Watershed nitrogen inputs

Kelly, J. M., and J. F. Meagher. 1986. Nitrogen input/output relationships for three forested watersheds in eastern Tennessee. In Watershed Research Perspectives (D. L. Correll, Ed.). Smithsonian Press, Washington, DC. 

Nitrogen inputs to watersheds, which results in DIN export to coastal systems, often leads to enhanced primary production, since many estuaries are N limited. This can result in the formation of HABs as well as hypoxia, and in worst case scenarios anoxic water columns see other effects in chapter 15. 

Relationship between Nitrogen Inputs to Watersheds and Riverine... 

Figure 36.2 Nitrogen fluxes per area of watershed to the coastal ocean as a function of net anthropogenic nitrogen inputs to the landscape per unit area. Reprinted from Howarth et al. (1996).

Figure 36-4 Relationship between nitrogen inputs and riverine N fluxes from watersheds spanning multiple scales and biomes of the world. Nitrogen inputs are expressed as (A) net total nitrogen inputs and (B) net anthropogenic nitrogen inputs. Modified from Boyer et al. (2006).

Figure 36.10 Comparison of anthropogenic nitrogen inputs and associated riverine N exports (kg per km of watershed area per year) in two contrasting water regions of the USA The northeast USA and the Mississippi River hasin. Modified from NRC (2000) based on data from Howarth et al. (1996).

The calculations (Howarth, 1996) showed that for the North Atlantic Ocean watershed the percentage of sewage and wastewaters to total nitrogen input varies among the regions, from a high of 34% in the North Sea region to virtually none in the Amazon basin (Table 6). 

Table 7. Major net anthropogenic nitrogen inputs to the North Atlantic watersheds by region, kg/krri /year.

Figure 5. Application of nitrogen in fertilizer and by atmospheric deposition ofFiOy to 13 of the watershed regions. Values are in kg N/km /year. Fertilizer inputs dominate net anthropogenic nitrogen inputs in most, but not all, regions of the North Atlantic basin (Howarth, 1996).

Results from studies investigating nitrogen input to some other estuarine and coastal regions are summarized in Table 3. In this table atmospheric sources for nitrogen are compared with all other sources, where possible. The atmospheric input ranges from 10% to almost 70% of the total. Note that some estimates compare only direct atmospheric deposition with all other sources and some include as part of the atmospheric input the portion of the deposition to the watershed that reaches the estuary or coast. 

Some of the nitrogen inputs from atmospheric deposition is nitrogen deposited directly onto the surface of coastal waters. This direct deposition to surface waters often contributes between 1% to 40% of the total nitrogen inputs to coastal ecosystems (Nixon et al. 1996 Paerl 1997 Howarth 1998 Paerl and Whitall 1999 Valigura et al. 2000). The direct deposition is most significant in very large systems, such as the Baltic Sea (Nixon et al. 1996) or in coastal systems which have relatively small watersheds in comparison to the area of the surface waters, such as Tampa Bay (Zarbock et al. 1996). 

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