Terrestrial nitrogen loading

As a result of their geographical circumstances and these various historical developments, many estuaries appear to be among the most heavily loaded ecosystems in the world in terms of the amount of nitrogen and phosphorus received per unit area (Fig. 5). Even the farm fields of the American corn belt do not receive the fertilization common to many estuaries, and the nutrient inputs to natural terrestrial systems and fresh waters appear to be orders of magnitude lower. There are fewer data available on the inputs of metals and other pollutants to estuaries, but the evidence at hand suggests that the delivery of heavy metals to estuaries may equal or exceed that deposited from the atmosphere on heavily industrialized urban areas and exceed that deposited on rural terrestrial ecosystems by orders of magnitude (Fig. 6). 

Figure 8. Exceedances of critical loads of sulfur and nitrogen Jor terrestrial ecosystems of Ukraine in 1992 deposition.

As a consec [uence of population growth and rapid economic development, Increasing loads of nitrogen and sulfur on the terrestrial ecosystems might well not remain limited to the known hotspots in Europe and North America but could expand and become critical as well for tropical and subtropical regions. 

The rates of precipitation deposition are mapped today and allow a comparison to be made with the results of the forest decline inquiry. To get below the critical load limit for the acid input under the present conditions of reduced input of alkaline dust, it is necessary to reduce not only the S02-emission but also the emission of NO and NHj. The reduction of the nitrogen compounds is also necessary in view of a reduction of the possible eutrophication of terrestrial ecosystems, the nitrate load of the groundwater and surface waters, as well as a reduction of the ozone load. 

Figure 5.4. The Nitrogen Saturation Hypothesis (from Aber et al. 1989 1998, reprinted with permission, copyright American Institute of Biological Sciences), a conceptual model of terrestrial ecosystem responses to chronically elevated nitrogen (N) deposition. This model identifies four stages of ecosystem response to N loading. Soil responses are shown in the upper panel, and plant responses in the lower panel. See text for discussion...

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