Testing a Conceptual Model of Soil Emissions of Nitrous and Nitric Oxides

Investigators from many diverse disciplines—agrono mists, atmospheric chemists, ecologists, geochemists, meteorologists, and microbiologists—all study emissions of nitrous oxide (N2O) and nitric oxide (NO) from soils. Their common interest in soil emissions of nitrogen oxides stems from the attempt to answer the following questions: ... In this article, we briefly review the disciplinary research on soil emissions of N2O and NO. We describe a mechanistically based conceptual model—the “hole-in-the-pipe” (HIP) model (Firestone and Davidson 1989)—that integrates the results of these disciplinary studies and that relates emissions of both nitrogen oxides to common soil processes. We then test the model predictions, using data from our recent studies in Costa Rica (Veldkamp et al. 1999), Brazil (Verchot et al. 1999), and Puerto Rico (Erickson, etal. in press) and additional data from the literature for forest ecosystems throughout the world. Nitrous and nitric oxides are often studied separately by atmospheric chemists because they play such different roles in the atmosphere. N2O is a stable greenhouse gas in the lower atmosphere (the troposphere; Ramanathan et al. 1985), but it participates in photochemical reactions in the upper atmosphere (the stratosphere) that destroy ozone (Crutzen 1970). In contrast, NO participates in photochemical reactions in the troposphere that produce ozone. Tropospheric ozone, also a greenhouse gas (Lammel and Graßl 1995), is a threat to human health and it decreases crop yield (NAS 1991). Although they have different atmospheric fates and consequences, NO and N2O are produced in soils primarily by nitrifying and denitrifying bacteria (Figure 1); the same environmental factors affect the production of both nitrogen oxides (see Firestone and Davidson 1989, Davidson 1991, Williams et al. 1992, Granli and Bøckman 1994).