A53N-0343: A model of nitrogen isotope fractionation during leaf-atmosphere NH3(g) exchange

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Authors: Jennifer E Johnson1, 2, Christopher B Field1, Joseph A Berry1

Author Institutions: 1. Department of Global Ecology, Carnegie Institution for Science, Stanford, CA, USA; 2. Department of Biological Sciences, Stanford University, Stanford, CA, USA

To date, the isotopic composition of NH3(g) in the atmosphere has been ascribed to two processes: volatilization from soils that generates isotopically depleted ammonia and precipitation events that scavenge isotopically enriched ammonium and deposit it to the land surface, leaving residual, isotopically depleted ammonia aloft. Here, we present a mathematical model describing nitrogen isotope fractionation during exchange between NH3(aq) in leaves and NH3(g) in the atmosphere. Two approaches are used to derive the model. The first is a classical, substitution-based method that yields only the equilibrium solution. The second is a matrix-based method that also yields the complete solution. With both approaches, the model predicts that efflux of NH3(g) from leaves isotopically depletes atmospheric NH3(g), whereas uptake isotopically enriches residual atmospheric NH3(g). When there is bidirectional exchange of NH3(g) (i.e., when atmospheric NH3(g) is close to the foliar NH3(g) compensation point), the model predicts that atmospheric NH3(g) can become either isotopically enriched or depleted. These results suggest that exchange between NH3(aq) in leaves and NH3(g) in the atmosphere contributes to variation in the nitrogen isotopic composition of NH3(g) in the atmosphere as well as organic nitrogen in plants, and should be considered in analyses of the distribution of nitrogen isotopes at natural abundance and tracer levels. Thus, the model may be useful for distinguishing between multiple sources and sinks for ammonia in the atmosphere, studying processes within the atmospheric reactive nitrogen cycle, and interpreting records of the isotopic composition of nitrogen-containing atmospheric gases (e.g., N2O) and particles (e.g., NH4+ and NO3 ˆ’ aerosols). In addition, the matrix-based modeling approach that is introduced may be useful for quantifying isotope dynamics in other complex systems in which the classical approach for determining equilibrium solutions becomes unwieldy, or dynamics are of particular interest.

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