There are many factors controlling the movement of NH 3 through the forest ecosystem making it difficult to assess to what degree is the forest a source or sink of NH 3. However, the exchange of NH 3 in forested regions has been shown to be dependent on the atmospheric concentrations. Soil NH x can also undergo nitrification as part of the natural N cycling occurring within the soil leading to the production of NO 2 − and NO 3 −, as shown in Scheme 1.ĭue to their N sensitivity, forest ecosystems are generally believed to be a sink for NH 3, which has led to a large body of research focusing on what are the critical loads for these systems. NH 4 + can be chemically fixed to clay minerals and organic matter, physically sorbed to colloids, or dissolved in soil pore water.
In contrast, the fate and mobility of NH x (dissolved NH 3 + NH 4 +) found in soils is dictated by soil type, moisture content, and local microbiota. As a result, the lifetime of NH x, the sum of gas phase NH 3 and particle phase NH 4 +, can range from several hours to several days before being removed from the atmosphere by dry or wet deposition.
NH 3 is also the main neutralizing agent for atmospheric trace acidic gases, such as HCl, HNO 3, or H 2SO 4, so it can also be found in particulate form (NH 4 +), allowing it to travel long distances. A large portion of NH 3 sources are directly linked to human activities, such as agriculture, transportation, and industry. By accounting for the total pool in each compartment, the lifetime of NH 3 with respect to the surface-atmosphere exchange in the soil is on the order of years compared to much faster naturally occurring processes, i.e., mineralization and nitrification.Ītmospheric ammonia (NH 3) plays an important role in the global nitrogen cycle. Inferred fluxes (F g) from each soil type predicted intervals of emission and deposition. Two different soil types were measured multiple times throughout the study, in which Γ g ranged from 5 to 2122. Measurements were performed at the Manitou Experimental Forest Observatory (MEFO) site in the Colorado Front Range by continuous online monitoring of gas and particle phase NH 3-NH 4 + with an ambient ion monitoring system coupled with ion chromatographs (AIM-IC), direct measurements of and pH in soil extracts to determine ground emission potential (Γ g), and measurements of bulk in pine needles to derive leaf emission potential (Γ st). Compensation point modeling was used to evaluate the direction and magnitude of surface-atmosphere exchange. The goal of this study was to characterize this exchange by measuring the atmosphere, soil, and vegetation. However, NH 3 exchange is dynamic and difficult to measure. Understanding the NH 3 exchange between forest ecosystems and the atmosphere is important due to its role in the nitrogen cycle.
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