Regulating the Tempo of Nitrogen Turnover in Soils: Microbial and

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Nitrogen is the nutrient that limits plant growth in most terrestrial ecosystems, but in some cases excess nitrogen can have negative environmental impacts. Past work has emphasized inorganic forms of nitrogen; however, in many ecosystems dissolved organic nitrogen is a larger pool. Relatively little is known about the rates of production and consumption of the dissolved organic nitrogen pool. This study will develop and test a new method to study the dynamics of dissolved organic nitrogen in soil. The method is a modification of the "isotope dilution" technique commonly used to study the turnover of inorganic soil nitrogen pools. The new method will produce dissolved organic nitrogen that is labeled with a stable isotope (15N). The labeled dissolved organic nitrogen will be added to soil to follow its consumption, by tracing the label into other nitrogen pools, and production, by measuring the dilution of the added (labeled) dissolved organic nitrogen as it is replaced by unlabeled dissolved organic nitrogen formed by natural microbial activity. If successful, this new application of the isotope dilution approach will provide a powerful tool for ecosystem scientists to study an important, but poorly understood aspect of the nitrogen cycle. It will have applications to both natural and managed ecosystems. More broadly, the insights that may be provided by the use of this approach may enable us to enhance plant productivity while minimizing detrimental environmental effects.