The use of stable isotopes to constrain the nitrogen cycle

Krystin M Riha, Purdue University


Nitrogen (N) is a crucial element which is essential for life and is necessary for all organisms to live and grow. However, N compounds have also been acknowledged for their many detrimental impacts on the environment. Human activities have dramatically altered the global N cycle through energy production (fossil fuel combustion), production of synthetic fertilizers, and cultivation of legumes and other crops. In this study we aim to resolve sources of reactive N and its fate in semi-arid urban environments using stable isotopes abundances. We have shown conclusively, for the first time, that in semi-arid urban environments the fractional contributions of atmospheric nitrate dominate compared to biologically derived nitrate observed in runoff. This study employs this use of nitrogen and triple oxygen isotopes of nitrate (NO3-) to infer changes in the nitrogen biogeochemical cycle (e.g. NO3- source appointment, processing, and atmospheric chemistry) in semi-arid urban environments. However, analytical and isotopic approaches come with their caveats many of which are overlooked. The recent isotopic analysis method which employs the use of denitrifying bacteria coupled with subsequent gold tube thermal decomposition has several shortcomings. Adaptations have been made to: simultaneously analyze nitrogen and oxygen isotopes, discuss detection limits, determine the isotopic effects of sample preparation as well as improve calibration curves to encompass the full range of environmental samples and eliminate the need for extrapolation and improper corrections. One way to determine the sources of nitrogen input to these environments is through the use of multiple isotope analysis (δ15N, δ 18O and Δ17O). However, the commonly used 'dual isotope' approach for NO3- source appointment is based on of limited studies conducted in forested and coastal ecosystems and is not conclusive of all ecosystem NO3- values. Poor separation also occurs between NO3-sources, particularly atmospheric and nitrification, as well as misinterpretation of NO3- values due to fractionation occurring during NO3- processing. However, many N biogeochemical studies still employ this approach which can lead to inconclusive or incorrect results. Improvements to the dual isotope approach presented in this dissertation include isotopic constraints of NO3- sources including atmospheric samples from a variety of locations across the globe, a larger set of fertilizer NO3- samples, modeled nitrification NO3- δ18O values, and inclusion of an alternative dual isotope approach using Δ17O which allows for better source separation. These NO3- source constraints were employed in a case study to determine the effects of urbanization on the coupled nitrogen hydrologic cycle in the semi-arid urban environment of Tucson, AZ. It was found that, contrary to an abundant amount of literature, variations in atmospherically derived NO3- was not controlled by changes in NO x source emissions but rather by shifts in meteorological conditions and atmospheric chemistry. Regardless of Δ17O or δ 18O approach, the fraction of atmospheric NO3- exported from all the urban catchments, throughout the study period, were sustainably higher than in nearly all other ecosystems. Most studies trying to quantify atmospheric NO3- export have attempted to use elevated δ18O values as a tracer of atmospheric NO3- with focus on forested and alpine ecosystems and have shown minimal to no atmospheric contribution to surrounding waterways. The variability in the fractional contribution in the study catchments changes over the course of the storm events suggesting different pools of nitrate are being mobilized during changing hydrologic conditions. The isotope data suggests that type of drainage substrate, such as concrete or vegetated washes, influences N cycling within the individual catchments.




Michalski, Purdue University.

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