Soil-air exchange of nitric oxide: An overview of processes, dnvironmentalvactors, and modeling studies

Terrestrial ecosystems with their main elements soil and plants may act, in principle, as both source and sink for atmospheric nitric oxide (NO). The net exchange between ecosystems and the atmosphere, however, is globally do minated by biogenic emissions of NO from soils. Consequently the soil-air e xchange of NO is the focus of the following overview. Particular emphasis i s placed on the major processes that are responsible for NO production in s oils (nitrification, denitrification) and their regulation by environmental factors (nitrogen availability, soil water content, soil temperature, ambi ent NO concentration). It is shown that interactions of these factors are a major reason for the broad range that exists in published data on NO fluxe s. This variability makes it difficult to predict the magnitude of NO fluxe s on relevant spatial and temporal scales. To overcome the problem various generalization procedures for scaling up in space and time have been develo ped, and the potential and limitations of the different approaches is discu ssed.