the investigation of the aqueous phase yield of light alkyl nitrates. Laboratory studies were conducted to determine if the oxidation of organic matter in solution is a significant source of light alkyl nitrates in the atmosphere by photolyzing solutions of methyl iodide that were saturated with nitric oxide and oxygen. Samples were irradiated in a custom-designed chamber and analyzed using gas chromatography with an electron capture detector and a purge and trap pre-concentrator. Results showed a significantly higher yield than is physically possible (∼200%), leading to the discovery of a co-eluting, non-photochemical product, which to-date was unidentifiable. This chapter discusses the necessary modifications to the project, and potential methods for identification of the side reaction products. (2) Chapter three describes the quantification of the fraction of soil nitrate that is directly deposited from the atmosphere. To do this, a distinctive isotopic signal, D17O, was used to trace atmospheric nitrate through the ecosystem. Soil, cloud water, and precipitation were collected in a nitrogen limited, temperate forest located at the University of Michigan Biological Station (UMBS) in northern Lower Michigan. Isotopic analysis (D17O, d18O, d17O) of extractable nitrate was conducted using an isotope ratio mass spectrometer and the results show that, on average, 9% of the soil solution NO3 - is unprocessed (no microbial turnover) nitrogen derived directly from the atmosphere. This chapter involves in-depth discussion of the impact of this 9% with respect to the forest nitrogen cycle and forest productivity. (3) Chapter four discusses the production of isoprene nitrates from both hydroxyl and nitrate radical reactions with isoprene and its oxidation products. Synthesized standards of isoprene hydroxynitrates were analyzed in combination with photochemical chamber studies to identify isomer elution order and quantify the relative yields. Results showed that the previously published yields and elution order were incorrect. Nitrate radical reaction products were also identified. Ambient measurements using a gas chromatograph electron capture detector (GC/ECD) were conducted above a forest canopy at UMBS in an attempt to measure speciated isoprene nitrate production in the atmosphere. These results showed that there was too much chemical interference to isolate the nitrate yields using the GC/ECD. However, as detailed in chapter 5, a zero dimensional (0-D, solely chemical kinetics) photochemical model was constructed to investigate the fate of nitrogen during isoprene oxidation. From this model, it was determined that isoprene nitrates are not a significant permanent sink for NOx. Rather, the secondary products from oxidation of isoprene nitrates are more important in influencing the fate and long-range transport of atmospheric nitrogen.
Degree
Ph.D.
Advisors
Shepson, Purdue University.
Subject Area
Atmospheric Chemistry|Biogeochemistry|Environmental science
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