Nitrogen watersheds

Peterson BJ, Wollheim WM, MuUiolland PJ, Webster JR, Meyer JL, Tank JL, Marti E, Bowden WB, Valett HM, Hershey AE, McDowell WH, Dodds WK, Hamilton SK, Gregory SV, Morral DD (2001) Control of nitrogen export from watersheds by headwater streams. Science 292 86-90... 

Garten, C.T., Jr. 1993 Variation in foliar abundance and the availability of soil nitrogen on Walker Branch watershed. Ecology 74 2098-2113. 

Similarly to N, most S pools are found in organic form in forest floor and soil humus. However, unlike nitrogen, there are important abiotic processes, especially sulfate sorption processes, which play a critical role in regulating sulfate dynamics in forest ecosystems. An example of this type of exposure pathway was shown in the Habbard Brook whole-tree harvesting experiment, where the decrease in sulfate output from the watershed was attributed to sulfate adsorption, which was enhanced by soil acidification from nitrification (see above). 

Long-Term Changes in Watershed Retention of Nitrogen... 

Watersheds are generally several orders of magnitude larger than the surface waters that drain them. Thus most of the atmospheric deposition that may potentially enter aquatic systems falls first on some portion of the watershed. Nitrogen may be deposited to the watershed or directly to water surfaces... 

Atmospheric N can enter aquatic systems either as direct deposition to water surfaces or as N deposition to the terrestrial portions of a watershed. Nitrogen deposited to the watershed is routed and transformed by watershed processes. It may eventually reach aquatic systems in forms only indirectly related to the original deposition. The transformations that N undergoes within the watershed (e.g., in soils, by microbial action, and in plants) play a major role in determining what forms and amounts of N eventually reach surface waters. Much of the challenge of determining when N deposition is... 

Figure 1. A simplified watershed nitrogen cycle, with major pathways (arrows) and their effects on the watershed hydrogen budget (numbers in circles) shown. Circled numbers represent the number of hydrogen ions transferred to the soil solution or surface water ( +1) or from the soil solution or surface water (-l)for every molecule of N03 or NH4 + that follows a given pathway. For example, nitrification follows the pathway for NH4 + assimilation into microbial biomass ( + l) and is leached out as N03 ( + 1), for a total hydrogen ion production of +2 for every molecule of N03 produced.

Much of the debate over whether aquatic systems are being affected by N deposition centers on the concept of nitrogen saturation of forested watersheds. Nitrogen saturation can be defined as a situation in which the... 

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