Sources of N

Biogeochemical effects

Physiological responses

Community responses

Case studies

Future study

References

Links

Nitrogen Cycle

Controlled Fertilization Experiments
There have been several long-term nitrogen fertilization experiments in Europe (Gundersen, 1991; Wright et al., 1995; Boxman et al., 1998) and the United States (Gilliam et al., 1996; Magill et al., 1997; Aber et al., 1998).  The advantage of this study design is that it allows for direct comparison of fertilized plots to non-fertilized control plots.  However, the application of liquid fertilizer may differ from natural nitrogen deposition.

Nitrogen Deposition Gradient Studies
Natural gradients in nitrogen deposition have been identified along longitudinal and elevational gradients (Friedland et al., 1991; McNulty et al., 1991; McNulty et al., 1996; Lovett and Rueth, 1999).  Natural gradients allow researchers to study long-term impacts of nitrogen deposition directly.  However, because of other confounding factors, it may be difficult to draw strong conclusions from correlative data.

The response of any particular site to nitrogen saturation is affected by numerous factors including species composition, nitrogen availability, soil characteristics, and land-use history.  While many of these factors are discussed elsewhere on this site, the importance of land-use history as a determining factor in nitrogen status should be emphasized here.  Aber et al (1998) believe that the stage of nitrogen saturation for the sites included in their review was most strongly influenced by its history.  For example, young forest stands growing on abandoned agriculture land may have a higher nitrogen status due to previous fertilization.  

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