Studies of canopy nitrogen chemistry and atmospheric inputs of nitrogen to the forest

Project Overview
Project Abstract: 
Studying the nature, chemistry, and impact of atmospheric reactive nitrogen is essential in understanding the fate of NOx and its effect on the biosphere. This project worked to advance this knowledge by observing the sequestration of reactive nitrogen through deposition to soil at UMBS. Our hypothesis stated that while some canopy surface photochemistry can re-release deposited nitrogen to the atmosphere in the form of HONO or NOx, some processing commits that nitrogen to the forest floor where it can be utilized by the biosphere. Further, a significant fraction of nitrogen that is in the soil is deposited from the atmosphere. To investigate this hypothesis and quantify the fraction of nitrate from the atmosphere, we collected and evaluated samples of soil, rain water, and cloud water for their nitrate concentration. This nitrate was further analyzed using an isotope ratio mass spectrometer (IRMS) to determine the major and minor isotopic ratios of oxygen. This ratio allows for the quantification of the atmospheric contribution though the mass independent fractionation (MIF) of oxygen. Oxygen’s isoptopic ratios show an observational and theoretical mass dependent relationship between the d17O and d18O values which is d17O ≈ 0.52×d18O. The notable exception to this rule is the observed equal enrichment of 17O and 18O during the formation of ozone. This 17O “excess” is quantified by d17O = d17O - 0.52d18O. It is from this unique response that we accurately quantified the proportion of atmospheric nitrate in soils by applying isotopic and mass balance. At the UMBS site we were able to determine that the fraction of the soil solution NO3− pool directly from atmospheric deposition was on average 9%. Meaning that this fraction of the soil solution is unprocessed (no microbial turnover) nitrate derived directly from the atmosphere. This points to the potential importance of anthropogenic nitrate deposition and contributes to the long-standing need to improve our understanding of the impacts of atmospheric nitrogen processing and deposition on forest ecosystems and forest productivity.
Investigator Info
Funding agency: 
NSF-IGERT
Years research project active: 
2008 to 2009