BACKGROUND

Nitrogen Saturation

Nitrogen saturation of forests occurs when the availability of inorganic nitrogen exceeds demand by plants and microbes (Aber et al. 1989). It is a response to human-caused increases in nitrogen deposition - specifically oxides of nitrogen (nitrate and NOx) mainly from fossil fuel emissions and also ammonium from production and use of fertilizers. With increasing recognition of this problem ecologists have focused more effort on the capacity of forests to retain high inputs of nitrogen and also effects of excess nitrogen on vegetation. In addition, some of these studies target effects of high nitrogen loading in receiving waters.

Two aspects of the Aber et al. paper are especially important to emphasize to students: 1) the non-linear response of key system phenomena (e.g. N cycling processes, N loss to ground and surface waters) to N inputs that is due to a "saturation" point beyond which fundamental changes take place and 2) a shift from positive fertilization effects of N to negative effects. The "good-bad" aspect of N loading to temperate forests may be confusing to students.

High rates of nitrogen deposition to forests lead to a cascade of effects (Vitousek et al. 1997). Increasing concentration of ammonium in soil stimulates nitrifiyers with an accompanying rise in hydrogen ions. Nitrate is mobile in soils and its leaching also results in loss of cations including calcium, magnesium, and potassium. Loss of these nutrients can lead to stunted tree growth and tree mortality as a result of nutrient imbalances in tree roots and leaves. Finally, with calcium depletion and soil acidification, aluminum ions become mobile and are a potential threat to tree roots or aquatic organisms.

Since the 1980's, numerous nitrogen saturation studies have documented: 1) increase in nitrate concentrations in streams and rivers, 2) increased loss of nutrient cations from soils, 3) nutrient imbalances, higher rates of insect and pathogen damage, and reduced frost hardiness, 4) declines in tree growth, especially of evergreens, and 5) higher rates of nitrous oxide emissions (Peterjohn et al. 1999).

The impacts of nitrogen saturation were first seem in Europe in the early 1980's with observations of increased nitrate in some streams and rivers and also yellowing plus needle loss in spruce and other conifers. The problem is most severe in parts of northern Europe compared to North America because nitrogen deposition rates are higher. Japanese watersheds are also experiencing N saturation in urbanized areas (Ohrui and Mitchell 1997). Effects in the U.S. are more limited to high elevations with high inputs and shallow, poorly buffered soils (Vitousek et al. 1997).

The Broader Context

As Jefferies and Maron (1997) point out, a century ago scientists debated whether atmospheric inputs of N were sufficient for plant growth. Today we recognize that more N is fixed each year by humanity than by natural processes in terrestrial ecosystems (see also Data Set 1 of this Issue). This shifts the ecological focus to fate of this anthropogenic N and its effects on systems.

Galloway et al (2003) take the phenomenon of nitrogen saturation further with their "nitrogen cascade" idea. They divide N compounds into reactive and nonreactive, and they propose that in prehuman times, reactive N did not accumulate due to a balance of nitrogen fixation and denitrification. In contrast today reactive N is accumulating on all spatial scales.

Results from numerous studies done in the 1990's now allow more synthetic analysis of nitrogen saturation in terrestrial ecosystems. For example, in a return to the question "Is the N status of northeastern forests being altered by N deposition?" Aber et al. (2003) say: "… our analysis suggests that the answer to this question is yes, although the degree of response varied greatly across the three different categories of indicators we examined. The surface water data suggest a strong relationship between NO3- concentration and flux across the N deposition gradient. The soil data show strong relationships between N deposition, soil C:N ratio, and nitrification in several cases, but the strength and significance of these trends differed among forest types and soil horizons. Finally, in the foliar data set, significant relationships with N deposition did not emerge beyond the covarying effects of climate and elevation".

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References

• Aber, J. D., J. M. Melillo, K. J. Nadelhoffer, J. Pastor, and R. D. Boone. 1991. Factors controlling nitrogen cycling and nitrogen saturation in northern temperate forest ecosystems. 1991. Ecological Applications 1: 303-315.


• Aber, J. D., et al. 2003. Is nitrogen deposition altering the nitrogen status of northeastern forests? BioScience 53: 3750389.


• Galloway, J. N., et al. 2003. The nitrogen cascade. BioScience 53: 341-356.


• Jefferies, R. L., and J. L. Maron. 1999. The embarrassment of riches: atmospheric deposition of nitrogen and community and ecosystem processes. Trends in Ecology and Evolution 12: 74-78.


• Ohrui, K., and M. J. Mitchell. 1997. Nitrogen saturation in Japanese forested watersheds. Ecological Applications 7: 391-401.


• Peterjohn, W. T., C. J. Foster, M. J. Christ, and M. B. Adams. 1999. Patterns of nitrogen availability within a forested watershed exhibiting symptoms of nitrogen saturation. Forest Ecology and Management 119: 247-257.


• Vitoursek, P. M., J. D. Aber, R. W. Howarth, G. E. Likens, P. A. Matson, D. W. Schindler, W. H. Schlesinger, and D. G. Tilman, 1997. Human alteration of the global nitrogen cycle: causes and consequences. Issues in Ecology 1: 1-15.

WWW Sites

See Resources section of this Issue for internet sites from LTER studies, figures for use in class, EPA, and utility sites.