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, (15)N/(14)N, 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 (15)N/(14)N is a linear function of a fraction of gaseous N losses. In the second, underexpression of denitrification's isotope effect is considered, and soil (15)N/(14)N is determined by both the fraction of gaseous losses and the proportion of NO(3)(-) 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 (15)N/(14)N. Under most conditions (mean annual precipitation (MAP) < 4050 mm), modeled soil (15)N/(14)N ratios agreed well with measurements (r(2) = 0.89), consistent with full expression of denitrification's isotope effect (scenario 1). In very wet sites (MAP >= 4050 mm), locally complete NO(3)(-) consumption appears to lower the isotopic expression of denitrification at ecosystem levels, resulting in soil (15)N/(14)N 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 (NO(x)+N(2)O) resulted in consistently lower estimates of soil (15)N/(14)N ratios across the forests. This points to a missing gas N loss term (i.e., N(2)), 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.