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Coupled isotopic and process‐based modeling of gaseous nitrogen losses from tropical rain forests

Gaseous nitrogen (N) losses remove fixed N from the biosphere and play an important role in regulating Earth's climate system. Current techniques for measuring gaseous N fluxes are still limited, however, and many uncertainties remain. We used the natural isotopes of N, 15N/14N, to constrain process‐based model (DAYCENT, the daily version of CENTURY) estimates of gaseous N emissions from terrestrial ecosystems. The isotope model considers two scenarios. In the first, soil 15N/14N is a linear function of a fraction of gaseous N losses. In the second, underexpression of denitrification's isotope effect is considered, and soil 15N/14N is determined by both the fraction of gaseous losses and the proportion of NO3− consumed locally by denitrification. We examined the coupled process‐ and isotope‐based model along two Hawaiian rain forest gradients which span a range of tropical climates, soil biogeochemical ages, and ecosystem 15N/14N. Under most conditions (mean annual precipitation (MAP) <4050 mm), modeled soil 15N/14N ratios agreed well with measurements (r2 = 0.89), consistent with full expression of denitrification's isotope effect (scenario 1). In very wet sites (MAP ≥ 4050 mm), locally complete NO3− consumption appears to lower the isotopic expression of denitrification at ecosystem levels, resulting in soil 15N/14N ratios that approach those of the N inputs (i.e., scenario 2). Replacing modeled gaseous N emissions with field‐based measures of oxidized N, gas fluxes (NOx + N2O) resulted in consistently lower estimates of soil 15N/14N ratios across the forests. This points to a missing gas N loss term (i.e., N2), inadequate coverage of spatial and temporal heterogeneity by empirical measures, or both. These results demonstrate the potential for soil N isotopes to constrain N gas fluxes at large geographic scales, providing a quantitative tracer of gaseous N emissions from land.